[RFC] fcntl-linux.h: add new definitions and manual updates for open file description locks
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Message ID 1398253172-9221-1-git-send-email-jlayton@redhat.com
State New
Headers show

Commit Message

Jeff Layton April 23, 2014, 11:39 a.m. UTC
(I haven't yet sent the patch to rename these to Linus, but here's a
 new draft version of the glibc header/manual updates for open file
 description locks. Please speak up if you see any issues or object
 to the new name.)

Open file description locks have been merged into the Linux kernel for
v3.15.  Add the appropriate command-value definitions and an update to
the manual that describes their usage.

ChangeLog:

2014-04-23  Jeff Layton  <jlayton@redhat.com>

	[BZ#16839]
	* manual/llio.texi: add section about open file description locks

	* sysdeps/unix/sysv/linux/bits/fcntl-linux.h:
	  (F_OFD_GETLK, F_OFD_SETLK, F_OFD_SETLKW): New macros.
---
 manual/examples/ofdlocks.c                 |  75 +++++++++
 manual/llio.texi                           | 237 ++++++++++++++++++++++++++++-
 sysdeps/unix/sysv/linux/bits/fcntl-linux.h |  17 +++
 3 files changed, 326 insertions(+), 3 deletions(-)
 create mode 100644 manual/examples/ofdlocks.c

Comments

Michael Kerrisk (man-pages) April 23, 2014, 12:37 p.m. UTC | #1
Hi Jeff,

Comments below.

On Wed, Apr 23, 2014 at 1:39 PM, Jeff Layton <jlayton@redhat.com> wrote:
> (I haven't yet sent the patch to rename these to Linus, but here's a
>  new draft version of the glibc header/manual updates for open file
>  description locks. Please speak up if you see any issues or object
>  to the new name.)
>
> Open file description locks have been merged into the Linux kernel for
> v3.15.  Add the appropriate command-value definitions and an update to
> the manual that describes their usage.
>
> ChangeLog:
>
> 2014-04-23  Jeff Layton  <jlayton@redhat.com>
>
>         [BZ#16839]
>         * manual/llio.texi: add section about open file description locks
>
>         * sysdeps/unix/sysv/linux/bits/fcntl-linux.h:
>           (F_OFD_GETLK, F_OFD_SETLK, F_OFD_SETLKW): New macros.
> ---
>  manual/examples/ofdlocks.c                 |  75 +++++++++
>  manual/llio.texi                           | 237 ++++++++++++++++++++++++++++-
>  sysdeps/unix/sysv/linux/bits/fcntl-linux.h |  17 +++
>  3 files changed, 326 insertions(+), 3 deletions(-)
>  create mode 100644 manual/examples/ofdlocks.c
>
> diff --git a/manual/examples/ofdlocks.c b/manual/examples/ofdlocks.c
> new file mode 100644
> index 000000000000..544001769272
> --- /dev/null
> +++ b/manual/examples/ofdlocks.c
> @@ -0,0 +1,75 @@
> +/* Open File Description Locks Usage Example
> +   Copyright (C) 1991-2014 Free Software Foundation, Inc.
> +
> +   This program is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU General Public License
> +   as published by the Free Software Foundation; either version 2
> +   of the License, or (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU General Public License for more details.
> +
> +   You should have received a copy of the GNU General Public License
> +   along with this program; if not, if not, see <http://www.gnu.org/licenses/>.

s/if not, if not/if not/

> +*/
> +
> +#define _GNU_SOURCE
> +#include <stdio.h>
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +#include <unistd.h>
> +#include <fcntl.h>
> +#include <pthread.h>
> +
> +#define FILENAME       "/tmp/foo"
> +#define NUM_THREADS    3
> +#define ITERATIONS     5
> +
> +void *
> +thread_start(void *arg)
> +{
> +       int i, fd, len;
> +       long tid = (long)arg;
s/)/) /

> +       char buf[256];
> +       struct flock lck = {
> +               .l_whence       = SEEK_SET,
> +               .l_start        = 0,
> +               .l_len          = 1,
> +       };
> +
> +       fd = open("/tmp/foo", O_RDWR|O_CREAT, 0666);

s/|/ | /

> +
> +       for (i = 0; i < ITERATIONS; i++) {

I am no lover of the GNU coding style, but I suspect this program
should be respecting it with respect to the brace style...

> +               lck.l_type = F_WRLCK;
> +               fcntl(fd, F_OFD_SETLKW, &lck);
> +
> +               len = sprintf(buf, "%d: tid=%ld fd=%d\n", i, tid, fd);
> +
> +               lseek(fd, 0, SEEK_END);
> +               write(fd, buf, len);
> +               fsync(fd);
> +
> +               lck.l_type = F_UNLCK;
> +               fcntl(fd, F_OFD_SETLK, &lck);
> +
> +               usleep(1);

I think the usleep() call needs to be either removed or explained.

> +       }
> +       pthread_exit(NULL);
> +}
> +
> +int
> +main(int argc, char **argv)
> +{
> +       long i;
> +       pthread_t       threads[NUM_THREADS];

Why the large white space gap?

> +
> +       truncate(FILENAME, 0);
> +
> +       for (i = 0; i < NUM_THREADS; i++)
> +               pthread_create(&threads[i], NULL, thread_start, (void *)i);
> +
> +       pthread_exit(NULL);
> +       return 0;

The "return 0" isn't needed.

> +}
> diff --git a/manual/llio.texi b/manual/llio.texi
> index 6f8adfc607d7..b5c2d07ad929 100644
> --- a/manual/llio.texi
> +++ b/manual/llio.texi
> @@ -57,6 +57,10 @@ directly.)
>                                           flags associated with open files.
>  * File Locks::                          Fcntl commands for implementing
>                                           file locking.
> +* Open File Description Locks::         Fcntl commands for implementing
> +                                         open file description locking.
> +* Open File Description Locks Example:: An example of open file description lock
> +                                         usage
>  * Interrupt Input::                     Getting an asynchronous signal when
>                                           input arrives.
>  * IOCTLs::                              Generic I/O Control operations.
> @@ -2890,7 +2894,7 @@ Get flags associated with the open file.  @xref{File Status Flags}.
>  Set flags associated with the open file.  @xref{File Status Flags}.
>
>  @item F_GETLK
> -Get a file lock.  @xref{File Locks}.
> +Test a file lock.  @xref{File Locks}.
>
>  @item F_SETLK
>  Set or clear a file lock.  @xref{File Locks}.
> @@ -2898,6 +2902,18 @@ Set or clear a file lock.  @xref{File Locks}.
>  @item F_SETLKW
>  Like @code{F_SETLK}, but wait for completion.  @xref{File Locks}.
>
> +@item F_OFD_GETLK
> +Test a open file description lock.  @xref{Open File Description Locks}.
> +Specific to Linux.
> +
> +@item F_OFD_SETLK
> +Set or clear a file lock.  @xref{Open File Description Locks}.
> +Specific to Linux.
> +
> +@item F_OFD_SETLKW
> +Like @code{F_OFD_SETLK}, but wait for completion.
> +@xref{Open File Description Locks}.  Specific to Linux.
> +
>  @item F_GETOWN
>  Get process or process group ID to receive @code{SIGIO} signals.
>  @xref{Interrupt Input}.
> @@ -3576,6 +3592,10 @@ set_nonblock_flag (int desc, int value)
>
>  @cindex file locks
>  @cindex record locking
> +This section describes record locks that are associated with the process.
> +There is also a different type of record lock that is associated with the
> +open description instead of the process.  @xref{Open File Description Locks}.

s/open/open file/

> +
>  The remaining @code{fcntl} commands are used to support @dfn{record
>  locking}, which permits multiple cooperating programs to prevent each
>  other from simultaneously accessing parts of a file in error-prone
> @@ -3641,7 +3661,10 @@ the file.
>  @item pid_t l_pid
>  This field is the process ID (@pxref{Process Creation Concepts}) of the
>  process holding the lock.  It is filled in by calling @code{fcntl} with
> -the @code{F_GETLK} command, but is ignored when making a lock.
> +the @code{F_GETLK} command, but is ignored when making a lock.  If the
> +conflicting lock is an open file description lock
> +(@pxref{Open File Description Locks}), then this field will be set to
> +@math{-1}.
>  @end table
>  @end deftp
>
> @@ -3813,10 +3836,218 @@ that part of the file for writing.
>
>  @c ??? This section could use an example program.
>
> -Remember that file locks are only a @emph{voluntary} protocol for
> +Remember that file locks are only an @emph{advisory} protocol for
>  controlling access to a file.  There is still potential for access to
>  the file by programs that don't use the lock protocol.
>
> +@node Open File Description Locks
> +@section Open File Description Locks
> +
> +In contrast to process-associated record locks (@pxref{File Locks}),
> +open file description record locks are associated with an open file
> +description rather than a process.
> +
> +Using @code{fcntl} to apply a open file description lock on a region that
> +already has an existing open file description lock that was created via the
> +same file descriptor will never cause a lock conflict.
> +
> +Open file description locks are also inherited by child processes across
> +@code{fork}, or @code{clone} with @code{CLONE_FILES} set
> +(@pxref{Creating a Process}), along with the file descriptor.
> +
> +It is important to distinguish between the file @emph{description} (an

s/file/open file/

> +instance of an open file, usually created by a call to @code{open}) and
> +a file @emph{descriptor}, which is a numeric value that refers to the
> +former.  The locks described here are associated with the file

I'd s/former/open file description/ for clarity

and
s/file/open file/

> +@emph{description} and not the file @emph{descriptor}.
> +
> +Using @code{dup} (@pxref{Duplicating Descriptors}) to copy a file
> +descriptor does not give you an entirely new file description , but

s/entirely//
(it's redundant, and possibly even confusing.)


> +rather copies a reference to an existing file description and assigns it

s/file/open file/

> +to a new file descriptor.  Thus, open file description locks set on a file
> +descriptor cloned by @code{dup} will never conflict with
> +open file description locks set on the original descriptor since they refer
> +to the same file description.  Depending on the range and type of lock

s/file/open file/

> +involved, the original lock may be modified by a @code{F_OFD_SETLK} or
> +@code{F_OFD_SETLKW} command in this situation however.
> +
> +Open file description locks always conflict with process-associated locks,
> +even if acquired by the same process or on the same open file
> +descriptor.
> +
> +Open file description locks use the same @code{struct flock} as
> +process-associated locks as an argument (@pxref{File Locks}) and the
> +macros for the cmd values are also declared in the header file

s/cmd/@code{command}/

> +@file{fcntl.h}. To use them, the macro @code{_GNU_SOURCE} must be
> +defined prior to including @file{fcntl.h}.

s/prior to including @file{fcntl.h}/before including any header file/

(See http://man7.org/linux/man-pages/man7/feature_test_macros.7.html )

> +
> +In contrast to process-associated locks, any @code{struct flock} used as
> +an argument to open file description lock commands must have the @code{l_pid}
> +value set to @math{0}.   Also, when returning information about a

s/s/an/

> +open file description lock in a @code{F_GETLK} or @code{F_OFD_GETLK} request,
> +the @code{l_pid} field in @code{struct flock} will be set to @math{-1}
> +to indicate that a lock is not associated with a process.
> +
> +When the same struct flock is reused as an argument to a

s/struct flock/@code{struct flock}/

> +@code{F_OFD_SETLK} or @code{F_OFD_SETLKW} request after being used for an
> +@code{F_OFD_GETLK} request, it is necessary to inspect and reset the
> +@code{l_pid} field to @math{0}.
> +
> +@pindex fcntl.h.
> +
> +@deftypevr Macro int F_OFD_GETLK
> +This macro is used as the @var{command} argument to @code{fcntl}, to
> +specify that it should get information about a lock.  This command
> +requires a third argument of type @w{@code{struct flock *}} to be passed
> +to @code{fcntl}, so that the form of the call is:
> +
> +@smallexample
> +fcntl (@var{filedes}, F_OFD_GETLK, @var{lockp})
> +@end smallexample
> +
> +If there is a lock already in place that would block the lock described
> +by the @var{lockp} argument, information about that lock overwrites

s/overwrites/is written to/

> +@code{*@var{lockp}}.  Existing locks are not reported if they are
> +compatible with making a new lock as specified.  Thus, you should
> +specify a lock type of @code{F_WRLCK} if you want to find out about both
> +read and write locks, or @code{F_RDLCK} if you want to find out about
> +write locks only.
> +
> +There might be more than one lock affecting the region specified by the
> +@var{lockp} argument, but @code{fcntl} only returns information about
> +one of them. Which lock is returned in this situation is undefined.
> +
> +The @code{l_whence} member of the @var{lockp} structure is set to
> +@code{SEEK_SET} and the @code{l_start} and @code{l_len} fields set to identify

s/set/are set/

> +the locked region.
> +
> +If no conflicting lock exists, the only change to the @var{lockp} structure
> +is to update the @code{l_type} field to the value @code{F_UNLCK}.
> +
> +The normal return value from @code{fcntl} with this command is either @math{0}
> +on success or @math{-1}, which indicates an error. The following @code{errno}
> +error conditions are defined for this command:
> +
> +@table @code
> +@item EBADF
> +The @var{filedes} argument is invalid.
> +
> +@item EINVAL
> +Either the @var{lockp} argument doesn't specify valid lock information,
> +the operating system kernel doesn't support open file description locks, or the file
> +associated with @var{filedes} doesn't support locks.
> +@end table
> +@end deftypevr
> +
> +@comment fcntl.h
> +@comment POSIX.1
> +@deftypevr Macro int F_OFD_SETLK
> +This macro is used as the @var{command} argument to @code{fcntl}, to
> +specify that it should set or clear a lock.  This command requires a
> +third argument of type @w{@code{struct flock *}} to be passed to
> +@code{fcntl}, so that the form of the call is:
> +
> +@smallexample
> +fcntl (@var{filedes}, F_OFD_SETLK, @var{lockp})
> +@end smallexample
> +
> +If the opened file already has a lock on any part of the

s/opened/open/

> +region, the old lock on that part is replaced with the new lock.  You
> +can remove a lock by specifying a lock type of @code{F_UNLCK}.
> +
> +If the lock cannot be set, @code{fcntl} returns immediately with a value
> +of @math{-1}.  This function does not wait for other tasks

s/function/command/

> +to release locks.  If @code{fcntl} succeeds, it returns @math{0}.
> +
> +The following @code{errno} error conditions are defined for this
> +function:
> +
> +@table @code
> +@item EAGAIN
> +The lock cannot be set because it is blocked by an existing lock on the
> +file.
> +
> +@item EBADF
> +Either: the @var{filedes} argument is invalid; you requested a read lock
> +but the @var{filedes} is not open for read access; or, you requested a
> +write lock but the @var{filedes} is not open for write access.
> +
> +@item EINVAL
> +Either the @var{lockp} argument doesn't specify valid lock information,
> +the operating system kernel doesn't support open file description locks, or the
> +file associated with @var{filedes} doesn't support locks.
> +
> +@item ENOLCK
> +The system has run out of file lock resources; there are already too
> +many file locks in place.
> +
> +Well-designed file systems never report this error, because they have no
> +limitation on the number of locks.  However, you must still take account
> +of the possibility of this error, as it could result from network access
> +to a file system on another machine.
> +@end table
> +@end deftypevr
> +
> +@comment fcntl.h
> +@comment POSIX.1
> +@deftypevr Macro int F_OFD_SETLKW
> +This macro is used as the @var{command} argument to @code{fcntl}, to
> +specify that it should set or clear a lock.  It is just like the
> +@code{F_OFD_SETLK} command, but causes the process to wait until the request
> +can be completed.
> +
> +This command requires a third argument of type @code{struct flock *}, as
> +for the @code{F_OFD_SETLK} command.
> +
> +The @code{fcntl} return values and errors are the same as for the
> +@code{F_OFD_SETLK} command, but these additional @code{errno} error conditions
> +are defined for this command:
> +
> +@table @code
> +@item EINTR
> +The function was interrupted by a signal while it was waiting.
> +@xref{Interrupted Primitives}.
> +
> +@end table
> +@end deftypevr
> +
> +Open file description locks are useful in the same sorts of situations as
> +process-associated locks. They can also be used to synchronize file
> +access between threads within the same process by having each thread perform
> +its own @code{open} of the file, to obtain its own file description.

s/file/open file/

> +
> +Because they are only released automatically when the last reference to
> +file description is closed, open file description locks avoid the possibility

s/file description/open file description/

But, I'd rewrite the sentence a little:

Because open file description locks are automatically freed only upon
closing the last file descriptor that refers to the open file
description, this locking mechanism avoids the possibility...

> +that locks are released due to a library routine opening and closing a file
> +without the application being aware.
> +
> +As with process-associated locks, open file description locks are advisory.
> +
> +@node Open File Description Locks Example
> +@section Open File Description Locks Example
> +
> +Here is an example of using open file description locks in a threaded program. If
> +this program used process-associated locks, then then it would be

s/then then/then/

> +subject to data corruption due since multiple threads are running within
> +the same process.

That last piece is a biut confusing. How about

...because process-associated locks are shared by the threads inside a
process, and thus cannot be used by one thread to lock out another
thread in the same process.

> Proper error handling has been omitted in the following
> +program for brevity.
> +
> +@smallexample
> +@include ofdlocks.c.texi
> +@end smallexample
> +
> +This example spawns three threads and has each do five iterations of
> +appending to a file.

Better:

This example creates three threads, each of which loops five times,
appending to the file.

> Access to that file is serialized via open file

s/taht/the/

> +description locks. If we compile and run the above program, we'll end up
> +with /tmp/foo that has 15 lines in it.
> +
> +If we, however, were to replace the @code{F_OFD_SETLK} and @code{F_OFD_SETLKW}
> +commands with their process-associated lock equivalents, the locking
> +essentially becomes a noop since it is all done within the context of
> +the same process. That leads to data corruption (typically manifested
> +as missing lines) as some threads race in and overwrite the data from

s/from/written by/

> +others.
> +
>  @node Interrupt Input
>  @section Interrupt-Driven Input
>
> diff --git a/sysdeps/unix/sysv/linux/bits/fcntl-linux.h b/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
> index 915eb3ede560..455389cd2c2a 100644
> --- a/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
> +++ b/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
> @@ -117,6 +117,23 @@
>  # define F_SETLKW64    14      /* Set record locking info (blocking).  */
>  #endif
>
> +/* open file description locks.
> +
> +   Usually record locks held by a process are released on *any* close and are
> +   not inherited across a fork.
> +
> +   These cmd values will set locks that conflict with process-associated record
> +   locks, but are "owned" by the opened file description, not the process.
> +   This means that they are inherited across fork or clone with CLONE_FILES
> +   like BSD (flock) locks, and they are only released automatically when the
> +   last reference to the the file description against which they were acquired
> +   is put. */
> +#if __USE_GNU
> +# define F_OFD_GETLK   36
> +# define F_OFD_SETLK   37
> +# define F_OFD_SETLKW  38
> +#endif
> +
>  #ifdef __USE_LARGEFILE64
>  # define O_LARGEFILE __O_LARGEFILE
>  #endif
> --
> 1.9.0
>

Cheers,

Michael
Alfred M. Szmidt April 23, 2014, 3:09 p.m. UTC | #2
@@ -2890,7 +2894,7 @@ Get flags associated with the open file.  @xref{File Status Flags}.
    Set flags associated with the open file.  @xref{File Status Flags}.

    @item F_GETLK
   -Get a file lock.  @xref{File Locks}.
   +Test a file lock.  @xref{File Locks}.

F_GETLK does get the (first) lock which blocks; it doesn't test for
it.  "Retrieves information about the first blocking lock ..." or
something might be better than the original

   @@ -2898,6 +2902,18 @@ Set or clear a file lock.  @xref{File Locks}.
    @item F_SETLKW
    Like @code{F_SETLK}, but wait for completion.  @xref{File Locks}.

   +@item F_OFD_GETLK
   +Test a open file description lock.  @xref{Open File Description Locks}.
   +Specific to Linux.

Likewise.  You infact write that it does get the lock information
later in the document wrt. F_OFD_GETLK.
Jeff Layton April 23, 2014, 3:28 p.m. UTC | #3
On Wed, 23 Apr 2014 11:09:51 -0400
ams@gnu.org (Alfred M. Szmidt) wrote:

>    @@ -2890,7 +2894,7 @@ Get flags associated with the open file.  @xref{File Status Flags}.
>     Set flags associated with the open file.  @xref{File Status Flags}.
> 
>     @item F_GETLK
>    -Get a file lock.  @xref{File Locks}.
>    +Test a file lock.  @xref{File Locks}.
> 
> F_GETLK does get the (first) lock which blocks; it doesn't test for
> it.  "Retrieves information about the first blocking lock ..." or
> something might be better than the original
> 
>    @@ -2898,6 +2902,18 @@ Set or clear a file lock.  @xref{File Locks}.
>     @item F_SETLKW
>     Like @code{F_SETLK}, but wait for completion.  @xref{File Locks}.
> 
>    +@item F_OFD_GETLK
>    +Test a open file description lock.  @xref{Open File Description Locks}.
>    +Specific to Linux.
> 
> Likewise.  You infact write that it does get the lock information
> later in the document wrt. F_OFD_GETLK.

Sorry, I disagree here...GETLK is really a misnomer, IMO. TESTLK would
have been a better name.

It's a way to test whether a particular lock can be applied, and to
return information about a conflicting lock if it can't. If, for
instance there is no conflicting lock, then you don't "get" any lock
information back (l_type just gets reset to F_UNLCK).
Alfred M. Szmidt April 23, 2014, 7 p.m. UTC | #4
> Likewise.  You infact write that it does get the lock information
   > later in the document wrt. F_OFD_GETLK.

   Sorry, I disagree here...GETLK is really a misnomer, IMO. TESTLK
   would have been a better name.

   GETLK are used is to "get the first lock".

   It's a way to test whether a particular lock can be applied, and to
   return information about a conflicting lock if it can't. If, for
   instance there is no conflicting lock, then you don't "get" any
   lock information back (l_type just gets reset to F_UNLCK).

While I kinda see your point, it isn't what GETLK does; it really does
get you information about the first lock -- you're not testing
anything.  It is also the terminology used in the POSIX standard.
Michael Kerrisk (man-pages) April 23, 2014, 7:13 p.m. UTC | #5
On Wed, Apr 23, 2014 at 9:00 PM, Alfred M. Szmidt <ams@gnu.org> wrote:
>    > Likewise.  You infact write that it does get the lock information
>    > later in the document wrt. F_OFD_GETLK.
>
>    Sorry, I disagree here...GETLK is really a misnomer, IMO. TESTLK
>    would have been a better name.

I'm inclined to agree.

>    GETLK are used is to "get the first lock".
>
>    It's a way to test whether a particular lock can be applied, and to
>    return information about a conflicting lock if it can't. If, for
>    instance there is no conflicting lock, then you don't "get" any
>    lock information back (l_type just gets reset to F_UNLCK).
>
> While I kinda see your point, it isn't what GETLK does; it really does
> get you information about the first lock -- you're not testing
> anything.  It is also the terminology used in the POSIX standard.

The POSIX wording is a little confused. For example, what does "first"
mean in this context? F_GETLK returns information about one
(arbitrarily selected) lock that blocks a lock you would like to
place. So, I'm inclined to agree with Jeff -- this really is a "test"
(or "can I lock it") operation.

Of course, the operation has no reliable use: by the time it returns
the information might already be out of date. I suspect that it was
designed to solve the problem: "My F_GETLK operation failed. Who's
blocking me?"

Cheers,

Michael
Jeff Layton April 23, 2014, 7:23 p.m. UTC | #6
On Wed, 23 Apr 2014 15:00:06 -0400
ams@gnu.org (Alfred M. Szmidt) wrote:

>    > Likewise.  You infact write that it does get the lock information
>    > later in the document wrt. F_OFD_GETLK.
> 
>    Sorry, I disagree here...GETLK is really a misnomer, IMO. TESTLK
>    would have been a better name.
> 
>    GETLK are used is to "get the first lock".
> 
>    It's a way to test whether a particular lock can be applied, and to
>    return information about a conflicting lock if it can't. If, for
>    instance there is no conflicting lock, then you don't "get" any
>    lock information back (l_type just gets reset to F_UNLCK).
> 
> While I kinda see your point, it isn't what GETLK does; it really does
> get you information about the first lock -- you're not testing
> anything.  It is also the terminology used in the POSIX standard.


You *are* testing a lock.

For instance, a process has locked bytes 0-5 for read in the file. I
then submit a F_GETLK request from another process and set:

    l_type = F_WRLCK
    l_start = 7
    l_len = 1

...this range does not overlap with the original range, and so no lock
will be returned even though one is being held on the file. In order to
determine whether it should return information about a lock it has to
first _test_ whether it conflicts with the information in the struct
flock that was passed down.

Similarly, if the struct flock I submit to the F_GETLK request has this:

   l_type = F_RDLCK
   l_start = 0
   l_len = 1

...then I also will not get any information about a lock back. The
information in the lock request does not conflict with the one being
held on the file (because they are both read locks).

If F_GETLK were just "getting" a lock, then there would be no test
involved, but that's not how this works. F_GETLK has to test and see
whether there is a conflicting lock before it can return anything.

If all you're objecting to is the change in verbiage on those two
pieces, then I'll back that part out in the interest of wrapping this
up.

I still think I'm correct though ;)

Patch
diff mbox

diff --git a/manual/examples/ofdlocks.c b/manual/examples/ofdlocks.c
new file mode 100644
index 000000000000..544001769272
--- /dev/null
+++ b/manual/examples/ofdlocks.c
@@ -0,0 +1,75 @@ 
+/* Open File Description Locks Usage Example
+   Copyright (C) 1991-2014 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or
+   modify it under the terms of the GNU General Public License
+   as published by the Free Software Foundation; either version 2
+   of the License, or (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, if not, see <http://www.gnu.org/licenses/>.
+*/
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <pthread.h>
+
+#define FILENAME	"/tmp/foo"
+#define NUM_THREADS	3
+#define ITERATIONS	5
+
+void *
+thread_start(void *arg)
+{
+	int i, fd, len;
+	long tid = (long)arg;
+	char buf[256];
+	struct flock lck = {
+		.l_whence	= SEEK_SET,
+		.l_start	= 0,
+		.l_len		= 1,
+	};
+
+	fd = open("/tmp/foo", O_RDWR|O_CREAT, 0666);
+
+	for (i = 0; i < ITERATIONS; i++) {
+		lck.l_type = F_WRLCK;
+		fcntl(fd, F_OFD_SETLKW, &lck);
+
+		len = sprintf(buf, "%d: tid=%ld fd=%d\n", i, tid, fd);
+
+		lseek(fd, 0, SEEK_END);
+		write(fd, buf, len);
+		fsync(fd);
+
+		lck.l_type = F_UNLCK;
+		fcntl(fd, F_OFD_SETLK, &lck);
+
+		usleep(1);
+	}
+	pthread_exit(NULL);
+}
+
+int
+main(int argc, char **argv)
+{
+	long i;
+	pthread_t	threads[NUM_THREADS];
+
+	truncate(FILENAME, 0);
+
+	for (i = 0; i < NUM_THREADS; i++)
+		pthread_create(&threads[i], NULL, thread_start, (void *)i);
+
+	pthread_exit(NULL);
+	return 0;
+}
diff --git a/manual/llio.texi b/manual/llio.texi
index 6f8adfc607d7..b5c2d07ad929 100644
--- a/manual/llio.texi
+++ b/manual/llio.texi
@@ -57,6 +57,10 @@  directly.)
                                          flags associated with open files.
 * File Locks::                          Fcntl commands for implementing
                                          file locking.
+* Open File Description Locks::         Fcntl commands for implementing
+                                         open file description locking.
+* Open File Description Locks Example:: An example of open file description lock
+                                         usage
 * Interrupt Input::                     Getting an asynchronous signal when
                                          input arrives.
 * IOCTLs::                              Generic I/O Control operations.
@@ -2890,7 +2894,7 @@  Get flags associated with the open file.  @xref{File Status Flags}.
 Set flags associated with the open file.  @xref{File Status Flags}.
 
 @item F_GETLK
-Get a file lock.  @xref{File Locks}.
+Test a file lock.  @xref{File Locks}.
 
 @item F_SETLK
 Set or clear a file lock.  @xref{File Locks}.
@@ -2898,6 +2902,18 @@  Set or clear a file lock.  @xref{File Locks}.
 @item F_SETLKW
 Like @code{F_SETLK}, but wait for completion.  @xref{File Locks}.
 
+@item F_OFD_GETLK
+Test a open file description lock.  @xref{Open File Description Locks}.
+Specific to Linux.
+
+@item F_OFD_SETLK
+Set or clear a file lock.  @xref{Open File Description Locks}.
+Specific to Linux.
+
+@item F_OFD_SETLKW
+Like @code{F_OFD_SETLK}, but wait for completion.
+@xref{Open File Description Locks}.  Specific to Linux.
+
 @item F_GETOWN
 Get process or process group ID to receive @code{SIGIO} signals.
 @xref{Interrupt Input}.
@@ -3576,6 +3592,10 @@  set_nonblock_flag (int desc, int value)
 
 @cindex file locks
 @cindex record locking
+This section describes record locks that are associated with the process.
+There is also a different type of record lock that is associated with the
+open description instead of the process.  @xref{Open File Description Locks}.
+
 The remaining @code{fcntl} commands are used to support @dfn{record
 locking}, which permits multiple cooperating programs to prevent each
 other from simultaneously accessing parts of a file in error-prone
@@ -3641,7 +3661,10 @@  the file.
 @item pid_t l_pid
 This field is the process ID (@pxref{Process Creation Concepts}) of the
 process holding the lock.  It is filled in by calling @code{fcntl} with
-the @code{F_GETLK} command, but is ignored when making a lock.
+the @code{F_GETLK} command, but is ignored when making a lock.  If the
+conflicting lock is an open file description lock
+(@pxref{Open File Description Locks}), then this field will be set to
+@math{-1}.
 @end table
 @end deftp
 
@@ -3813,10 +3836,218 @@  that part of the file for writing.
 
 @c ??? This section could use an example program.
 
-Remember that file locks are only a @emph{voluntary} protocol for
+Remember that file locks are only an @emph{advisory} protocol for
 controlling access to a file.  There is still potential for access to
 the file by programs that don't use the lock protocol.
 
+@node Open File Description Locks
+@section Open File Description Locks
+
+In contrast to process-associated record locks (@pxref{File Locks}),
+open file description record locks are associated with an open file
+description rather than a process.
+
+Using @code{fcntl} to apply a open file description lock on a region that
+already has an existing open file description lock that was created via the
+same file descriptor will never cause a lock conflict.
+
+Open file description locks are also inherited by child processes across
+@code{fork}, or @code{clone} with @code{CLONE_FILES} set
+(@pxref{Creating a Process}), along with the file descriptor.
+
+It is important to distinguish between the file @emph{description} (an
+instance of an open file, usually created by a call to @code{open}) and
+a file @emph{descriptor}, which is a numeric value that refers to the
+former.  The locks described here are associated with the file
+@emph{description} and not the file @emph{descriptor}.
+
+Using @code{dup} (@pxref{Duplicating Descriptors}) to copy a file
+descriptor does not give you an entirely new file description , but
+rather copies a reference to an existing file description and assigns it
+to a new file descriptor.  Thus, open file description locks set on a file
+descriptor cloned by @code{dup} will never conflict with
+open file description locks set on the original descriptor since they refer
+to the same file description.  Depending on the range and type of lock
+involved, the original lock may be modified by a @code{F_OFD_SETLK} or
+@code{F_OFD_SETLKW} command in this situation however.
+
+Open file description locks always conflict with process-associated locks,
+even if acquired by the same process or on the same open file
+descriptor.
+
+Open file description locks use the same @code{struct flock} as
+process-associated locks as an argument (@pxref{File Locks}) and the
+macros for the cmd values are also declared in the header file
+@file{fcntl.h}. To use them, the macro @code{_GNU_SOURCE} must be
+defined prior to including @file{fcntl.h}.
+
+In contrast to process-associated locks, any @code{struct flock} used as
+an argument to open file description lock commands must have the @code{l_pid}
+value set to @math{0}.  Also, when returning information about a
+open file description lock in a @code{F_GETLK} or @code{F_OFD_GETLK} request,
+the @code{l_pid} field in @code{struct flock} will be set to @math{-1}
+to indicate that a lock is not associated with a process.
+
+When the same struct flock is reused as an argument to a
+@code{F_OFD_SETLK} or @code{F_OFD_SETLKW} request after being used for an
+@code{F_OFD_GETLK} request, it is necessary to inspect and reset the
+@code{l_pid} field to @math{0}.
+
+@pindex fcntl.h.
+
+@deftypevr Macro int F_OFD_GETLK
+This macro is used as the @var{command} argument to @code{fcntl}, to
+specify that it should get information about a lock.  This command
+requires a third argument of type @w{@code{struct flock *}} to be passed
+to @code{fcntl}, so that the form of the call is:
+
+@smallexample
+fcntl (@var{filedes}, F_OFD_GETLK, @var{lockp})
+@end smallexample
+
+If there is a lock already in place that would block the lock described
+by the @var{lockp} argument, information about that lock overwrites
+@code{*@var{lockp}}.  Existing locks are not reported if they are
+compatible with making a new lock as specified.  Thus, you should
+specify a lock type of @code{F_WRLCK} if you want to find out about both
+read and write locks, or @code{F_RDLCK} if you want to find out about
+write locks only.
+
+There might be more than one lock affecting the region specified by the
+@var{lockp} argument, but @code{fcntl} only returns information about
+one of them. Which lock is returned in this situation is undefined.
+
+The @code{l_whence} member of the @var{lockp} structure is set to
+@code{SEEK_SET} and the @code{l_start} and @code{l_len} fields set to identify
+the locked region.
+
+If no conflicting lock exists, the only change to the @var{lockp} structure
+is to update the @code{l_type} field to the value @code{F_UNLCK}.
+
+The normal return value from @code{fcntl} with this command is either @math{0}
+on success or @math{-1}, which indicates an error. The following @code{errno}
+error conditions are defined for this command:
+
+@table @code
+@item EBADF
+The @var{filedes} argument is invalid.
+
+@item EINVAL
+Either the @var{lockp} argument doesn't specify valid lock information,
+the operating system kernel doesn't support open file description locks, or the file
+associated with @var{filedes} doesn't support locks.
+@end table
+@end deftypevr
+
+@comment fcntl.h
+@comment POSIX.1
+@deftypevr Macro int F_OFD_SETLK
+This macro is used as the @var{command} argument to @code{fcntl}, to
+specify that it should set or clear a lock.  This command requires a
+third argument of type @w{@code{struct flock *}} to be passed to
+@code{fcntl}, so that the form of the call is:
+
+@smallexample
+fcntl (@var{filedes}, F_OFD_SETLK, @var{lockp})
+@end smallexample
+
+If the opened file already has a lock on any part of the
+region, the old lock on that part is replaced with the new lock.  You
+can remove a lock by specifying a lock type of @code{F_UNLCK}.
+
+If the lock cannot be set, @code{fcntl} returns immediately with a value
+of @math{-1}.  This function does not wait for other tasks
+to release locks.  If @code{fcntl} succeeds, it returns @math{0}.
+
+The following @code{errno} error conditions are defined for this
+function:
+
+@table @code
+@item EAGAIN
+The lock cannot be set because it is blocked by an existing lock on the
+file.
+
+@item EBADF
+Either: the @var{filedes} argument is invalid; you requested a read lock
+but the @var{filedes} is not open for read access; or, you requested a
+write lock but the @var{filedes} is not open for write access.
+
+@item EINVAL
+Either the @var{lockp} argument doesn't specify valid lock information,
+the operating system kernel doesn't support open file description locks, or the
+file associated with @var{filedes} doesn't support locks.
+
+@item ENOLCK
+The system has run out of file lock resources; there are already too
+many file locks in place.
+
+Well-designed file systems never report this error, because they have no
+limitation on the number of locks.  However, you must still take account
+of the possibility of this error, as it could result from network access
+to a file system on another machine.
+@end table
+@end deftypevr
+
+@comment fcntl.h
+@comment POSIX.1
+@deftypevr Macro int F_OFD_SETLKW
+This macro is used as the @var{command} argument to @code{fcntl}, to
+specify that it should set or clear a lock.  It is just like the
+@code{F_OFD_SETLK} command, but causes the process to wait until the request
+can be completed.
+
+This command requires a third argument of type @code{struct flock *}, as
+for the @code{F_OFD_SETLK} command.
+
+The @code{fcntl} return values and errors are the same as for the
+@code{F_OFD_SETLK} command, but these additional @code{errno} error conditions
+are defined for this command:
+
+@table @code
+@item EINTR
+The function was interrupted by a signal while it was waiting.
+@xref{Interrupted Primitives}.
+
+@end table
+@end deftypevr
+
+Open file description locks are useful in the same sorts of situations as
+process-associated locks. They can also be used to synchronize file
+access between threads within the same process by having each thread perform
+its own @code{open} of the file, to obtain its own file description.
+
+Because they are only released automatically when the last reference to
+file description is closed, open file description locks avoid the possibility
+that locks are released due to a library routine opening and closing a file
+without the application being aware.
+
+As with process-associated locks, open file description locks are advisory.
+
+@node Open File Description Locks Example
+@section Open File Description Locks Example
+
+Here is an example of using open file description locks in a threaded program. If
+this program used process-associated locks, then then it would be
+subject to data corruption due since multiple threads are running within
+the same process. Proper error handling has been omitted in the following
+program for brevity.
+
+@smallexample
+@include ofdlocks.c.texi
+@end smallexample
+
+This example spawns three threads and has each do five iterations of
+appending to a file. Access to that file is serialized via open file
+description locks. If we compile and run the above program, we'll end up
+with /tmp/foo that has 15 lines in it.
+
+If we, however, were to replace the @code{F_OFD_SETLK} and @code{F_OFD_SETLKW}
+commands with their process-associated lock equivalents, the locking
+essentially becomes a noop since it is all done within the context of
+the same process. That leads to data corruption (typically manifested
+as missing lines) as some threads race in and overwrite the data from
+others.
+
 @node Interrupt Input
 @section Interrupt-Driven Input
 
diff --git a/sysdeps/unix/sysv/linux/bits/fcntl-linux.h b/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
index 915eb3ede560..455389cd2c2a 100644
--- a/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
+++ b/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
@@ -117,6 +117,23 @@ 
 # define F_SETLKW64	14	/* Set record locking info (blocking).	*/
 #endif
 
+/* open file description locks.
+
+   Usually record locks held by a process are released on *any* close and are
+   not inherited across a fork.
+
+   These cmd values will set locks that conflict with process-associated record
+   locks, but are "owned" by the opened file description, not the process.
+   This means that they are inherited across fork or clone with CLONE_FILES
+   like BSD (flock) locks, and they are only released automatically when the
+   last reference to the the file description against which they were acquired
+   is put. */
+#if __USE_GNU
+# define F_OFD_GETLK	36
+# define F_OFD_SETLK	37
+# define F_OFD_SETLKW	38
+#endif
+
 #ifdef __USE_LARGEFILE64
 # define O_LARGEFILE __O_LARGEFILE
 #endif