diff mbox series

i386: Use pthread_barrier for synchronization on tst-bz21269

Message ID 20230215124354.2069895-1-adhemerval.zanella@linaro.org
State New
Headers show
Series i386: Use pthread_barrier for synchronization on tst-bz21269 | expand

Commit Message

Adhemerval Zanella Netto Feb. 15, 2023, 12:43 p.m. UTC 
To improve the false negative in patchwork buildbot.

Checked on i686-linux-gnu.
---
 sysdeps/unix/sysv/linux/i386/tst-bz21269.c | 58 +++++++---------------
 1 file changed, 17 insertions(+), 41 deletions(-)

Comments

DJ Delorie Feb. 20, 2023, 10:55 p.m. UTC | #1 
The resulting test has two calls to pthread_barrier_wait() in the
subthread but three calls in main().  These will quickly get out of
sync.


Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org> writes:
> -   - C11 atomics instead of plain access.
> +   - Use pthread_barrier instead of atomic and futexes.

Is this true relative to the original testcase?  Still, merely a
comment, so OK.

> -#include <stdatomic.h>
> -
>  #include <asm/ldt.h>
> -#include <linux/futex.h>

Ok.

> +#include <support/xsignal.h>

Ok.

> -static int
> -futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
> -	int val3)
> -{
> -  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
> -}

We remove all calls to futex, so no longer need this.  Ok.

> -  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
> +  xsigaction (sig, &sa, 0);

Ok.

> -/* Possible values of futex:
> -   0: thread is idle.
> -   1: thread armed.
> -   2: thread should clear LDT entry 0.
> -   3: thread should exit.  */
> -static atomic_uint ftx;

Ok.

> +static pthread_barrier_t barrier;

New, ok.

>  static void *
>  threadproc (void *ctx)
>  {
> -  while (1)
> +  for (int i = 0; i < 5; i++)

This matches the loop in main.  Ok.  In the future, a #define loop limit
would be appropriate, to prevent these getting out of sync.  Or a
comment that it has to match main().

>      {
> -      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
> -      while (atomic_load (&ftx) != 2)
> -	{
> -	  if (atomic_load (&ftx) >= 3)
> -	    return NULL;
> -	}
> +      xpthread_barrier_wait (&barrier);

First barrier, ok.

>        /* clear LDT entry 0.  */
>        const struct user_desc desc = { 0 };
>        xmodify_ldt (1, &desc, sizeof (desc));

Leave the stuff the thread is actually testing ;-)

> -      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
> -      if (atomic_fetch_add (&ftx, -2) != 2)
> -	return NULL;
> +      /* Wait for 'ss' set in main thread.  */
> +      xpthread_barrier_wait (&barrier);

Second barrier, ok.

>      }
> +
> +  return NULL;

Ok, moved from above.

>  }
>  
>  
> @@ -180,6 +162,8 @@ do_test (void)
>    /* Some kernels send SIGBUS instead.  */
>    xsethandler (SIGBUS, sigsegv_handler, 0);
>  
> +  xpthread_barrier_init (&barrier, NULL, 2);

Initialize; must have two callers.  Ok - main and thread both call.

>    thread = xpthread_create (0, threadproc, 0);
>  
>    asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
> @@ -190,8 +174,7 @@ do_test (void)
>  	continue;
>  
>        /* Make sure the thread is ready after the last test. */
> -      while (atomic_load (&ftx) != 0)
> -	;
> +      xpthread_barrier_wait (&barrier);

First barrier, ok.

>        struct user_desc desc = {
>  	.entry_number       = 0,
> @@ -207,28 +190,21 @@ do_test (void)
>  
>        xmodify_ldt (0x11, &desc, sizeof (desc));
>  
> -      /* Arm the thread.  */
> -      ftx = 1;
> -      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
> +      xpthread_barrier_wait (&barrier);

Second barrier, ok.
  
>        asm volatile ("mov %0, %%ss" : : "r" (0x7));
>  
> -      /* Fire up thread modify_ldt call.  */
> -      atomic_store (&ftx, 2);
> -
> -      while (atomic_load (&ftx) != 0)
> -	;
> +      xpthread_barrier_wait (&barrier);

Third barrier?  This puts main and the thread out of sync.

>        /* On success, modify_ldt will segfault us synchronously and we will
>  	 escape via siglongjmp.  */
>        support_record_failure ();
>      }
>  
> -  atomic_store (&ftx, 100);
> -  futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
> -

Ok.

>    xpthread_join (thread);
>  
> +  xpthread_barrier_destroy (&barrier);
> +

Ok.
Adhemerval Zanella Netto Feb. 21, 2023, 7:22 p.m. UTC | #2 
On 20/02/23 19:55, DJ Delorie wrote:
> 
> The resulting test has two calls to pthread_barrier_wait() in the
> subthread but three calls in main().  These will quickly get out of
> sync.
> 
> 
> Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org> writes:
>> -   - C11 atomics instead of plain access.
>> +   - Use pthread_barrier instead of atomic and futexes.
> 
> Is this true relative to the original testcase?  Still, merely a
> comment, so OK.

The original refers to the kernel tests in here.

> 
>> -#include <stdatomic.h>
>> -
>>  #include <asm/ldt.h>
>> -#include <linux/futex.h>
> 
> Ok.
> 
>> +#include <support/xsignal.h>
> 
> Ok.
> 
>> -static int
>> -futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
>> -	int val3)
>> -{
>> -  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
>> -}
> 
> We remove all calls to futex, so no longer need this.  Ok.
> 
>> -  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
>> +  xsigaction (sig, &sa, 0);
> 
> Ok.
> 
>> -/* Possible values of futex:
>> -   0: thread is idle.
>> -   1: thread armed.
>> -   2: thread should clear LDT entry 0.
>> -   3: thread should exit.  */
>> -static atomic_uint ftx;
> 
> Ok.
> 
>> +static pthread_barrier_t barrier;
> 
> New, ok.
> 
>>  static void *
>>  threadproc (void *ctx)
>>  {
>> -  while (1)
>> +  for (int i = 0; i < 5; i++)
> 
> This matches the loop in main.  Ok.  In the future, a #define loop limit
> would be appropriate, to prevent these getting out of sync.  Or a
> comment that it has to match main().

Ack.

> 
>>      {
>> -      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
>> -      while (atomic_load (&ftx) != 2)
>> -	{
>> -	  if (atomic_load (&ftx) >= 3)
>> -	    return NULL;
>> -	}
>> +      xpthread_barrier_wait (&barrier);
> 
> First barrier, ok.
> 
>>        /* clear LDT entry 0.  */
>>        const struct user_desc desc = { 0 };
>>        xmodify_ldt (1, &desc, sizeof (desc));
> 
> Leave the stuff the thread is actually testing ;-)
> 
>> -      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
>> -      if (atomic_fetch_add (&ftx, -2) != 2)
>> -	return NULL;
>> +      /* Wait for 'ss' set in main thread.  */
>> +      xpthread_barrier_wait (&barrier);
> 
> Second barrier, ok.
> 
>>      }
>> +
>> +  return NULL;
> 
> Ok, moved from above.
> 
>>  }
>>  
>>  
>> @@ -180,6 +162,8 @@ do_test (void)
>>    /* Some kernels send SIGBUS instead.  */
>>    xsethandler (SIGBUS, sigsegv_handler, 0);
>>  
>> +  xpthread_barrier_init (&barrier, NULL, 2);
> 
> Initialize; must have two callers.  Ok - main and thread both call.
> 
>>    thread = xpthread_create (0, threadproc, 0);
>>  
>>    asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
>> @@ -190,8 +174,7 @@ do_test (void)
>>  	continue;
>>  
>>        /* Make sure the thread is ready after the last test. */
>> -      while (atomic_load (&ftx) != 0)
>> -	;
>> +      xpthread_barrier_wait (&barrier);
> 
> First barrier, ok.
> 
>>        struct user_desc desc = {
>>  	.entry_number       = 0,
>> @@ -207,28 +190,21 @@ do_test (void)
>>  
>>        xmodify_ldt (0x11, &desc, sizeof (desc));
>>  
>> -      /* Arm the thread.  */
>> -      ftx = 1;
>> -      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
>> +      xpthread_barrier_wait (&barrier);
> 
> Second barrier, ok.
>   
>>        asm volatile ("mov %0, %%ss" : : "r" (0x7));
>>  
>> -      /* Fire up thread modify_ldt call.  */
>> -      atomic_store (&ftx, 2);
>> -
>> -      while (atomic_load (&ftx) != 0)
>> -	;
>> +      xpthread_barrier_wait (&barrier);
> 
> Third barrier?  This puts main and the thread out of sync.

Indeed, this is wrong. I will remove it.

> 
>>        /* On success, modify_ldt will segfault us synchronously and we will
>>  	 escape via siglongjmp.  */
>>        support_record_failure ();
>>      }
>>  
>> -  atomic_store (&ftx, 100);
>> -  futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
>> -
> 
> Ok.
> 
>>    xpthread_join (thread);
>>  
>> +  xpthread_barrier_destroy (&barrier);
>> +
> 
> Ok.
>
diff mbox series

Patch

diff --git a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
index 1850240c34..07880650c6 100644
--- a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
+++ b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
@@ -20,16 +20,13 @@ 
    more specifically in do_multicpu_tests function.  The main changes
    are:
 
-   - C11 atomics instead of plain access.
+   - Use pthread_barrier instead of atomic and futexes.
    - Remove x86_64 support which simplifies the syscall handling
      and fallbacks.
    - Replicate only the test required to trigger the issue for the
      BZ#21269.  */
 
-#include <stdatomic.h>
-
 #include <asm/ldt.h>
-#include <linux/futex.h>
 
 #include <setjmp.h>
 #include <signal.h>
@@ -40,6 +37,7 @@ 
 #include <support/xunistd.h>
 #include <support/check.h>
 #include <support/xthread.h>
+#include <support/xsignal.h>
 
 static int
 xset_thread_area (struct user_desc *u_info)
@@ -55,13 +53,6 @@  xmodify_ldt (int func, const void *ptr, unsigned long bytecount)
   TEST_VERIFY_EXIT (syscall (SYS_modify_ldt, 1, ptr, bytecount) == 0);
 }
 
-static int
-futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
-	int val3)
-{
-  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
-}
-
 static void
 xsethandler (int sig, void (*handler)(int, siginfo_t *, void *), int flags)
 {
@@ -69,7 +60,7 @@  xsethandler (int sig, void (*handler)(int, siginfo_t *, void *), int flags)
   sa.sa_sigaction = handler;
   sa.sa_flags = SA_SIGINFO | flags;
   TEST_VERIFY_EXIT (sigemptyset (&sa.sa_mask) == 0);
-  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
+  xsigaction (sig, &sa, 0);
 }
 
 static jmp_buf jmpbuf;
@@ -123,33 +114,24 @@  setup_low_user_desc (void)
   low_user_desc_clear->seg_not_present = 1;
 }
 
-/* Possible values of futex:
-   0: thread is idle.
-   1: thread armed.
-   2: thread should clear LDT entry 0.
-   3: thread should exit.  */
-static atomic_uint ftx;
+static pthread_barrier_t barrier;
 
 static void *
 threadproc (void *ctx)
 {
-  while (1)
+  for (int i = 0; i < 5; i++)
     {
-      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
-      while (atomic_load (&ftx) != 2)
-	{
-	  if (atomic_load (&ftx) >= 3)
-	    return NULL;
-	}
+      xpthread_barrier_wait (&barrier);
 
       /* clear LDT entry 0.  */
       const struct user_desc desc = { 0 };
       xmodify_ldt (1, &desc, sizeof (desc));
 
-      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
-      if (atomic_fetch_add (&ftx, -2) != 2)
-	return NULL;
+      /* Wait for 'ss' set in main thread.  */
+      xpthread_barrier_wait (&barrier);
     }
+
+  return NULL;
 }
 
 
@@ -180,6 +162,8 @@  do_test (void)
   /* Some kernels send SIGBUS instead.  */
   xsethandler (SIGBUS, sigsegv_handler, 0);
 
+  xpthread_barrier_init (&barrier, NULL, 2);
+
   thread = xpthread_create (0, threadproc, 0);
 
   asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
@@ -190,8 +174,7 @@  do_test (void)
 	continue;
 
       /* Make sure the thread is ready after the last test. */
-      while (atomic_load (&ftx) != 0)
-	;
+      xpthread_barrier_wait (&barrier);
 
       struct user_desc desc = {
 	.entry_number       = 0,
@@ -207,28 +190,21 @@  do_test (void)
 
       xmodify_ldt (0x11, &desc, sizeof (desc));
 
-      /* Arm the thread.  */
-      ftx = 1;
-      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+      xpthread_barrier_wait (&barrier);
 
       asm volatile ("mov %0, %%ss" : : "r" (0x7));
 
-      /* Fire up thread modify_ldt call.  */
-      atomic_store (&ftx, 2);
-
-      while (atomic_load (&ftx) != 0)
-	;
+      xpthread_barrier_wait (&barrier);
 
       /* On success, modify_ldt will segfault us synchronously and we will
 	 escape via siglongjmp.  */
       support_record_failure ();
     }
 
-  atomic_store (&ftx, 100);
-  futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
-
   xpthread_join (thread);
 
+  xpthread_barrier_destroy (&barrier);
+
   return 0;
 }