diff mbox series

[v2,6/9] nptl: Move cancel type out of cancelhandling

Message ID 20210527172823.3461314-7-adhemerval.zanella@linaro.org
State New
Headers show
Series nptl: pthread cancellation refactor | expand

Commit Message

Adhemerval Zanella May 27, 2021, 5:28 p.m. UTC
Now that the thread cancellation type is not accessed concurrently
anymore, it is possible to move it out the cancelhandling.

By removing the cancel state out of the internal thread cancel handling
state there is no need to check if cancelled bit was set in CAS
operation.

It allows simplifing the cancellation wrappers and the
CANCEL_CANCELED_AND_ASYNCHRONOUS is removed.

Checked on x86_64-linux-gnu and aarch64-linux-gnu.
---
 nptl/allocatestack.c          |  1 +
 nptl/cancellation.c           | 51 +++++++++--------------------------
 nptl/cleanup_defer.c          | 46 ++++---------------------------
 nptl/descr.h                  | 14 +++-------
 nptl/libc-cleanup.c           | 44 +++---------------------------
 nptl/pthread_cancel.c         |  4 +--
 nptl/pthread_setcanceltype.c  | 42 ++++-------------------------
 sysdeps/nptl/dl-tls_init_tp.c |  1 +
 8 files changed, 34 insertions(+), 169 deletions(-)

Comments

Florian Weimer June 1, 2021, 12:37 p.m. UTC | #1
* Adhemerval Zanella via Libc-alpha:

> Now that the thread cancellation type is not accessed concurrently
> anymore, it is possible to move it out the cancelhandling.
>
> By removing the cancel state out of the internal thread cancel handling
> state there is no need to check if cancelled bit was set in CAS
> operation.
>
> It allows simplifing the cancellation wrappers and the
> CANCEL_CANCELED_AND_ASYNCHRONOUS is removed.

Patch looks okay to me.  Maybe this should be merged with the other
patch that changes the cancel state handling.

Thanks,
Florian
Adhemerval Zanella June 2, 2021, 1:11 p.m. UTC | #2
On 01/06/2021 09:37, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-alpha:
> 
>> Now that the thread cancellation type is not accessed concurrently
>> anymore, it is possible to move it out the cancelhandling.
>>
>> By removing the cancel state out of the internal thread cancel handling
>> state there is no need to check if cancelled bit was set in CAS
>> operation.
>>
>> It allows simplifing the cancellation wrappers and the
>> CANCEL_CANCELED_AND_ASYNCHRONOUS is removed.
> 
> Patch looks okay to me.  Maybe this should be merged with the other
> patch that changes the cancel state handling.

I don't have a strong preference, I split just to make easier to review
and see the logical changes.  I will merge when commit then.
Adhemerval Zanella June 9, 2021, 5:06 p.m. UTC | #3
On 02/06/2021 10:11, Adhemerval Zanella wrote:
> 
> 
> On 01/06/2021 09:37, Florian Weimer wrote:
>> * Adhemerval Zanella via Libc-alpha:
>>
>>> Now that the thread cancellation type is not accessed concurrently
>>> anymore, it is possible to move it out the cancelhandling.
>>>
>>> By removing the cancel state out of the internal thread cancel handling
>>> state there is no need to check if cancelled bit was set in CAS
>>> operation.
>>>
>>> It allows simplifing the cancellation wrappers and the
>>> CANCEL_CANCELED_AND_ASYNCHRONOUS is removed.
>>
>> Patch looks okay to me.  Maybe this should be merged with the other
>> patch that changes the cancel state handling.
> 
> I don't have a strong preference, I split just to make easier to review
> and see the logical changes.  I will merge when commit then.
> 

In fact merging will result in a larger, which might be harder for
a reader (and there is no direct gain in doing so). I will keep the
change in two patches.
diff mbox series

Patch

diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
index 54e95baad7..9be6c42894 100644
--- a/nptl/allocatestack.c
+++ b/nptl/allocatestack.c
@@ -161,6 +161,7 @@  get_cached_stack (size_t *sizep, void **memp)
   /* Cancellation handling is back to the default.  */
   result->cancelhandling = 0;
   result->cancelstate = PTHREAD_CANCEL_ENABLE;
+  result->canceltype = PTHREAD_CANCEL_DEFERRED;
   result->cleanup = NULL;
   result->setup_failed = 0;
 
diff --git a/nptl/cancellation.c b/nptl/cancellation.c
index ce00603109..05962784d5 100644
--- a/nptl/cancellation.c
+++ b/nptl/cancellation.c
@@ -31,31 +31,19 @@  int
 __pthread_enable_asynccancel (void)
 {
   struct pthread *self = THREAD_SELF;
-  int oldval = THREAD_GETMEM (self, cancelhandling);
 
-  while (1)
-    {
-      int newval = oldval | CANCELTYPE_BITMASK;
-
-      if (newval == oldval)
-	break;
-
-      int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
-					      oldval);
-      if (__glibc_likely (curval == oldval))
-	{
-	  if (self->cancelstate == PTHREAD_CANCEL_ENABLE
-	      && CANCEL_CANCELED_AND_ASYNCHRONOUS (newval))
-	    {
-	      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
-	      __do_cancel ();
-	    }
+  int oldval = THREAD_GETMEM (self, canceltype);
+  THREAD_SETMEM (self, canceltype, PTHREAD_CANCEL_ASYNCHRONOUS);
 
-	  break;
-	}
+  int ch = THREAD_GETMEM (self, cancelhandling);
 
-      /* Prepare the next round.  */
-      oldval = curval;
+  if (self->cancelstate == PTHREAD_CANCEL_ENABLE
+      && (ch & CANCELED_BITMASK)
+      && !(ch & EXITING_BITMASK)
+      && !(ch & TERMINATED_BITMASK))
+    {
+      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
+      __do_cancel ();
     }
 
   return oldval;
@@ -69,25 +57,10 @@  __pthread_disable_asynccancel (int oldtype)
 {
   /* If asynchronous cancellation was enabled before we do not have
      anything to do.  */
-  if (oldtype & CANCELTYPE_BITMASK)
+  if (oldtype == PTHREAD_CANCEL_ASYNCHRONOUS)
     return;
 
   struct pthread *self = THREAD_SELF;
-  int newval;
-
-  int oldval = THREAD_GETMEM (self, cancelhandling);
-
-  while (1)
-    {
-      newval = oldval & ~CANCELTYPE_BITMASK;
-
-      int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
-					      oldval);
-      if (__glibc_likely (curval == oldval))
-	break;
-
-      /* Prepare the next round.  */
-      oldval = curval;
-    }
+  self->canceltype = PTHREAD_CANCEL_DEFERRED;
 }
 libc_hidden_def (__pthread_disable_asynccancel)
diff --git a/nptl/cleanup_defer.c b/nptl/cleanup_defer.c
index 873ab007fd..7e858d0df0 100644
--- a/nptl/cleanup_defer.c
+++ b/nptl/cleanup_defer.c
@@ -31,27 +31,9 @@  ___pthread_register_cancel_defer (__pthread_unwind_buf_t *buf)
   ibuf->priv.data.prev = THREAD_GETMEM (self, cleanup_jmp_buf);
   ibuf->priv.data.cleanup = THREAD_GETMEM (self, cleanup);
 
-  int cancelhandling = THREAD_GETMEM (self, cancelhandling);
-
   /* Disable asynchronous cancellation for now.  */
-  if (__glibc_unlikely (cancelhandling & CANCELTYPE_BITMASK))
-    while (1)
-      {
-	int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling,
-						cancelhandling
-						& ~CANCELTYPE_BITMASK,
-						cancelhandling);
-	if (__glibc_likely (curval == cancelhandling))
-	  /* Successfully replaced the value.  */
-	  break;
-
-	/* Prepare for the next round.  */
-	cancelhandling = curval;
-      }
-
-  ibuf->priv.data.canceltype = (cancelhandling & CANCELTYPE_BITMASK
-				? PTHREAD_CANCEL_ASYNCHRONOUS
-				: PTHREAD_CANCEL_DEFERRED);
+  ibuf->priv.data.canceltype = THREAD_GETMEM (self, canceltype);
+  THREAD_SETMEM (self, canceltype, PTHREAD_CANCEL_DEFERRED);
 
   /* Store the new cleanup handler info.  */
   THREAD_SETMEM (self, cleanup_jmp_buf, (struct pthread_unwind_buf *) buf);
@@ -73,27 +55,9 @@  ___pthread_unregister_cancel_restore (__pthread_unwind_buf_t *buf)
 
   THREAD_SETMEM (self, cleanup_jmp_buf, ibuf->priv.data.prev);
 
-  int cancelhandling;
-  if (ibuf->priv.data.canceltype != PTHREAD_CANCEL_DEFERRED
-      && ((cancelhandling = THREAD_GETMEM (self, cancelhandling))
-	  & CANCELTYPE_BITMASK) == 0)
-    {
-      while (1)
-	{
-	  int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling,
-						  cancelhandling
-						  | CANCELTYPE_BITMASK,
-						  cancelhandling);
-	  if (__glibc_likely (curval == cancelhandling))
-	    /* Successfully replaced the value.  */
-	    break;
-
-	  /* Prepare for the next round.  */
-	  cancelhandling = curval;
-	}
-
-      __pthread_testcancel ();
-    }
+  THREAD_SETMEM (self, canceltype, ibuf->priv.data.canceltype);
+  if (ibuf->priv.data.canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
+    __pthread_testcancel ();
 }
 versioned_symbol (libc, ___pthread_unregister_cancel_restore,
 		  __pthread_unregister_cancel_restore, GLIBC_2_34);
diff --git a/nptl/descr.h b/nptl/descr.h
index 35f5330e7f..c85778d449 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -277,9 +277,6 @@  struct pthread
 
   /* Flags determining processing of cancellation.  */
   int cancelhandling;
-  /* Bit set if asynchronous cancellation mode is selected.  */
-#define CANCELTYPE_BIT		1
-#define CANCELTYPE_BITMASK	(0x01 << CANCELTYPE_BIT)
   /* Bit set if canceled.  */
 #define CANCELED_BIT		3
 #define CANCELED_BITMASK	(0x01 << CANCELED_BIT)
@@ -292,13 +289,6 @@  struct pthread
   /* Bit set if thread is supposed to change XID.  */
 #define SETXID_BIT		6
 #define SETXID_BITMASK		(0x01 << SETXID_BIT)
-  /* Mask for the rest.  Helps the compiler to optimize.  */
-#define CANCEL_RESTMASK		0xffffff80
-
-#define CANCEL_CANCELED_AND_ASYNCHRONOUS(value) \
-  (((value) & (CANCELTYPE_BITMASK | CANCELED_BITMASK    \
-	       | EXITING_BITMASK | CANCEL_RESTMASK | TERMINATED_BITMASK))     \
-   == (CANCELTYPE_BITMASK | CANCELED_BITMASK))
 
   /* Flags.  Including those copied from the thread attribute.  */
   int flags;
@@ -402,6 +392,10 @@  struct pthread
      PTHREAD_CANCEL_DISABLE).  */
   unsigned char cancelstate;
 
+  /* Thread cancel type (PTHREAD_CANCEL_DEFERRED or
+     PTHREAD_CANCEL_ASYNCHRONOUS).  */
+  unsigned char canceltype;
+
   /* Used on strsignal.  */
   struct tls_internal_t tls_state;
 
diff --git a/nptl/libc-cleanup.c b/nptl/libc-cleanup.c
index 6286b8b525..180d15bc9e 100644
--- a/nptl/libc-cleanup.c
+++ b/nptl/libc-cleanup.c
@@ -27,27 +27,9 @@  __libc_cleanup_push_defer (struct _pthread_cleanup_buffer *buffer)
 
   buffer->__prev = THREAD_GETMEM (self, cleanup);
 
-  int cancelhandling = THREAD_GETMEM (self, cancelhandling);
-
   /* Disable asynchronous cancellation for now.  */
-  if (__glibc_unlikely (cancelhandling & CANCELTYPE_BITMASK))
-    while (1)
-      {
-	int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling,
-						cancelhandling
-						& ~CANCELTYPE_BITMASK,
-						cancelhandling);
-	if (__glibc_likely (curval == cancelhandling))
-	  /* Successfully replaced the value.  */
-	  break;
-
-	/* Prepare for the next round.  */
-	cancelhandling = curval;
-      }
-
-  buffer->__canceltype = (cancelhandling & CANCELTYPE_BITMASK
-			  ? PTHREAD_CANCEL_ASYNCHRONOUS
-			  : PTHREAD_CANCEL_DEFERRED);
+  buffer->__canceltype = THREAD_GETMEM (self, canceltype);
+  THREAD_SETMEM (self, canceltype, PTHREAD_CANCEL_DEFERRED);
 
   THREAD_SETMEM (self, cleanup, buffer);
 }
@@ -60,26 +42,8 @@  __libc_cleanup_pop_restore (struct _pthread_cleanup_buffer *buffer)
 
   THREAD_SETMEM (self, cleanup, buffer->__prev);
 
-  int cancelhandling;
-  if (__builtin_expect (buffer->__canceltype != PTHREAD_CANCEL_DEFERRED, 0)
-      && ((cancelhandling = THREAD_GETMEM (self, cancelhandling))
-	  & CANCELTYPE_BITMASK) == 0)
-    {
-      while (1)
-	{
-	  int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling,
-						  cancelhandling
-						  | CANCELTYPE_BITMASK,
-						  cancelhandling);
-	  if (__glibc_likely (curval == cancelhandling))
-	    /* Successfully replaced the value.  */
-	    break;
-
-	  /* Prepare for the next round.  */
-	  cancelhandling = curval;
-	}
-
+  THREAD_SETMEM (self, canceltype, buffer->__canceltype);
+  if (buffer->__canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
       __pthread_testcancel ();
-    }
 }
 libc_hidden_def (__libc_cleanup_pop_restore)
diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index 69b57c8808..c6086b6c3c 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -53,7 +53,7 @@  sigcancel_handler (int sig, siginfo_t *si, void *ctx)
   /* Set the return value.  */
   THREAD_SETMEM (self, result, PTHREAD_CANCELED);
   /* Make sure asynchronous cancellation is still enabled.  */
-  if ((ch & CANCELTYPE_BITMASK) != 0)
+  if (self->canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
     __do_cancel ();
 }
 
@@ -104,7 +104,7 @@  __pthread_cancel (pthread_t th)
 #endif
 
       THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
-      if ((oldch & CANCELTYPE_BITMASK) != 0)
+      if (pd->canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
 	__do_cancel ();
       return 0;
     }
diff --git a/nptl/pthread_setcanceltype.c b/nptl/pthread_setcanceltype.c
index ae5df1d591..e7b24ae733 100644
--- a/nptl/pthread_setcanceltype.c
+++ b/nptl/pthread_setcanceltype.c
@@ -29,43 +29,11 @@  __pthread_setcanceltype (int type, int *oldtype)
 
   volatile struct pthread *self = THREAD_SELF;
 
-  int oldval = THREAD_GETMEM (self, cancelhandling);
-  while (1)
-    {
-      int newval = (type == PTHREAD_CANCEL_ASYNCHRONOUS
-		    ? oldval | CANCELTYPE_BITMASK
-		    : oldval & ~CANCELTYPE_BITMASK);
-
-      /* Store the old value.  */
-      if (oldtype != NULL)
-	*oldtype = ((oldval & CANCELTYPE_BITMASK)
-		    ? PTHREAD_CANCEL_ASYNCHRONOUS : PTHREAD_CANCEL_DEFERRED);
-
-      /* Avoid doing unnecessary work.  The atomic operation can
-	 potentially be expensive if the memory has to be locked and
-	 remote cache lines have to be invalidated.  */
-      if (oldval == newval)
-	break;
-
-      /* Update the cancel handling word.  This has to be done
-	 atomically since other bits could be modified as well.  */
-      int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
-					      oldval);
-      if (__glibc_likely (curval == oldval))
-	{
-	  if (self->cancelstate == PTHREAD_CANCEL_ENABLE
-	      && CANCEL_CANCELED_AND_ASYNCHRONOUS (newval))
-	    {
-	      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
-	      __do_cancel ();
-	    }
-
-	  break;
-	}
-
-      /* Prepare for the next round.  */
-      oldval = curval;
-    }
+  if (oldtype != NULL)
+    *oldtype = self->canceltype;
+  self->canceltype = type;
+  if (type == PTHREAD_CANCEL_ASYNCHRONOUS)
+    __pthread_testcancel ();
 
   return 0;
 }
diff --git a/sysdeps/nptl/dl-tls_init_tp.c b/sysdeps/nptl/dl-tls_init_tp.c
index 378b26a2f5..b7b3bb1cdb 100644
--- a/sysdeps/nptl/dl-tls_init_tp.c
+++ b/sysdeps/nptl/dl-tls_init_tp.c
@@ -96,4 +96,5 @@  __tls_init_tp (void)
   THREAD_SETMEM (pd, stackblock_size, (size_t) __libc_stack_end);
 
   THREAD_SETMEM (pd, cancelstate, PTHREAD_CANCEL_ENABLE);
+  THREAD_SETMEM (pd, canceltype, PTHREAD_CANCEL_DEFERRED);
 }