[v2,5/9] nptl: Move cancel state out of cancelhandling

Message ID 20210527172823.3461314-6-adhemerval.zanella@linaro.org
State New
Headers show
Series
  • nptl: pthread cancellation refactor
Related show

Commit Message

David Howells via Libc-alpha May 27, 2021, 5:28 p.m.
Now that thread cancellation state is not accessed concurrently anymore,
it is possible to move it out the 'cancelhandling'.

The code is also simplified: the CANCELLATION_P is replaced with a
internal pthread_testcancel call and the CANCELSTATE_BIT{MASK} is
removed.

The second part of this patchset also keeps the pthread_setcanceltype as
cancellation entrypoint by calling pthread_testcancel if the new type
is PTHREAD_CANCEL_ASYNCHRONOUS.

With this behavior pthread_setcancelstate does not require to act on
cancellation if cancel type is asynchronous (is already handled either
by pthread_setcanceltype or by the signal handler).

Checked on x86_64-linux-gnu and aarch64-linux-gnu.
---
 manual/pattern.texi           |  1 -
 manual/process.texi           |  3 +--
 nptl/allocatestack.c          |  1 +
 nptl/cancellation.c           |  3 ++-
 nptl/cleanup_defer.c          |  2 +-
 nptl/descr.h                  | 14 ++++++--------
 nptl/libc-cleanup.c           |  2 +-
 nptl/pthreadP.h               | 12 ------------
 nptl/pthread_cancel.c         |  2 +-
 nptl/pthread_join_common.c    |  5 ++++-
 nptl/pthread_setcancelstate.c | 36 +++--------------------------------
 nptl/pthread_setcanceltype.c  |  3 ++-
 nptl/pthread_testcancel.c     | 11 ++++++++++-
 sysdeps/nptl/dl-tls_init_tp.c |  2 ++
 14 files changed, 34 insertions(+), 63 deletions(-)

-- 
2.30.2

Comments

David Howells via Libc-alpha June 1, 2021, 9:58 a.m. | #1
* Adhemerval Zanella via Libc-alpha:

> Now that thread cancellation state is not accessed concurrently anymore,

> it is possible to move it out the 'cancelhandling'.

>

> The code is also simplified: the CANCELLATION_P is replaced with a


Drop “the”?

> internal pthread_testcancel call and the CANCELSTATE_BIT{MASK} is

> removed.

>

> The second part of this patchset also keeps the pthread_setcanceltype as

> cancellation entrypoint by calling pthread_testcancel if the new type

> is PTHREAD_CANCEL_ASYNCHRONOUS.


What do you mean by “patchset” in this context?  It's in the same
commit, right?

> With this behavior pthread_setcancelstate does not require to act on

> cancellation if cancel type is asynchronous (is already handled either

> by pthread_setcanceltype or by the signal handler).


Is this really true?  What is supposed to happen with a pending
asynchronous cancellation request if cancellation is disabled for the
target thread?  Isn't it supposed to be queued if it's a self-cancel?

Thanks,
Florian
David Howells via Libc-alpha June 2, 2021, 1:09 p.m. | #2
On 01/06/2021 06:58, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-alpha:

> 

>> Now that thread cancellation state is not accessed concurrently anymore,

>> it is possible to move it out the 'cancelhandling'.

>>

>> The code is also simplified: the CANCELLATION_P is replaced with a

> 

> Drop “the”?


Ack.

> 

>> internal pthread_testcancel call and the CANCELSTATE_BIT{MASK} is

>> removed.

>>

>> The second part of this patchset also keeps the pthread_setcanceltype as

>> cancellation entrypoint by calling pthread_testcancel if the new type

>> is PTHREAD_CANCEL_ASYNCHRONOUS.

> 

> What do you mean by “patchset” in this context?  It's in the same

> commit, right?


I did not update this comment with recent rebase.  I will just remove 
this paragraph.

> 

>> With this behavior pthread_setcancelstate does not require to act on

>> cancellation if cancel type is asynchronous (is already handled either

>> by pthread_setcanceltype or by the signal handler).

> 

> Is this really true?  What is supposed to happen with a pending

> asynchronous cancellation request if cancellation is disabled for the

> target thread?  Isn't it supposed to be queued if it's a self-cancel?


POSIX does not really define pthread_setcancelstate as a cancellation
entrypoint [1].  And the pthread_cancel already queue the cancellation
by setting the CANCELED_BITMASK, the cancellation will be acted upon
on next cancellation entrypoint when cancellation is enabled.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html

Patch

diff --git a/manual/pattern.texi b/manual/pattern.texi
index 39ae97a3c4..4fa4c25525 100644
--- a/manual/pattern.texi
+++ b/manual/pattern.texi
@@ -1820,7 +1820,6 @@  the beginning of the vector.
 @c      (disable cancellation around exec_comm; it may do_cancel the
 @c       second time, if async cancel is enabled)
 @c     THREAD_ATOMIC_CMPXCHG_VAL dup ok
-@c     CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS dup ok
 @c     do_cancel @ascuplugin @ascuheap @acsmem
 @c      THREAD_ATOMIC_BIT_SET dup ok
 @c      pthread_unwind @ascuplugin @ascuheap @acsmem
diff --git a/manual/process.texi b/manual/process.texi
index 54e65f76c6..134d5c6143 100644
--- a/manual/process.texi
+++ b/manual/process.texi
@@ -68,7 +68,7 @@  until the subprogram terminates before you can do anything else.
 @c   CLEANUP_HANDLER @ascuplugin @ascuheap @acsmem
 @c    libc_cleanup_region_start @ascuplugin @ascuheap @acsmem
 @c     pthread_cleanup_push_defer @ascuplugin @ascuheap @acsmem
-@c      CANCELLATION_P @ascuplugin @ascuheap @acsmem
+@c      __pthread_testcancel @ascuplugin @ascuheap @acsmem
 @c       CANCEL_ENABLED_AND_CANCELED ok
 @c       do_cancel @ascuplugin @ascuheap @acsmem
 @c    cancel_handler ok
@@ -86,7 +86,6 @@  until the subprogram terminates before you can do anything else.
 @c  SINGLE_THREAD_P ok
 @c  LIBC_CANCEL_ASYNC @ascuplugin @ascuheap @acsmem
 @c   libc_enable_asynccancel @ascuplugin @ascuheap @acsmem
-@c    CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS dup ok
 @c    do_cancel dup @ascuplugin @ascuheap @acsmem
 @c  LIBC_CANCEL_RESET ok
 @c   libc_disable_asynccancel ok
diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
index 2114bd2e27..54e95baad7 100644
--- a/nptl/allocatestack.c
+++ b/nptl/allocatestack.c
@@ -160,6 +160,7 @@  get_cached_stack (size_t *sizep, void **memp)
 
   /* Cancellation handling is back to the default.  */
   result->cancelhandling = 0;
+  result->cancelstate = PTHREAD_CANCEL_ENABLE;
   result->cleanup = NULL;
   result->setup_failed = 0;
 
diff --git a/nptl/cancellation.c b/nptl/cancellation.c
index b15f25d8f6..ce00603109 100644
--- a/nptl/cancellation.c
+++ b/nptl/cancellation.c
@@ -44,7 +44,8 @@  __pthread_enable_asynccancel (void)
 					      oldval);
       if (__glibc_likely (curval == oldval))
 	{
-	  if (CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS (newval))
+	  if (self->cancelstate == PTHREAD_CANCEL_ENABLE
+	      && CANCEL_CANCELED_AND_ASYNCHRONOUS (newval))
 	    {
 	      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
 	      __do_cancel ();
diff --git a/nptl/cleanup_defer.c b/nptl/cleanup_defer.c
index 6d85359118..873ab007fd 100644
--- a/nptl/cleanup_defer.c
+++ b/nptl/cleanup_defer.c
@@ -92,7 +92,7 @@  ___pthread_unregister_cancel_restore (__pthread_unwind_buf_t *buf)
 	  cancelhandling = curval;
 	}
 
-      CANCELLATION_P (self);
+      __pthread_testcancel ();
     }
 }
 versioned_symbol (libc, ___pthread_unregister_cancel_restore,
diff --git a/nptl/descr.h b/nptl/descr.h
index a120365f88..35f5330e7f 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 cancellation is disabled.  */
-#define CANCELSTATE_BIT		0
-#define CANCELSTATE_BITMASK	(0x01 << CANCELSTATE_BIT)
   /* Bit set if asynchronous cancellation mode is selected.  */
 #define CANCELTYPE_BIT		1
 #define CANCELTYPE_BITMASK	(0x01 << CANCELTYPE_BIT)
@@ -298,11 +295,8 @@  struct pthread
   /* Mask for the rest.  Helps the compiler to optimize.  */
 #define CANCEL_RESTMASK		0xffffff80
 
-#define CANCEL_ENABLED_AND_CANCELED(value) \
-  (((value) & (CANCELSTATE_BITMASK | CANCELED_BITMASK | EXITING_BITMASK	      \
-	       | CANCEL_RESTMASK | TERMINATED_BITMASK)) == CANCELED_BITMASK)
-#define CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS(value) \
-  (((value) & (CANCELSTATE_BITMASK | CANCELTYPE_BITMASK | CANCELED_BITMASK    \
+#define CANCEL_CANCELED_AND_ASYNCHRONOUS(value) \
+  (((value) & (CANCELTYPE_BITMASK | CANCELED_BITMASK    \
 	       | EXITING_BITMASK | CANCEL_RESTMASK | TERMINATED_BITMASK))     \
    == (CANCELTYPE_BITMASK | CANCELED_BITMASK))
 
@@ -404,6 +398,10 @@  struct pthread
   /* Indicates whether is a C11 thread created by thrd_creat.  */
   bool c11;
 
+  /* Thread cancel state (PTHREAD_CANCEL_ENABLE or
+     PTHREAD_CANCEL_DISABLE).  */
+  unsigned char cancelstate;
+
   /* Used on strsignal.  */
   struct tls_internal_t tls_state;
 
diff --git a/nptl/libc-cleanup.c b/nptl/libc-cleanup.c
index 14ccfe9285..6286b8b525 100644
--- a/nptl/libc-cleanup.c
+++ b/nptl/libc-cleanup.c
@@ -79,7 +79,7 @@  __libc_cleanup_pop_restore (struct _pthread_cleanup_buffer *buffer)
 	  cancelhandling = curval;
 	}
 
-      CANCELLATION_P (self);
+      __pthread_testcancel ();
     }
 }
 libc_hidden_def (__libc_cleanup_pop_restore)
diff --git a/nptl/pthreadP.h b/nptl/pthreadP.h
index 48d48e7008..3e7a4f52ab 100644
--- a/nptl/pthreadP.h
+++ b/nptl/pthreadP.h
@@ -245,18 +245,6 @@  libc_hidden_proto (__pthread_current_priority)
 #define INVALID_TD_P(pd) __builtin_expect ((pd)->tid <= 0, 0)
 #define INVALID_NOT_TERMINATED_TD_P(pd) __builtin_expect ((pd)->tid < 0, 0)
 
-/* Cancellation test.  */
-#define CANCELLATION_P(self) \
-  do {									      \
-    int cancelhandling = THREAD_GETMEM (self, cancelhandling);		      \
-    if (CANCEL_ENABLED_AND_CANCELED (cancelhandling))			      \
-      {									      \
-	THREAD_SETMEM (self, result, PTHREAD_CANCELED);			      \
-	__do_cancel ();							      \
-      }									      \
-  } while (0)
-
-
 extern void __pthread_unwind (__pthread_unwind_buf_t *__buf)
      __cleanup_fct_attribute __attribute ((__noreturn__))
 #if !defined SHARED && !IS_IN (libpthread)
diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index 8dfbcff8c3..69b57c8808 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -45,7 +45,7 @@  sigcancel_handler (int sig, siginfo_t *si, void *ctx)
 
   int ch = atomic_load_relaxed (&self->cancelhandling);
   /* Cancelation not enabled, not cancelled, or already exitting.  */
-  if ((ch & CANCELSTATE_BITMASK) != 0
+  if (self->cancelstate == PTHREAD_CANCEL_DISABLE
       || (ch & CANCELED_BITMASK) == 0
       || (ch & EXITING_BITMASK) != 0)
     return;
diff --git a/nptl/pthread_join_common.c b/nptl/pthread_join_common.c
index f842c91a08..7303069316 100644
--- a/nptl/pthread_join_common.c
+++ b/nptl/pthread_join_common.c
@@ -59,7 +59,10 @@  __pthread_clockjoin_ex (pthread_t threadid, void **thread_return,
 	   && (pd->cancelhandling
 	       & (CANCELED_BITMASK | EXITING_BITMASK
 		  | TERMINATED_BITMASK)) == 0))
-      && !CANCEL_ENABLED_AND_CANCELED (self->cancelhandling))
+      && !(self->cancelstate == PTHREAD_CANCEL_ENABLE
+	   && (pd->cancelhandling & (CANCELED_BITMASK | EXITING_BITMASK
+				     | TERMINATED_BITMASK))
+	       == CANCELED_BITMASK))
     /* This is a deadlock situation.  The threads are waiting for each
        other to finish.  Note that this is a "may" error.  To be 100%
        sure we catch this error we would have to lock the data
diff --git a/nptl/pthread_setcancelstate.c b/nptl/pthread_setcancelstate.c
index e3696ca348..7e2b6e4974 100644
--- a/nptl/pthread_setcancelstate.c
+++ b/nptl/pthread_setcancelstate.c
@@ -31,39 +31,9 @@  __pthread_setcancelstate (int state, int *oldstate)
 
   self = THREAD_SELF;
 
-  int oldval = THREAD_GETMEM (self, cancelhandling);
-  while (1)
-    {
-      int newval = (state == PTHREAD_CANCEL_DISABLE
-		    ? oldval | CANCELSTATE_BITMASK
-		    : oldval & ~CANCELSTATE_BITMASK);
-
-      /* Store the old value.  */
-      if (oldstate != NULL)
-	*oldstate = ((oldval & CANCELSTATE_BITMASK)
-		     ? PTHREAD_CANCEL_DISABLE : PTHREAD_CANCEL_ENABLE);
-
-      /* 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 (CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS (newval))
-	    __do_cancel ();
-
-	  break;
-	}
-
-      /* Prepare for the next round.  */
-      oldval = curval;
-    }
+  if (oldstate != NULL)
+    *oldstate = self->cancelstate;
+  self->cancelstate = state;
 
   return 0;
 }
diff --git a/nptl/pthread_setcanceltype.c b/nptl/pthread_setcanceltype.c
index 5f061d512b..ae5df1d591 100644
--- a/nptl/pthread_setcanceltype.c
+++ b/nptl/pthread_setcanceltype.c
@@ -53,7 +53,8 @@  __pthread_setcanceltype (int type, int *oldtype)
 					      oldval);
       if (__glibc_likely (curval == oldval))
 	{
-	  if (CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS (newval))
+	  if (self->cancelstate == PTHREAD_CANCEL_ENABLE
+	      && CANCEL_CANCELED_AND_ASYNCHRONOUS (newval))
 	    {
 	      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
 	      __do_cancel ();
diff --git a/nptl/pthread_testcancel.c b/nptl/pthread_testcancel.c
index a9e941ddb7..920374643a 100644
--- a/nptl/pthread_testcancel.c
+++ b/nptl/pthread_testcancel.c
@@ -23,7 +23,16 @@ 
 void
 ___pthread_testcancel (void)
 {
-  CANCELLATION_P (THREAD_SELF);
+  struct pthread *self = THREAD_SELF;
+  int cancelhandling = THREAD_GETMEM (self, cancelhandling);
+  if (self->cancelstate == PTHREAD_CANCEL_ENABLE
+      && (cancelhandling & CANCELED_BITMASK)
+      && !(cancelhandling & EXITING_BITMASK)
+      && !(cancelhandling & TERMINATED_BITMASK))
+    {
+      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
+      __do_cancel ();
+    }
 }
 versioned_symbol (libc, ___pthread_testcancel, pthread_testcancel, GLIBC_2_34);
 versioned_symbol (libc, ___pthread_testcancel, __pthread_testcancel,
diff --git a/sysdeps/nptl/dl-tls_init_tp.c b/sysdeps/nptl/dl-tls_init_tp.c
index 1f7790297f..378b26a2f5 100644
--- a/sysdeps/nptl/dl-tls_init_tp.c
+++ b/sysdeps/nptl/dl-tls_init_tp.c
@@ -94,4 +94,6 @@  __tls_init_tp (void)
      It will be bigger than it actually is, but for unwind.c/pt-longjmp.c
      purposes this is good enough.  */
   THREAD_SETMEM (pd, stackblock_size, (size_t) __libc_stack_end);
+
+  THREAD_SETMEM (pd, cancelstate, PTHREAD_CANCEL_ENABLE);
 }