nptl: Remove CANCELING_BITMASK

The CANCELING_BITMASK is used as an optimization to avoid sending
the signal when pthread_cancel is called in a concurrent manner.

This requires then to put both the cancellation state and type on a
shared state (cancelhandling), since 'pthread_cancel' checks whether
cancellation is enabled and asynchrnous to either cancel itself of
 sending the signal.

It also requires handle the CANCELING_BITMASK on
__pthread_disable_asynccancel, however this incurs in the same issues
described on BZ#12683: the cancellation is acted upon even *after*
syscall returns with user visible side-effects.

This patch removes this optimization and simplifies the pthread
cancellation implementation: pthread_cancel now first checks if
cancellation is already pending and if not always, sends a signal
if the target is not itself.  The SIGCANCEL handler is also simpified
since there is not need to setup a CAS loop.

It also allows to move both the cancellation state and mode out of
'cancelhadling' (it is done in subsequent patches).

Checked on x86_64-linux-gnu and aarch64-linux-gnu.

nptl/cancellation.c

@@ -88,17 +88,5 @@ __pthread_disable_asynccancel (int oldtype)
       /* Prepare the next round.  */
       oldval = curval;
     }
-
-  /* We cannot return when we are being canceled.  Upon return the
-     thread might be things which would have to be undone.  The
-     following loop should loop until the cancellation signal is
-     delivered.  */
-  while (__builtin_expect ((newval & (CANCELING_BITMASK | CANCELED_BITMASK))
-			   == CANCELING_BITMASK, 0))
-    {
-      futex_wait_simple ((unsigned int *) &self->cancelhandling, newval,
-			 FUTEX_PRIVATE);
-      newval = THREAD_GETMEM (self, cancelhandling);
-    }
 }
 libc_hidden_def (__pthread_disable_asynccancel)

nptl/descr.h

@@ -283,9 +283,6 @@ struct pthread
   /* Bit set if asynchronous cancellation mode is selected.  */
 #define CANCELTYPE_BIT		1
 #define CANCELTYPE_BITMASK	(0x01 << CANCELTYPE_BIT)
-  /* Bit set if canceling has been initiated.  */
-#define CANCELING_BIT		2
-#define CANCELING_BITMASK	(0x01 << CANCELING_BIT)
   /* Bit set if canceled.  */
 #define CANCELED_BIT		3
 #define CANCELED_BITMASK	(0x01 << CANCELED_BIT)

nptl/pthread_cancel.c

@@ -43,35 +43,18 @@ sigcancel_handler (int sig, siginfo_t *si, void *ctx)
 
   struct pthread *self = THREAD_SELF;
 
-  int oldval = THREAD_GETMEM (self, cancelhandling);
+  int ch = atomic_load_relaxed (&self->cancelhandling);
-  while (1)
+  /* Cancelation not enabled, not cancelled, or already exitting.  */
-    {
+  if ((ch & CANCELSTATE_BITMASK) != 0
-      /* We are canceled now.  When canceled by another thread this flag
+      || (ch & CANCELED_BITMASK) == 0
-	 is already set but if the signal is directly send (internally or
+      || (ch & EXITING_BITMASK) != 0)
-	 from another process) is has to be done here.  */
+    return;
-      int newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
 
-      if (oldval == newval || (oldval & EXITING_BITMASK) != 0)
-	/* Already canceled or exiting.  */
-	break;
-
-      int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
-					      oldval);
-      if (curval == oldval)
-	{
   /* Set the return value.  */
   THREAD_SETMEM (self, result, PTHREAD_CANCELED);
-
   /* Make sure asynchronous cancellation is still enabled.  */
-	  if ((newval & CANCELTYPE_BITMASK) != 0)
+  if ((ch & CANCELTYPE_BITMASK) != 0)
-	    /* Run the registered destructors and terminate the thread.  */
     __do_cancel ();
-
-	  break;
-	}
-
-      oldval = curval;
-    }
 }
 
 int
@@ -104,72 +87,34 @@ __pthread_cancel (pthread_t th)
 		    " must be installed for pthread_cancel to work\n");
   }
 #endif
-  int result = 0;
-  int oldval;
-  int newval;
-  do
-    {
-    again:
-      oldval = pd->cancelhandling;
-      newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
 
-      /* Avoid doing unnecessary work.  The atomic operation can
+  int oldch = atomic_fetch_or_acquire (&pd->cancelhandling, CANCELED_BITMASK);
-	 potentially be expensive if the bug has to be locked and
+  if ((oldch & CANCELED_BITMASK) != 0)
-	 remote cache lines have to be invalidated.  */
+    return 0;
-      if (oldval == newval)
-	break;
-
-      /* If the cancellation is handled asynchronously just send a
-	 signal.  We avoid this if possible since it's more
-	 expensive.  */
-      if (CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS (newval))
-	{
-	  /* Mark the cancellation as "in progress".  */
-	  if (atomic_compare_and_exchange_bool_acq (&pd->cancelhandling,
-						    oldval | CANCELING_BITMASK,
-						    oldval))
-	    goto again;
 
   if (pd == THREAD_SELF)
-	    /* This is not merely an optimization: An application may
-	       call pthread_cancel (pthread_self ()) without calling
-	       pthread_create, so the signal handler may not have been
-	       set up for a self-cancel.  */
     {
-	      THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
+      /* A single-threaded process should be able to kill itself, since there
-	      if ((newval & CANCELTYPE_BITMASK) != 0)
+	 is nothing in the POSIX specification that says that it cannot.  So
-		__do_cancel ();
+	 we set multiple_threads to true so that cancellation points get
-	    }
+	 executed.  */
-	  else
-	    {
-	      /* The cancellation handler will take care of marking the
-		 thread as canceled.  */
-	      pid_t pid = __getpid ();
-
-	      int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid,
-					       SIGCANCEL);
-	      if (INTERNAL_SYSCALL_ERROR_P (val))
-		result = INTERNAL_SYSCALL_ERRNO (val);
-	    }
-
-	  break;
-	}
-
-	/* A single-threaded process should be able to kill itself, since
-	   there is nothing in the POSIX specification that says that it
-	   cannot.  So we set multiple_threads to true so that cancellation
-	   points get executed.  */
       THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
 #ifndef TLS_MULTIPLE_THREADS_IN_TCB
       __libc_multiple_threads = 1;
 #endif
-    }
-  /* Mark the thread as canceled.  This has to be done
-     atomically since other bits could be modified as well.  */
-  while (atomic_compare_and_exchange_bool_acq (&pd->cancelhandling, newval,
-					       oldval));
 
-  return result;
+      THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
+      if ((oldch & CANCELSTATE_BITMASK) == 0
+	  && (oldch & CANCELTYPE_BITMASK) != 0)
+	__do_cancel ();
+      return 0;
+    }
+
+  pid_t pid = __getpid ();
+  int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid, SIGCANCEL);
+  return INTERNAL_SYSCALL_ERROR_P (val)
+	 ? INTERNAL_SYSCALL_ERRNO (val)
+	 : 0;
 }
 versioned_symbol (libc, __pthread_cancel, pthread_cancel, GLIBC_2_34);
 

nptl/pthread_join_common.c

@@ -57,7 +57,7 @@ __pthread_clockjoin_ex (pthread_t threadid, void **thread_return,
   if ((pd == self
        || (self->joinid == pd
 	   && (pd->cancelhandling
-	       & (CANCELING_BITMASK | CANCELED_BITMASK | EXITING_BITMASK
+	       & (CANCELED_BITMASK | EXITING_BITMASK
 		  | TERMINATED_BITMASK)) == 0))
       && !CANCEL_ENABLED_AND_CANCELED (self->cancelhandling))
     /* This is a deadlock situation.  The threads are waiting for each