nptl: Handle spurious EINTR when thread cancellation is disabled (BZ#29029)

Some Linux interfaces never restart after being interrupted by a signal
handler, regardless of the use of SA_RESTART [1].  It means that for
pthread cancellation, if the target thread disables cancellation with
pthread_setcancelstate and calls such interfaces (like poll or select),
it should not see spurious EINTR failures due the internal SIGCANCEL.

However recent changes made pthread_cancel to always sent the internal
signal, regardless of the target thread cancellation status or type.
To fix it, the previous semantic is restored, where the cancel signal
is only sent if the target thread has cancelation enabled in
asynchronous mode.

The cancel state and cancel type is moved back to cancelhandling
and atomic operation are used to synchronize between threads.  The
patch essentially revert the following commits:

  8c1c0aae20 nptl: Move cancel type out of cancelhandling
  2b51742531 nptl: Move cancel state out of cancelhandling
  26cfbb7162 nptl: Remove CANCELING_BITMASK

However I changed the atomic operation to follow the internal C11
semantic and removed the MACRO usage, it simplifies a bit the
resulting code (and removes another usage of the old atomic macros).

Checked on x86_64-linux-gnu, i686-linux-gnu, aarch64-linux-gnu,
and powerpc64-linux-gnu.

[1] https://man7.org/linux/man-pages/man7/signal.7.html

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>

10 files changed, 272 insertions, 85 deletions

diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
index 34a33164ff..01a282f3f6 100644
--- a/nptl/allocatestack.c
+++ b/nptl/allocatestack.c
@@ -119,8 +119,6 @@ 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 8d54a6add1..f4b08902b2 100644
--- a/nptl/cancellation.c
+++ b/nptl/cancellation.c
@@ -30,19 +30,26 @@ int
 __pthread_enable_asynccancel (void)
 {
   struct pthread *self = THREAD_SELF;
+  int oldval = atomic_load_relaxed (&self->cancelhandling);
 
-  int oldval = THREAD_GETMEM (self, canceltype);
-  THREAD_SETMEM (self, canceltype, PTHREAD_CANCEL_ASYNCHRONOUS);
+  while (1)
+    {
+      int newval = oldval | CANCELTYPE_BITMASK;
 
-  int ch = THREAD_GETMEM (self, cancelhandling);
+      if (newval == oldval)
+	break;
 
-  if (self->cancelstate == PTHREAD_CANCEL_ENABLE
-      && (ch & CANCELED_BITMASK)
-      && !(ch & EXITING_BITMASK)
-      && !(ch & TERMINATED_BITMASK))
-    {
-      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
-      __do_cancel ();
+      if (atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						&oldval, newval))
+	{
+	  if (cancel_enabled_and_canceled_and_async (newval))
+	    {
+	      self->result = PTHREAD_CANCELED;
+	      __do_cancel ();
+	    }
+
+	  break;
+	}
     }
 
   return oldval;
@@ -56,10 +63,29 @@ __pthread_disable_asynccancel (int oldtype)
 {
   /* If asynchronous cancellation was enabled before we do not have
      anything to do.  */
-  if (oldtype == PTHREAD_CANCEL_ASYNCHRONOUS)
+  if (oldtype & CANCELTYPE_BITMASK)
     return;
 
   struct pthread *self = THREAD_SELF;
-  self->canceltype = PTHREAD_CANCEL_DEFERRED;
+  int newval;
+  int oldval = atomic_load_relaxed (&self->cancelhandling);
+  do
+    {
+      newval = oldval & ~CANCELTYPE_BITMASK;
+    }
+  while (!atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						&oldval, newval));
+
+  /* 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 (__glibc_unlikely ((newval & (CANCELING_BITMASK | CANCELED_BITMASK))
+			   == CANCELING_BITMASK))
+    {
+      futex_wait_simple ((unsigned int *) &self->cancelhandling, newval,
+			 FUTEX_PRIVATE);
+      newval = atomic_load_relaxed (&self->cancelhandling);
+    }
 }
 libc_hidden_def (__pthread_disable_asynccancel)
diff --git a/nptl/cleanup_defer.c b/nptl/cleanup_defer.c
index f8181a40e8..eb0bc77740 100644
--- a/nptl/cleanup_defer.c
+++ b/nptl/cleanup_defer.c
@@ -30,9 +30,22 @@ ___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);
 
-  /* Disable asynchronous cancellation for now.  */
-  ibuf->priv.data.canceltype = THREAD_GETMEM (self, canceltype);
-  THREAD_SETMEM (self, canceltype, PTHREAD_CANCEL_DEFERRED);
+  int cancelhandling = atomic_load_relaxed (&self->cancelhandling);
+  if (__glibc_unlikely (cancelhandling & CANCELTYPE_BITMASK))
+    {
+      int newval;
+      do
+	{
+	  newval = cancelhandling & ~CANCELTYPE_BITMASK;
+	}
+      while (!atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						    &cancelhandling,
+						    newval));
+    }
+
+  ibuf->priv.data.canceltype = (cancelhandling & CANCELTYPE_BITMASK
+				? PTHREAD_CANCEL_ASYNCHRONOUS
+				: PTHREAD_CANCEL_DEFERRED);
 
   /* Store the new cleanup handler info.  */
   THREAD_SETMEM (self, cleanup_jmp_buf, (struct pthread_unwind_buf *) buf);
@@ -54,9 +67,26 @@ ___pthread_unregister_cancel_restore (__pthread_unwind_buf_t *buf)
 
   THREAD_SETMEM (self, cleanup_jmp_buf, ibuf->priv.data.prev);
 
-  THREAD_SETMEM (self, canceltype, ibuf->priv.data.canceltype);
-  if (ibuf->priv.data.canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
-    __pthread_testcancel ();
+  if (ibuf->priv.data.canceltype == PTHREAD_CANCEL_DEFERRED)
+    return;
+
+  int cancelhandling = atomic_load_relaxed (&self->cancelhandling);
+  if (cancelhandling & CANCELTYPE_BITMASK)
+    {
+      int newval;
+      do
+	{
+	  newval = cancelhandling | CANCELTYPE_BITMASK;
+	}
+      while (!atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						    &cancelhandling, newval));
+
+      if (cancel_enabled_and_canceled (cancelhandling))
+	{
+	  self->result = PTHREAD_CANCELED;
+	  __do_cancel ();
+	}
+    }
 }
 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 ea8aca08e6..bb46b5958e 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -279,18 +279,27 @@ struct pthread
 
   /* Flags determining processing of cancellation.  */
   int cancelhandling;
+  /* Bit set if cancellation is disabled.  */
+#define CANCELSTATE_BIT		0
+#define CANCELSTATE_BITMASK	(1 << CANCELSTATE_BIT)
+  /* Bit set if asynchronous cancellation mode is selected.  */
+#define CANCELTYPE_BIT		1
+#define CANCELTYPE_BITMASK	(1 << CANCELTYPE_BIT)
+  /* Bit set if canceling has been initiated.  */
+#define CANCELING_BIT		2
+#define CANCELING_BITMASK	(1 << CANCELING_BIT)
   /* Bit set if canceled.  */
 #define CANCELED_BIT		3
-#define CANCELED_BITMASK	(0x01 << CANCELED_BIT)
+#define CANCELED_BITMASK	(1 << CANCELED_BIT)
   /* Bit set if thread is exiting.  */
 #define EXITING_BIT		4
-#define EXITING_BITMASK		(0x01 << EXITING_BIT)
+#define EXITING_BITMASK		(1 << EXITING_BIT)
   /* Bit set if thread terminated and TCB is freed.  */
 #define TERMINATED_BIT		5
-#define TERMINATED_BITMASK	(0x01 << TERMINATED_BIT)
+#define TERMINATED_BITMASK	(1 << TERMINATED_BIT)
   /* Bit set if thread is supposed to change XID.  */
 #define SETXID_BIT		6
-#define SETXID_BITMASK		(0x01 << SETXID_BIT)
+#define SETXID_BITMASK		(1 << SETXID_BIT)
 
   /* Flags.  Including those copied from the thread attribute.  */
   int flags;
@@ -390,14 +399,6 @@ 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;
-
-  /* Thread cancel type (PTHREAD_CANCEL_DEFERRED or
-     PTHREAD_CANCEL_ASYNCHRONOUS).  */
-  unsigned char canceltype;
-
   /* Used in __pthread_kill_internal to detected a thread that has
      exited or is about to exit.  exit_lock must only be acquired
      after blocking signals.  */
@@ -417,6 +418,22 @@ struct pthread
   (sizeof (struct pthread) - offsetof (struct pthread, end_padding))
 } __attribute ((aligned (TCB_ALIGNMENT)));
 
+static inline bool
+cancel_enabled_and_canceled (int value)
+{
+  return (value & (CANCELSTATE_BITMASK | CANCELED_BITMASK | EXITING_BITMASK
+		   | TERMINATED_BITMASK))
+    == CANCELED_BITMASK;
+}
+
+static inline bool
+cancel_enabled_and_canceled_and_async (int value)
+{
+  return ((value) & (CANCELSTATE_BITMASK | CANCELTYPE_BITMASK | CANCELED_BITMASK
+		     | EXITING_BITMASK | TERMINATED_BITMASK))
+    == (CANCELTYPE_BITMASK | CANCELED_BITMASK);
+}
+
 /* This yields the pointer that TLS support code calls the thread pointer.  */
 #if TLS_TCB_AT_TP
 # define TLS_TPADJ(pd) (pd)
diff --git a/nptl/libc-cleanup.c b/nptl/libc-cleanup.c
index cb4c226281..c4a83591bf 100644
--- a/nptl/libc-cleanup.c
+++ b/nptl/libc-cleanup.c
@@ -26,9 +26,24 @@ __libc_cleanup_push_defer (struct _pthread_cleanup_buffer *buffer)
 
   buffer->__prev = THREAD_GETMEM (self, cleanup);
 
+  int cancelhandling = atomic_load_relaxed (&self->cancelhandling);
+
   /* Disable asynchronous cancellation for now.  */
-  buffer->__canceltype = THREAD_GETMEM (self, canceltype);
-  THREAD_SETMEM (self, canceltype, PTHREAD_CANCEL_DEFERRED);
+  if (__glibc_unlikely (cancelhandling & CANCELTYPE_BITMASK))
+    {
+      int newval;
+      do
+	{
+	  newval = cancelhandling & ~CANCELTYPE_BITMASK;
+	}
+      while (!atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						    &cancelhandling,
+						    newval));
+    }
+
+  buffer->__canceltype = (cancelhandling & CANCELTYPE_BITMASK
+			  ? PTHREAD_CANCEL_ASYNCHRONOUS
+			  : PTHREAD_CANCEL_DEFERRED);
 
   THREAD_SETMEM (self, cleanup, buffer);
 }
@@ -41,8 +56,22 @@ __libc_cleanup_pop_restore (struct _pthread_cleanup_buffer *buffer)
 
   THREAD_SETMEM (self, cleanup, buffer->__prev);
 
-  THREAD_SETMEM (self, canceltype, buffer->__canceltype);
-  if (buffer->__canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
-      __pthread_testcancel ();
+  int cancelhandling = atomic_load_relaxed (&self->cancelhandling);
+  if (cancelhandling & CANCELTYPE_BITMASK)
+    {
+      int newval;
+      do
+	{
+	  newval = cancelhandling | CANCELTYPE_BITMASK;
+	}
+      while (!atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						    &cancelhandling, newval));
+
+      if (cancel_enabled_and_canceled (cancelhandling))
+	{
+	  self->result = PTHREAD_CANCELED;
+	  __do_cancel ();
+	}
+    }
 }
 libc_hidden_def (__libc_cleanup_pop_restore)
diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index 7524c7ce4d..c76882e279 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -42,18 +42,29 @@ sigcancel_handler (int sig, siginfo_t *si, void *ctx)
 
   struct pthread *self = THREAD_SELF;
 
-  int ch = atomic_load_relaxed (&self->cancelhandling);
-  /* Cancelation not enabled, not cancelled, or already exitting.  */
-  if (self->cancelstate == PTHREAD_CANCEL_DISABLE
-      || (ch & CANCELED_BITMASK) == 0
-      || (ch & EXITING_BITMASK) != 0)
-    return;
-
-  /* Set the return value.  */
-  THREAD_SETMEM (self, result, PTHREAD_CANCELED);
-  /* Make sure asynchronous cancellation is still enabled.  */
-  if (self->canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
-    __do_cancel ();
+  int oldval = atomic_load_relaxed (&self->cancelhandling);
+  while (1)
+    {
+      /* We are canceled now.  When canceled by another thread this flag
+	 is already set but if the signal is directly send (internally or
+	 from another process) is has to be done here.  */
+      int newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
+
+      if (oldval == newval || (oldval & EXITING_BITMASK) != 0)
+	/* Already canceled or exiting.  */
+	break;
+
+      if (atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						&oldval, newval))
+	{
+	  self->result = PTHREAD_CANCELED;
+
+	  /* Make sure asynchronous cancellation is still enabled.  */
+	  if ((oldval & CANCELTYPE_BITMASK) != 0)
+	    /* Run the registered destructors and terminate the thread.  */
+	    __do_cancel ();
+	}
+    }
 }
 
 int
@@ -92,29 +103,70 @@ __pthread_cancel (pthread_t th)
   }
 #endif
 
-  int oldch = atomic_fetch_or_acquire (&pd->cancelhandling, CANCELED_BITMASK);
-  if ((oldch & CANCELED_BITMASK) != 0)
-    return 0;
-
-  if (pd == THREAD_SELF)
+  /* Some syscalls are never restarted after being interrupted by a signal
+     handler, regardless of the use of SA_RESTART (they always fail with
+     EINTR).  So pthread_cancel cannot send SIGCANCEL unless the cancellation
+     is enabled and set as asynchronous (in this case the cancellation will
+     be acted in the cancellation handler instead by the syscall wrapper).
+     Otherwise the target thread is set as 'cancelling' (CANCELING_BITMASK)
+     by atomically setting 'cancelhandling' and the cancelation will be acted
+     upon on next cancellation entrypoing in the target thread.
+
+     It also requires to atomically check if cancellation is enabled and
+     asynchronous, so both cancellation state and type are tracked on
+     'cancelhandling'.  */
+
+  int result = 0;
+  int oldval = atomic_load_relaxed (&pd->cancelhandling);
+  int newval;
+  do
     {
-      /* 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);
+      newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
+      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_async (newval))
+	{
+	  /* Mark the cancellation as "in progress".  */
+	  int newval2 = oldval | CANCELING_BITMASK;
+	  if (!atomic_compare_exchange_weak_acquire (&pd->cancelhandling,
+						     &oldval, newval2))
+	    continue;
+
+	  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.  */
+	    {
+	      pd->result = PTHREAD_CANCELED;
+	      if ((newval & CANCELTYPE_BITMASK) != 0)
+		__do_cancel ();
+	    }
+	  else
+	    /* The cancellation handler will take care of marking the
+	       thread as canceled.  */
+	    result = __pthread_kill_internal (th, SIGCANCEL);
+
+	  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
-
-      THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
-      if (pd->cancelstate == PTHREAD_CANCEL_ENABLE
-	  && pd->canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
-	__do_cancel ();
-      return 0;
     }
+  while (!atomic_compare_exchange_weak_acquire (&pd->cancelhandling, &oldval,
+						newval));
 
-  return __pthread_kill_internal (th, SIGCANCEL);
+  return result;
 }
 versioned_symbol (libc, __pthread_cancel, pthread_cancel, GLIBC_2_34);
 
diff --git a/nptl/pthread_join_common.c b/nptl/pthread_join_common.c
index a8e884f341..ca3245b0af 100644
--- a/nptl/pthread_join_common.c
+++ b/nptl/pthread_join_common.c
@@ -57,12 +57,9 @@ __pthread_clockjoin_ex (pthread_t threadid, void **thread_return,
   if ((pd == self
        || (self->joinid == pd
 	   && (pd->cancelhandling
-	       & (CANCELED_BITMASK | EXITING_BITMASK
+	       & (CANCELING_BITMASK | CANCELED_BITMASK | EXITING_BITMASK
 		  | TERMINATED_BITMASK)) == 0))
-      && !(self->cancelstate == PTHREAD_CANCEL_ENABLE
-	   && (pd->cancelhandling & (CANCELED_BITMASK | EXITING_BITMASK
-				     | TERMINATED_BITMASK))
-	       == CANCELED_BITMASK))
+      && !cancel_enabled_and_canceled (self->cancelhandling))
     /* 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 9905b12e4f..f8edf18fbe 100644
--- a/nptl/pthread_setcancelstate.c
+++ b/nptl/pthread_setcancelstate.c
@@ -30,9 +30,29 @@ __pthread_setcancelstate (int state, int *oldstate)
 
   self = THREAD_SELF;
 
-  if (oldstate != NULL)
-    *oldstate = self->cancelstate;
-  self->cancelstate = state;
+  int oldval = atomic_load_relaxed (&self->cancelhandling);
+  while (1)
+    {
+      int newval = (state == PTHREAD_CANCEL_DISABLE
+		    ? oldval | CANCELSTATE_BITMASK
+		    : oldval & ~CANCELSTATE_BITMASK);
+
+      if (oldstate != NULL)
+	*oldstate = ((oldval & CANCELSTATE_BITMASK)
+		     ? PTHREAD_CANCEL_DISABLE : PTHREAD_CANCEL_ENABLE);
+
+      if (oldval == newval)
+	break;
+
+      if (atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						&oldval, newval))
+	{
+	  if (cancel_enabled_and_canceled_and_async (newval))
+	    __do_cancel ();
+
+	  break;
+	}
+    }
 
   return 0;
 }
diff --git a/nptl/pthread_setcanceltype.c b/nptl/pthread_setcanceltype.c
index 94e56466de..1307d355c1 100644
--- a/nptl/pthread_setcanceltype.c
+++ b/nptl/pthread_setcanceltype.c
@@ -28,11 +28,32 @@ __pthread_setcanceltype (int type, int *oldtype)
 
   volatile struct pthread *self = THREAD_SELF;
 
-  if (oldtype != NULL)
-    *oldtype = self->canceltype;
-  self->canceltype = type;
-  if (type == PTHREAD_CANCEL_ASYNCHRONOUS)
-    __pthread_testcancel ();
+  int oldval = atomic_load_relaxed (&self->cancelhandling);
+  while (1)
+    {
+      int newval = (type == PTHREAD_CANCEL_ASYNCHRONOUS
+		    ? oldval | CANCELTYPE_BITMASK
+		    : oldval & ~CANCELTYPE_BITMASK);
+
+      if (oldtype != NULL)
+	*oldtype = ((oldval & CANCELTYPE_BITMASK)
+		    ? PTHREAD_CANCEL_ASYNCHRONOUS : PTHREAD_CANCEL_DEFERRED);
+
+      if (oldval == newval)
+	break;
+
+      if (atomic_compare_exchange_weak_acquire (&self->cancelhandling,
+						&oldval, newval))
+	{
+	  if (cancel_enabled_and_canceled_and_async (newval))
+	    {
+	      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
+	      __do_cancel ();
+	    }
+
+	  break;
+	}
+    }
 
   return 0;
 }
diff --git a/nptl/pthread_testcancel.c b/nptl/pthread_testcancel.c
index 13123608e8..b81928c000 100644
--- a/nptl/pthread_testcancel.c
+++ b/nptl/pthread_testcancel.c
@@ -23,13 +23,10 @@ void
 ___pthread_testcancel (void)
 {
   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))
+  int cancelhandling = atomic_load_relaxed (&self->cancelhandling);
+  if (cancel_enabled_and_canceled (cancelhandling))
     {
-      THREAD_SETMEM (self, result, PTHREAD_CANCELED);
+      self->result = PTHREAD_CANCELED;
       __do_cancel ();
     }
 }