commit 8f9450a0b7a9e78267e8ae1ab1000ebca08e473e

Author: Torvald Riegel <triegel@redhat.com>

Date:   Sat Dec 24 00:40:46 2016 +0100

    Add compiler barriers around modifications of the robust mutex list.

    Any changes to the per-thread list of robust mutexes currently acquired as

    well as the pending-operations entry are not simply sequential code but

    basically concurrent with any actions taken by the kernel when it tries

    to clean up after a crash.  This is not quite like multi-thread concurrency

    but more like signal-handler concurrency.

    This patch fixes latent bugs by adding compiler barriers where necessary so

    that it is ensured that the kernel crash handling sees consistent data.

    This is meant to be easy to backport, so we do not use C11-style signal

    fences yet.

            * nptl/descr.h (ENQUEUE_MUTEX_BOTH, DEQUEUE_MUTEX): Add compiler

            barriers and comments.

            * nptl/pthread_mutex_lock.c (__pthread_mutex_lock_full): Likewise.

            * nptl/pthread_mutex_timedlock.c (pthread_mutex_timedlock): Likewise.

            * nptl/pthread_mutex_unlock.c (__pthread_mutex_unlock_full): Likewise.

Index: glibc-2.17-c758a686/nptl/descr.h

===================================================================

--- glibc-2.17-c758a686.orig/nptl/descr.h

+++ glibc-2.17-c758a686/nptl/descr.h

@@ -180,7 +180,16 @@ struct pthread

      but the pointer to the next/previous element of the list points

      in the middle of the object, the __next element.  Whenever

      casting to __pthread_list_t we need to adjust the pointer

-     first.  */

+     first.

+     These operations are effectively concurrent code in that the thread

+     can get killed at any point in time and the kernel takes over.  Thus,

+     the __next elements are a kind of concurrent list and we need to

+     enforce using compiler barriers that the individual operations happen

+     in such a way that the kernel always sees a consistent list.  The

+     backward links (ie, the __prev elements) are not used by the kernel.

+     FIXME We should use relaxed MO atomic operations here and signal fences

+     because this kind of concurrency is similar to synchronizing with a

+     signal handler.  */

 # define QUEUE_PTR_ADJUST (offsetof (__pthread_list_t, __next))

 # define ENQUEUE_MUTEX_BOTH(mutex, val)					      \

@@ -192,6 +201,8 @@ struct pthread

     mutex->__data.__list.__next = THREAD_GETMEM (THREAD_SELF,		      \

 						 robust_head.list);	      \

     mutex->__data.__list.__prev = (void *) &THREAD_SELF->robust_head;	      \

+    /* Ensure that the new list entry is ready before we insert it.  */	      \

+    __asm ("" ::: "memory");						      \

     THREAD_SETMEM (THREAD_SELF, robust_head.list,			      \

 		   (void *) (((uintptr_t) &mutex->__data.__list.__next)	      \

 			     | val));					      \

@@ -206,6 +217,9 @@ struct pthread

       ((char *) (((uintptr_t) mutex->__data.__list.__prev) & ~1ul)	      \

        - QUEUE_PTR_ADJUST);						      \

     prev->__next = mutex->__data.__list.__next;				      \

+    /* Ensure that we remove the entry from the list before we change the     \

+       __next pointer of the entry, which is read by the kernel.  */	      \

+    __asm ("" ::: "memory");						      \

     mutex->__data.__list.__prev = NULL;					      \

     mutex->__data.__list.__next = NULL;					      \

   } while (0)

@@ -220,6 +234,8 @@ struct pthread

   do {									      \

     mutex->__data.__list.__next						      \

       = THREAD_GETMEM (THREAD_SELF, robust_list.__next);		      \

+    /* Ensure that the new list entry is ready before we insert it.  */	      \

+    __asm ("" ::: "memory");						      \

     THREAD_SETMEM (THREAD_SELF, robust_list.__next,			      \

 		   (void *) (((uintptr_t) &mutex->__data.__list) | val));     \

   } while (0)

@@ -240,6 +256,9 @@ struct pthread

 	  }								      \

 									      \

 	runp->__next = next->__next;					      \

+	/* Ensure that we remove the entry from the list before we change the \

+	   __next pointer of the entry, which is read by the kernel.  */      \

+	    __asm ("" ::: "memory");					      \

 	mutex->__data.__list.__next = NULL;				      \

       }									      \

   } while (0)

Index: glibc-2.17-c758a686/nptl/pthread_mutex_lock.c

===================================================================

--- glibc-2.17-c758a686.orig/nptl/pthread_mutex_lock.c

+++ glibc-2.17-c758a686/nptl/pthread_mutex_lock.c

@@ -181,6 +181,9 @@ __pthread_mutex_lock_full (pthread_mutex

     case PTHREAD_MUTEX_ROBUST_ADAPTIVE_NP:

       THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

 		     &mutex->__data.__list.__next);

+      /* We need to set op_pending before starting the operation.  Also

+	 see comments at ENQUEUE_MUTEX.  */

+      __asm ("" ::: "memory");

       oldval = mutex->__data.__lock;

       /* This is set to FUTEX_WAITERS iff we might have shared the

@@ -228,7 +231,12 @@ __pthread_mutex_lock_full (pthread_mutex

 	      /* But it is inconsistent unless marked otherwise.  */

 	      mutex->__data.__owner = PTHREAD_MUTEX_INCONSISTENT;

+	      /* We must not enqueue the mutex before we have acquired it.

+		 Also see comments at ENQUEUE_MUTEX.  */

+	      __asm ("" ::: "memory");

 	      ENQUEUE_MUTEX (mutex);

+	      /* We need to clear op_pending after we enqueue the mutex.  */

+	      __asm ("" ::: "memory");

 	      THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	      /* Note that we deliberately exit here.  If we fall

@@ -250,6 +258,8 @@ __pthread_mutex_lock_full (pthread_mutex

 	      int kind = PTHREAD_MUTEX_TYPE (mutex);

 	      if (kind == PTHREAD_MUTEX_ROBUST_ERRORCHECK_NP)

 		{

+		  /* We do not need to ensure ordering wrt another memory

+		     access.  Also see comments at ENQUEUE_MUTEX. */

 		  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

 				 NULL);

 		  return EDEADLK;

@@ -257,6 +267,8 @@ __pthread_mutex_lock_full (pthread_mutex

 	      if (kind == PTHREAD_MUTEX_ROBUST_RECURSIVE_NP)

 		{

+		  /* We do not need to ensure ordering wrt another memory

+		     access.  */

 		  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

 				 NULL);

@@ -309,12 +321,19 @@ __pthread_mutex_lock_full (pthread_mutex

 	  mutex->__data.__count = 0;

 	  int private = PTHREAD_ROBUST_MUTEX_PSHARED (mutex);

 	  lll_unlock (mutex->__data.__lock, private);

+	  /* FIXME This violates the mutex destruction requirements.  See

+	     __pthread_mutex_unlock_full.  */

 	  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	  return ENOTRECOVERABLE;

 	}

       mutex->__data.__count = 1;

+      /* We must not enqueue the mutex before we have acquired it.

+	 Also see comments at ENQUEUE_MUTEX.  */

+      __asm ("" ::: "memory");

       ENQUEUE_MUTEX (mutex);

+      /* We need to clear op_pending after we enqueue the mutex.  */

+      __asm ("" ::: "memory");

       THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

       break;

@@ -331,10 +350,15 @@ __pthread_mutex_lock_full (pthread_mutex

 	int robust = mutex->__data.__kind & PTHREAD_MUTEX_ROBUST_NORMAL_NP;

 	if (robust)

-	  /* Note: robust PI futexes are signaled by setting bit 0.  */

-	  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

-			 (void *) (((uintptr_t) &mutex->__data.__list.__next)

-				   | 1));

+	  {

+	    /* Note: robust PI futexes are signaled by setting bit 0.  */

+	    THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

+			   (void *) (((uintptr_t) &mutex->__data.__list.__next)

+				     | 1));

+	    /* We need to set op_pending before starting the operation.  Also

+	       see comments at ENQUEUE_MUTEX.  */

+	    __asm ("" ::: "memory");

+	  }

 	oldval = mutex->__data.__lock;

@@ -343,12 +367,16 @@ __pthread_mutex_lock_full (pthread_mutex

 	  {

 	    if (kind == PTHREAD_MUTEX_ERRORCHECK_NP)

 	      {

+		/* We do not need to ensure ordering wrt another memory

+		   access.  */

 		THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 		return EDEADLK;

 	      }

 	    if (kind == PTHREAD_MUTEX_RECURSIVE_NP)

 	      {

+		/* We do not need to ensure ordering wrt another memory

+		   access.  */

 		THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 		/* Just bump the counter.  */

@@ -411,7 +439,12 @@ __pthread_mutex_lock_full (pthread_mutex

 	    /* But it is inconsistent unless marked otherwise.  */

 	    mutex->__data.__owner = PTHREAD_MUTEX_INCONSISTENT;

+	    /* We must not enqueue the mutex before we have acquired it.

+	       Also see comments at ENQUEUE_MUTEX.  */

+	    __asm ("" ::: "memory");

 	    ENQUEUE_MUTEX_PI (mutex);

+	    /* We need to clear op_pending after we enqueue the mutex.  */

+	    __asm ("" ::: "memory");

 	    THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	    /* Note that we deliberately exit here.  If we fall

@@ -439,6 +472,8 @@ __pthread_mutex_lock_full (pthread_mutex

 						  PTHREAD_ROBUST_MUTEX_PSHARED (mutex)),

 			      0, 0);

+	    /* To the kernel, this will be visible after the kernel has

+	       acquired the mutex in the syscall.  */

 	    THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	    return ENOTRECOVERABLE;

 	  }

@@ -446,7 +481,12 @@ __pthread_mutex_lock_full (pthread_mutex

 	mutex->__data.__count = 1;

 	if (robust)

 	  {

+	    /* We must not enqueue the mutex before we have acquired it.

+	       Also see comments at ENQUEUE_MUTEX.  */

+	    __asm ("" ::: "memory");

 	    ENQUEUE_MUTEX_PI (mutex);

+	    /* We need to clear op_pending after we enqueue the mutex.  */

+	    __asm ("" ::: "memory");

 	    THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	  }

       }

Index: glibc-2.17-c758a686/nptl/pthread_mutex_timedlock.c

===================================================================

--- glibc-2.17-c758a686.orig/nptl/pthread_mutex_timedlock.c

+++ glibc-2.17-c758a686/nptl/pthread_mutex_timedlock.c

@@ -140,6 +140,9 @@ pthread_mutex_timedlock (pthread_mutex_t

     case PTHREAD_MUTEX_ROBUST_ADAPTIVE_NP:

       THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

 		     &mutex->__data.__list.__next);

+      /* We need to set op_pending before starting the operation.  Also

+	 see comments at ENQUEUE_MUTEX.  */

+      __asm ("" ::: "memory");

       oldval = mutex->__data.__lock;

       /* This is set to FUTEX_WAITERS iff we might have shared the

@@ -177,7 +180,12 @@ pthread_mutex_timedlock (pthread_mutex_t

 	      /* But it is inconsistent unless marked otherwise.  */

 	      mutex->__data.__owner = PTHREAD_MUTEX_INCONSISTENT;

+	      /* We must not enqueue the mutex before we have acquired it.

+		 Also see comments at ENQUEUE_MUTEX.  */

+	      __asm ("" ::: "memory");

 	      ENQUEUE_MUTEX (mutex);

+	      /* We need to clear op_pending after we enqueue the mutex.  */

+	      __asm ("" ::: "memory");

 	      THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	      /* Note that we deliberately exit here.  If we fall

@@ -193,6 +201,8 @@ pthread_mutex_timedlock (pthread_mutex_t

 	      int kind = PTHREAD_MUTEX_TYPE (mutex);

 	      if (kind == PTHREAD_MUTEX_ROBUST_ERRORCHECK_NP)

 		{

+		  /* We do not need to ensure ordering wrt another memory

+		     access.  Also see comments at ENQUEUE_MUTEX. */

 		  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

 				 NULL);

 		  return EDEADLK;

@@ -200,6 +210,8 @@ pthread_mutex_timedlock (pthread_mutex_t

 	      if (kind == PTHREAD_MUTEX_ROBUST_RECURSIVE_NP)

 		{

+		  /* We do not need to ensure ordering wrt another memory

+		     access.  */

 		  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

 				 NULL);

@@ -294,12 +306,19 @@ pthread_mutex_timedlock (pthread_mutex_t

 	  mutex->__data.__count = 0;

 	  int private = PTHREAD_ROBUST_MUTEX_PSHARED (mutex);

 	  lll_unlock (mutex->__data.__lock, private);

+	  /* FIXME This violates the mutex destruction requirements.  See

+	     __pthread_mutex_unlock_full.  */

 	  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	  return ENOTRECOVERABLE;

 	}

       mutex->__data.__count = 1;

+      /* We must not enqueue the mutex before we have acquired it.

+	 Also see comments at ENQUEUE_MUTEX.  */

+      __asm ("" ::: "memory");

       ENQUEUE_MUTEX (mutex);

+      /* We need to clear op_pending after we enqueue the mutex.  */

+      __asm ("" ::: "memory");

       THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

       break;

@@ -316,10 +335,15 @@ pthread_mutex_timedlock (pthread_mutex_t

 	int robust = mutex->__data.__kind & PTHREAD_MUTEX_ROBUST_NORMAL_NP;

 	if (robust)

-	  /* Note: robust PI futexes are signaled by setting bit 0.  */

-	  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

-			 (void *) (((uintptr_t) &mutex->__data.__list.__next)

-				   | 1));

+	  {

+	    /* Note: robust PI futexes are signaled by setting bit 0.  */

+	    THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

+			   (void *) (((uintptr_t) &mutex->__data.__list.__next)

+				     | 1));

+	    /* We need to set op_pending before starting the operation.  Also

+	       see comments at ENQUEUE_MUTEX.  */

+	    __asm ("" ::: "memory");

+	  }

 	oldval = mutex->__data.__lock;

@@ -328,12 +352,16 @@ pthread_mutex_timedlock (pthread_mutex_t

 	  {

 	    if (kind == PTHREAD_MUTEX_ERRORCHECK_NP)

 	      {

+		/* We do not need to ensure ordering wrt another memory

+		   access.  */

 		THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 		return EDEADLK;

 	      }

 	    if (kind == PTHREAD_MUTEX_RECURSIVE_NP)

 	      {

+		/* We do not need to ensure ordering wrt another memory

+		   access.  */

 		THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 		/* Just bump the counter.  */

@@ -420,7 +448,12 @@ pthread_mutex_timedlock (pthread_mutex_t

 	    /* But it is inconsistent unless marked otherwise.  */

 	    mutex->__data.__owner = PTHREAD_MUTEX_INCONSISTENT;

+	    /* We must not enqueue the mutex before we have acquired it.

+	       Also see comments at ENQUEUE_MUTEX.  */

+	    __asm ("" ::: "memory");

 	    ENQUEUE_MUTEX_PI (mutex);

+	    /* We need to clear op_pending after we enqueue the mutex.  */

+	    __asm ("" ::: "memory");

 	    THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	    /* Note that we deliberately exit here.  If we fall

@@ -443,6 +476,8 @@ pthread_mutex_timedlock (pthread_mutex_t

 						  PTHREAD_ROBUST_MUTEX_PSHARED (mutex)),

 			      0, 0);

+	    /* To the kernel, this will be visible after the kernel has

+	       acquired the mutex in the syscall.  */

 	    THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	    return ENOTRECOVERABLE;

 	  }

@@ -450,7 +485,12 @@ pthread_mutex_timedlock (pthread_mutex_t

 	mutex->__data.__count = 1;

 	if (robust)

 	  {

+	    /* We must not enqueue the mutex before we have acquired it.

+	       Also see comments at ENQUEUE_MUTEX.  */

+	    __asm ("" ::: "memory");

 	    ENQUEUE_MUTEX_PI (mutex);

+	    /* We need to clear op_pending after we enqueue the mutex.  */

+	    __asm ("" ::: "memory");

 	    THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

 	  }

 	}

Index: glibc-2.17-c758a686/nptl/pthread_mutex_unlock.c

===================================================================

--- glibc-2.17-c758a686.orig/nptl/pthread_mutex_unlock.c

+++ glibc-2.17-c758a686/nptl/pthread_mutex_unlock.c

@@ -143,6 +143,9 @@ __pthread_mutex_unlock_full (pthread_mut

       /* Remove mutex from the list.  */

       THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

 		     &mutex->__data.__list.__next);

+      /* We must set op_pending before we dequeue the mutex.  Also see

+	 comments at ENQUEUE_MUTEX.  */

+      __asm ("" ::: "memory");

       DEQUEUE_MUTEX (mutex);

       mutex->__data.__owner = newowner;

@@ -159,6 +162,14 @@ __pthread_mutex_unlock_full (pthread_mut

 			     & FUTEX_WAITERS) != 0))

 	lll_futex_wake (&mutex->__data.__lock, 1, private);

+      /* We must clear op_pending after we release the mutex.

+	 FIXME However, this violates the mutex destruction requirements

+	 because another thread could acquire the mutex, destroy it, and

+	 reuse the memory for something else; then, if this thread crashes,

+	 and the memory happens to have a value equal to the TID, the kernel

+	 will believe it is still related to the mutex (which has been

+	 destroyed already) and will modify some other random object.  */

+      __asm ("" ::: "memory");

       THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

       break;

@@ -223,6 +234,9 @@ __pthread_mutex_unlock_full (pthread_mut

 	  THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending,

 			 (void *) (((uintptr_t) &mutex->__data.__list.__next)

 				   | 1));

+	  /* We must set op_pending before we dequeue the mutex.  Also see

+	     comments at ENQUEUE_MUTEX.  */

+	  __asm ("" ::: "memory");

 	  DEQUEUE_MUTEX (mutex);

 	}

@@ -247,6 +261,9 @@ __pthread_mutex_unlock_full (pthread_mut

 			    __lll_private_flag (FUTEX_UNLOCK_PI, private));

 	}

+      /* This happens after the kernel releases the mutex but violates the

+	 mutex destruction requirements; see comments in the code handling

+	 PTHREAD_MUTEX_ROBUST_NORMAL_NP.  */

       THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL);

       break;