Fix missing wake-ups in pthread_rwlock_rdlock.

This adds wake-ups that would be missing if assuming that for a
non-writer-preferring rwlock, if one thread has acquired a rdlock and
does not release it, another thread will eventually acquire a rdlock too
despite concurrent write lock acquisition attempts.  BZ 14958 is about
supporting this assumption.  Strictly speaking, this isn't a valid
test case, but nonetheless worth supporting (see comment 7 of BZ 14958).

1 files changed, 38 insertions, 2 deletions

diff --git a/nptl/pthread_rwlock_rdlock.c b/nptl/pthread_rwlock_rdlock.c
index 0edca654a3..004a386fd5 100644
--- a/nptl/pthread_rwlock_rdlock.c
+++ b/nptl/pthread_rwlock_rdlock.c
@@ -23,6 +23,7 @@
 #include <pthreadP.h>
 #include <stap-probe.h>
 #include <elide.h>
+#include <stdbool.h>
 
 
 /* Acquire read lock for RWLOCK.  Slow path.  */
@@ -30,6 +31,7 @@ static int __attribute__((noinline))
 __pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
 {
   int result = 0;
+  bool wake = false;
 
   /* Lock is taken in caller.  */
 
@@ -81,7 +83,17 @@ __pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
 	      result = EAGAIN;
 	    }
 	  else
-	    LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
+	    {
+	      LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
+	      /* See pthread_rwlock_rdlock.  */
+	      if (rwlock->__data.__nr_readers == 1
+		  && rwlock->__data.__nr_readers_queued > 0
+		  && rwlock->__data.__nr_writers_queued > 0)
+		{
+		  ++rwlock->__data.__readers_wakeup;
+		  wake = true;
+		}
+	    }
 
 	  break;
 	}
@@ -90,6 +102,10 @@ __pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
   /* We are done, free the lock.  */
   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 
+  if (wake)
+    lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
+		    rwlock->__data.__shared);
+
   return result;
 }
 
@@ -100,6 +116,7 @@ int
 __pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
 {
   int result = 0;
+  bool wake = false;
 
   LIBC_PROBE (rdlock_entry, 1, rwlock);
 
@@ -126,11 +143,30 @@ __pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
 	  result = EAGAIN;
 	}
       else
-	LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
+	{
+	  LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
+	  /* If we are the first reader, and there are blocked readers and
+	     writers (which we don't prefer, see above), then it can be the
+	     case that we stole the lock from a writer that was already woken
+	     to acquire it.  That means that we need to take over the writer's
+	     responsibility to wake all readers (see pthread_rwlock_unlock).
+	     Thus, wake all readers in this case.  */
+	  if (rwlock->__data.__nr_readers == 1
+	      && rwlock->__data.__nr_readers_queued > 0
+	      && rwlock->__data.__nr_writers_queued > 0)
+	    {
+	      ++rwlock->__data.__readers_wakeup;
+	      wake = true;
+	    }
+	}
 
       /* We are done, free the lock.  */
       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 
+      if (wake)
+	lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
+			rwlock->__data.__shared);
+
       return result;
     }