[6/9] nptl: Use a single loop in pthread_cond_wait instaed of a nested loop

Message ID 20230128021829.7990-7-malteskarupke@fastmail.fm
State Superseded
Delegated to: Carlos O'Donell
Headers
Series Patch to fix glibc condition variable bug (BZ 25847) |

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent

Commit Message

develop--- via Libc-alpha Jan. 28, 2023, 2:18 a.m. UTC
  From: Malte Skarupke <malteskarupke@fastmail.fm>

The loop was a little more complicated than necessary. There was only one
break statement out of the inner loop, and the outer loop was nearly empty.
So just remove the outer loop, moving its code to the one break statement in
the inner loop. This allows us to replace all gotos with break statements.
---
 nptl/pthread_cond_wait.c | 41 +++++++++++++++++++---------------------
 1 file changed, 19 insertions(+), 22 deletions(-)
  

Patch

diff --git a/nptl/pthread_cond_wait.c b/nptl/pthread_cond_wait.c
index 8a9219e064..8b7db51148 100644
--- a/nptl/pthread_cond_wait.c
+++ b/nptl/pthread_cond_wait.c
@@ -382,17 +382,15 @@  __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
       return err;
     }
 
-  /* Now wait until a signal is available in our group or it is closed.
-     Acquire MO so that if we observe (signals == lowseq) after group
-     switching in __condvar_quiesce_and_switch_g1, we synchronize with that
-     store and will see the prior update of __g1_start done while switching
-     groups too.  */
-  unsigned int signals = atomic_load_acquire (cond->__data.__g_signals + g);
-
-  do
-    {
+
       while (1)
 	{
+	  /* Now wait until a signal is available in our group or it is closed.
+	     Acquire MO so that if we observe (signals == lowseq) after group
+	     switching in __condvar_quiesce_and_switch_g1, we synchronize with that
+	     store and will see the prior update of __g1_start done while switching
+	     groups too.  */
+	  unsigned int signals = atomic_load_acquire (cond->__data.__g_signals + g);
 	  uint64_t g1_start = __condvar_load_g1_start_relaxed (cond);
 	  unsigned int lowseq = (g1_start & 1) == g ? signals : g1_start & ~1U;
 
@@ -401,7 +399,7 @@  __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
 	      /* If the group is closed already,
 	         then this waiter originally had enough extra signals to
 	         consume, up until the time its group was closed.  */
-	       goto done;
+	       break;
 	    }
 
 	  /* If there is an available signal, don't block.
@@ -410,7 +408,16 @@  __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
 	     G2, but in either case we're allowed to consume the available
 	     signal and should not block anymore.  */
 	  if ((int)(signals - lowseq) >= 2)
-	    break;
+	    {
+	      /* Try to grab a signal.  See above for MO.  (if we do another loop
+		 iteration we need to see the correct value of g1_start)  */
+		      if (atomic_compare_exchange_weak_acquire (
+		      		cond->__data.__g_signals + g,
+			&signals, signals - 2))
+		      	break;
+		      else
+		      	continue;
+	    }
 
 	  // Now block.
 	  struct _pthread_cleanup_buffer buffer;
@@ -431,19 +438,9 @@  __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
 	      /* If we timed out, we effectively cancel waiting.  */
 	      __condvar_cancel_waiting (cond, seq, g, private);
 	      result = err;
-	      goto done;
+	      break;
 	    }
-
-	  /* Reload signals.  See above for MO.  */
-	  signals = atomic_load_acquire (cond->__data.__g_signals + g);
 	}
-    }
-  /* Try to grab a signal.  See above for MO.  (if we do another loop
-     iteration we need to see the correct value of g1_start)  */
-  while (!atomic_compare_exchange_weak_acquire (cond->__data.__g_signals + g,
-						&signals, signals - 2));
-
- done:
 
   /* Confirm that we have been woken.  We do that before acquiring the mutex
      to allow for execution of pthread_cond_destroy while having acquired the