[PATCH 4/9] nptl: Remove unnecessary quadruple check in pthread_cond_wait

[malteskarupke@fastmail.fm]

From: Malte Skarupke <malteskarupke@fastmail.fm>

pthread_cond_wait was checking whether it was in a closed group no less than
four times. Checking once is enough. Here are the four checks:

1. While spin-waiting. This was dead code: maxspin is set to 0 and has been
   for years.
2. Before deciding to go to sleep, and before incrementing grefs: I kept this
3. After incrementing grefs. There is no reason to think that the group would
   close while we do an atomic increment. Obviously it could close at any
   point, but that doesn't mean we have to recheck after every step. This
   check was equally good as check 2, except it has to do more work.
4. When we find ourselves in a group that has a signal. We only get here after
   we check that we're not in a closed group. There is no need to check again.
   The check would only have helped in cases where the compare_exchange in the
   next line would also have failed. Relying on the compare_exchange is fine.

Removing the duplicate checks clarifies the code.
---
 nptl/pthread_cond_wait.c | 49 ----------------------------------------
 1 file changed, 49 deletions(-)

diff --git a/nptl/pthread_cond_wait.c b/nptl/pthread_cond_wait.c
index cee1968756..47e834cade 100644
--- a/nptl/pthread_cond_wait.c
+++ b/nptl/pthread_cond_wait.c
@@ -366,7 +366,6 @@ static __always_inline int
 __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
     clockid_t clockid, const struct __timespec64 *abstime)
 {
-  const int maxspin = 0;
   int err;
   int result = 0;
 
@@ -425,33 +424,6 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
 	  uint64_t g1_start = __condvar_load_g1_start_relaxed (cond);
 	  unsigned int lowseq = (g1_start & 1) == g ? signals : g1_start & ~1U;
 
-	  /* Spin-wait first.
-	     Note that spinning first without checking whether a timeout
-	     passed might lead to what looks like a spurious wake-up even
-	     though we should return ETIMEDOUT (e.g., if the caller provides
-	     an absolute timeout that is clearly in the past).  However,
-	     (1) spurious wake-ups are allowed, (2) it seems unlikely that a
-	     user will (ab)use pthread_cond_wait as a check for whether a
-	     point in time is in the past, and (3) spinning first without
-	     having to compare against the current time seems to be the right
-	     choice from a performance perspective for most use cases.  */
-	  unsigned int spin = maxspin;
-	  while (spin > 0 && ((int)(signals - lowseq) < 2))
-	    {
-	      /* Check that we are not spinning on a group that's already
-		 closed.  */
-	      if (seq < (g1_start >> 1))
-		break;
-
-	      /* TODO Back off.  */
-
-	      /* Reload signals.  See above for MO.  */
-	      signals = atomic_load_acquire (cond->__data.__g_signals + g);
-	      g1_start = __condvar_load_g1_start_relaxed (cond);
-	      lowseq = (g1_start & 1) == g ? signals : g1_start & ~1U;
-	      spin--;
-	    }
-
 	  if (seq < (g1_start >> 1))
 	    {
 	      /* If the group is closed already,
@@ -482,24 +454,6 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
 	     an atomic read-modify-write operation and thus extend the release
 	     sequence.  */
 	  atomic_fetch_add_acquire (cond->__data.__g_refs + g, 2);
-	  signals = atomic_load_acquire (cond->__data.__g_signals + g);
-	  g1_start = __condvar_load_g1_start_relaxed (cond);
-	  lowseq = (g1_start & 1) == g ? signals : g1_start & ~1U;
-
-	  if (seq < (g1_start >> 1))
-	    {
-	      /* group is closed already, so don't block */
-	      __condvar_dec_grefs (cond, g, private);
-	      goto done;
-	    }
-
-	  if ((int)(signals - lowseq) >= 2)
-	    {
-	      /* a signal showed up or G1/G2 switched after we grabbed the
-	         refcount */
-	      __condvar_dec_grefs (cond, g, private);
-	      break;
-	    }
 
 	  // Now block.
 	  struct _pthread_cleanup_buffer buffer;
@@ -533,9 +487,6 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
 	  /* Reload signals.  See above for MO.  */
 	  signals = atomic_load_acquire (cond->__data.__g_signals + g);
 	}
-
-       if (seq < (__condvar_load_g1_start_relaxed (cond) >> 1))
-	 goto done;
     }
   /* 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)  */
-- 
2.25.1