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[2/5] nptl: Remove the signal-stealing code. It is no longer needed.

Message ID 20210116204950.16434-2-malteskarupke@web.de
State New
Delegated to: Carlos O'Donell
Headers show
Series [1/5] nptl: Fix pthread_cond_signal missing a sleeper (#BZ 25847) | expand

Commit Message

Malte Skarupke Jan. 16, 2021, 8:49 p.m. UTC
After my last change, stealing of signals can no longer happen. This
patch removes the code that handled the case when a signal was stolen.
---
 nptl/pthread_cond_wait.c | 63 ----------------------------------------
 1 file changed, 63 deletions(-)

--
2.17.1
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Patch

diff --git a/nptl/pthread_cond_wait.c b/nptl/pthread_cond_wait.c
index 0f50048c0b..7b825f81df 100644
--- a/nptl/pthread_cond_wait.c
+++ b/nptl/pthread_cond_wait.c
@@ -525,69 +525,6 @@  __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
   while (!atomic_compare_exchange_weak_acquire (cond->__data.__g_signals + g,
 						&signals, signals - 2));

-  /* We consumed a signal but we could have consumed from a more recent group
-     that aliased with ours due to being in the same group slot.  If this
-     might be the case our group must be closed as visible through
-     __g1_start.  */
-  uint64_t g1_start = __condvar_load_g1_start_relaxed (cond);
-  if (seq < (g1_start >> 1))
-    {
-      /* We potentially stole a signal from a more recent group but we do not
-	 know which group we really consumed from.
-	 We do not care about groups older than current G1 because they are
-	 closed; we could have stolen from these, but then we just add a
-	 spurious wake-up for the current groups.
-	 We will never steal a signal from current G2 that was really intended
-	 for G2 because G2 never receives signals (until it becomes G1).  We
-	 could have stolen a signal from G2 that was conservatively added by a
-	 previous waiter that also thought it stole a signal -- but given that
-	 that signal was added unnecessarily, it's not a problem if we steal
-	 it.
-	 Thus, the remaining case is that we could have stolen from the current
-	 G1, where "current" means the __g1_start value we observed.  However,
-	 if the current G1 does not have the same slot index as we do, we did
-	 not steal from it and do not need to undo that.  This is the reason
-	 for putting a bit with G2's index into__g1_start as well.  */
-      if (((g1_start & 1) ^ 1) == g)
-	{
-	  /* We have to conservatively undo our potential mistake of stealing
-	     a signal.  We can stop trying to do that when the current G1
-	     changes because other spinning waiters will notice this too and
-	     __condvar_quiesce_and_switch_g1 has checked that there are no
-	     futex waiters anymore before switching G1.
-	     Relaxed MO is fine for the __g1_start load because we need to
-	     merely be able to observe this fact and not have to observe
-	     something else as well.
-	     ??? Would it help to spin for a little while to see whether the
-	     current G1 gets closed?  This might be worthwhile if the group is
-	     small or close to being closed.  */
-	  unsigned int s = atomic_load_relaxed (cond->__data.__g_signals + g);
-	  while (__condvar_load_g1_start_relaxed (cond) == g1_start)
-	    {
-	      /* Try to add a signal.  We don't need to acquire the lock
-		 because at worst we can cause a spurious wake-up.  If the
-		 group is in the process of being closed (LSB is true), this
-		 has an effect similar to us adding a signal.  */
-	      if (((s & 1) != 0)
-		  || atomic_compare_exchange_weak_relaxed
-		       (cond->__data.__g_signals + g, &s, s + 2))
-		{
-		  /* If we added a signal, we also need to add a wake-up on
-		     the futex.  We also need to do that if we skipped adding
-		     a signal because the group is being closed because
-		     while __condvar_quiesce_and_switch_g1 could have closed
-		     the group, it might stil be waiting for futex waiters to
-		     leave (and one of those waiters might be the one we stole
-		     the signal from, which cause it to block using the
-		     futex).  */
-		  futex_wake (cond->__data.__g_signals + g, 1, private);
-		  break;
-		}
-	      /* TODO Back off.  */
-	    }
-	}
-    }
-
  done:

   /* Confirm that we have been woken.  We do that before acquiring the mutex