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pthread_cond_broadcast: Fix waiters-after-spinning case [BZ #23538]

Message ID c688066e-1dce-5f19-9786-233eb6ad33e2@redhat.com
State New
Headers show
Series pthread_cond_broadcast: Fix waiters-after-spinning case [BZ #23538] | expand

Commit Message

Florian Weimer Aug. 24, 2018, 9:59 a.m. UTC 
I intend to commit this on behalf of Martin and Torvald.

Per FSF guidelines, we do not need copyright assignment for this change 
because it is small enough.

Martin, Torvald, would you please double-check the summary in the 
subject and the ChangeLog entry.

Thanks,
Florian

Comments

Kuchta, Martin Aug. 24, 2018, 9:39 p.m. UTC | #1 
Florian Weimer <fweimer@redhat.com> wrote:
> Martin, Torvald, would you please double-check the summary in the 
> subject and the ChangeLog entry.


Thanks for picking this up. Both the summary and ChangeLog entry look
good to me.
Torvald Riegel Aug. 28, 2018, 1:22 p.m. UTC | #2 
On Fri, 2018-08-24 at 11:59 +0200, Florian Weimer wrote:
> I intend to commit this on behalf of Martin and Torvald.
> 
> Per FSF guidelines, we do not need copyright assignment for this change 
> because it is small enough.
> 
> Martin, Torvald, would you please double-check the summary in the 
> subject and the ChangeLog entry.

LGTM.  Thanks for applying the patch.
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Patch

From: Martin Kuchta <martin.kuchta@netapp.com>
Subject: [PATCH] pthread_cond_broadcast: Fix waiters-after-spinning case [BZ #23538]
To: libc-alpha@sourceware.org

2018-08-24 Martin Kuchta  <martin.kuchta@netapp.com>
	   Torvald Riegel  <triegel@redhat.com>

	[BZ #23538]
	* nptl/pthread_cond_common.c (__condvar_quiesce_and_switch_g1):
	Update r to include the set wake-request flag if waiters are
	remaining after spinning.

diff --git a/nptl/pthread_cond_common.c b/nptl/pthread_cond_common.c
index 8e425eb01e..479e54febb 100644
--- a/nptl/pthread_cond_common.c
+++ b/nptl/pthread_cond_common.c
@@ -405,8 +405,12 @@  __condvar_quiesce_and_switch_g1 (pthread_cond_t *cond, uint64_t wseq,
 	{
 	  /* There is still a waiter after spinning.  Set the wake-request
 	     flag and block.  Relaxed MO is fine because this is just about
-	     this futex word.  */
-	  r = atomic_fetch_or_relaxed (cond->__data.__g_refs + g1, 1);
+	     this futex word.
+
+	     Update r to include the set wake-request flag so that the upcoming
+	     futex_wait only blocks if the flag is still set (otherwise, we'd
+	     violate the basic client-side futex protocol).  */
+	  r = atomic_fetch_or_relaxed (cond->__data.__g_refs + g1, 1) | 1;
 
 	  if ((r >> 1) > 0)
 	    futex_wait_simple (cond->__data.__g_refs + g1, r, private);