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[PATCH] pthread_cond_broadcast: Fix waiters-after-spinning case [BZ #23538]
- From: fweimer at redhat dot com (Florian Weimer)
- To: libc-alpha at sourceware dot org
- Date: Mon, 27 Aug 2018 20:04:20 +0200
- Subject: [PATCH] pthread_cond_broadcast: Fix waiters-after-spinning case [BZ #23538]
From: Martin Kuchta <martin.kuchta@netapp.com>
(cherry picked from commit 99ea93ca31795469d2a1f1570f17a5c39c2eb7e2)
2018-08-27 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/NEWS b/NEWS
index 61ac02cd69..a868a652f7 100644
--- a/NEWS
+++ b/NEWS
@@ -146,6 +146,7 @@ The following bugs are resolved with this release:
[23236] Harden function pointers in _IO_str_fields
[23313] libio: Disable vtable validation in case of interposition
[23349] Various glibc headers no longer compatible with <linux/time.h>
+ [23538] pthread_cond_broadcast: Fix waiters-after-spinning case
[23363] stdio-common/tst-printf.c has non-free license
[23456] Wrong index_cpu_LZCNT
[23459] COMMON_CPUID_INDEX_80000001 isn't populated for Intel processors
diff --git a/nptl/pthread_cond_common.c b/nptl/pthread_cond_common.c
index ffbbde4106..c99435e439 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);