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musl: add upstream patch to fix race condition in file locking

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According to [1]:

  musl 1.1.20 introduced a regression in stdio FILE locking that can
  cause soft deadlocks with >2 threads contending for a
  FILE. Users/dists should apply
  https://git.musl-libc.org/cgit/musl/commit/?id=0db393d3a77bb9f300a356c6a5484fc2dddb161d.

[1] https://twitter.com/musllibc/status/1042292786568024070

This commit therefore adds the corresponding patch to the musl
package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
This commit is contained in:
Thomas Petazzoni 2018-09-19 08:44:24 +02:00 committed by Peter Korsgaard
parent adc98bfa26
commit 1f90976c88
1 changed files with 55 additions and 0 deletions
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55
package/musl/0002-fix-race-condition-in-file-locking.patch Normal file
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@ -0,0 +1,55 @@
From 0db393d3a77bb9f300a356c6a5484fc2dddb161d Mon Sep 17 00:00:00 2001
From: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
Date: Tue, 18 Sep 2018 10:03:27 +0300
Subject: [PATCH] fix race condition in file locking
The condition occurs when
- thread #1 is holding the lock
- thread #2 is waiting for it on __futexwait
- thread #1 is about to release the lock and performs a_swap
- thread #3 enters the __lockfile function and manages to grab the lock
before thread #1 calls __wake, resetting the MAYBE_WAITERS flag
- thread #1 calls __wake
- thread #2 wakes up but goes again to __futexwait as the lock is
held by thread #3
- thread #3 releases the lock but does not call __wake as the
MAYBE_WAITERS flag is not set
This condition results in thread #2 not being woken up. This patch fixes
the problem by making the woken up thread ensure that the flag is
properly set before going to sleep again.
Mainainer's note: This fixes a regression introduced in commit
c21f750727515602a9e84f2a190ee8a0a2aeb2a1.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
---
src/stdio/__lockfile.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/src/stdio/__lockfile.c b/src/stdio/__lockfile.c
index 2ff75d8a..0dcb2a42 100644
--- a/src/stdio/__lockfile.c
+++ b/src/stdio/__lockfile.c
@@ -8,13 +8,13 @@ int __lockfile(FILE *f)
int owner = f->lock, tid = __pthread_self()->tid;
if ((owner & ~MAYBE_WAITERS) == tid)
return 0;
- for (;;) {
- owner = a_cas(&f->lock, 0, tid);
- if (!owner) return 1;
- if (a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner) break;
+ owner = a_cas(&f->lock, 0, tid);
+ if (!owner) return 1;
+ while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS))) {
+ if ((owner & MAYBE_WAITERS) ||
+ a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner)
+ __futexwait(&f->lock, owner|MAYBE_WAITERS, 1);
}
- while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS)))
- __futexwait(&f->lock, owner, 1);
return 1;
}
--
2.14.4