55 lines
1.9 KiB
Diff
55 lines
1.9 KiB
Diff
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From 0db393d3a77bb9f300a356c6a5484fc2dddb161d Mon Sep 17 00:00:00 2001
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From: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
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Date: Tue, 18 Sep 2018 10:03:27 +0300
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Subject: fix race condition in file locking
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The condition occurs when
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- thread #1 is holding the lock
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- thread #2 is waiting for it on __futexwait
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- thread #1 is about to release the lock and performs a_swap
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- thread #3 enters the __lockfile function and manages to grab the lock
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before thread #1 calls __wake, resetting the MAYBE_WAITERS flag
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- thread #1 calls __wake
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- thread #2 wakes up but goes again to __futexwait as the lock is
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held by thread #3
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- thread #3 releases the lock but does not call __wake as the
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MAYBE_WAITERS flag is not set
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This condition results in thread #2 not being woken up. This patch fixes
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the problem by making the woken up thread ensure that the flag is
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properly set before going to sleep again.
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Mainainer's note: This fixes a regression introduced in commit
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c21f750727515602a9e84f2a190ee8a0a2aeb2a1.
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---
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src/stdio/__lockfile.c | 12 ++++++------
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1 file changed, 6 insertions(+), 6 deletions(-)
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diff --git a/src/stdio/__lockfile.c b/src/stdio/__lockfile.c
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index 2ff75d8a..0dcb2a42 100644
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--- a/src/stdio/__lockfile.c
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+++ b/src/stdio/__lockfile.c
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@@ -8,13 +8,13 @@ int __lockfile(FILE *f)
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int owner = f->lock, tid = __pthread_self()->tid;
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if ((owner & ~MAYBE_WAITERS) == tid)
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return 0;
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- for (;;) {
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- owner = a_cas(&f->lock, 0, tid);
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- if (!owner) return 1;
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- if (a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner) break;
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+ owner = a_cas(&f->lock, 0, tid);
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+ if (!owner) return 1;
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+ while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS))) {
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+ if ((owner & MAYBE_WAITERS) ||
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+ a_cas(&f->lock, owner, owner|MAYBE_WAITERS)==owner)
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+ __futexwait(&f->lock, owner|MAYBE_WAITERS, 1);
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}
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- while ((owner = a_cas(&f->lock, 0, tid|MAYBE_WAITERS)))
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- __futexwait(&f->lock, owner, 1);
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return 1;
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}
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--
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cgit v1.2.1
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