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[PATCH][BZ 13690] Do not violate mutex destruction requirements.
- From: Torvald Riegel <triegel at redhat dot com>
- To: GLIBC Devel <libc-alpha at sourceware dot org>
- Cc: "Carlos O'Donell" <carlos at redhat dot com>, David Miller <davem at davemloft dot net>
- Date: Tue, 14 Jul 2015 22:21:13 +0200
- Subject: [PATCH][BZ 13690] Do not violate mutex destruction requirements.
- Authentication-results: sourceware.org; auth=none
POSIX and C++11 require that a thread can destroy a mutex if no thread
owns the mutex, is blocked on the mutex, or will try to acquire it in
the future. After destroying the mutex, it can reuse or unmap the
underlying memory. Thus, we must not access a mutex' memory after
releasing it. Currently, we can load the private flag after releasing
the mutex, which is fixed by this patch.
See https://sourceware.org/bugzilla/show_bug.cgi?id=13690 for more
background.
We need to call futex_wake on the lock after releasing it, however.
This is by design, and can lead to spurious wake-ups on unrelated futex
words (e.g., when the mutex memory is reused for another mutex). This
behavior is documented in the glibc-internal futex API and in recent
drafts of the Linux kernel's futex documentation (see the draft_futex
branch of git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git).
Carlos, do you want to consider this for 2.22, or should this rather
target 2.23? The actual change is simple.
Not tested. Could someone test this on a non-x86-linux machine (I don't
have one handy). Dave, could you test on sparc, please?
2015-07-14 Torvald Riegel <triegel@redhat.com>
[BZ #13690]
* sysdeps/nptl/lowlevellock.h (__lll_unlock): Do not access the lock
after releasing it.
(__lll_robust_unlock): Likewise.
* nptl/pthread_mutex_unlock.c (__pthread_mutex_unlock_full): Likewise.
* sysdeps/unix/sysv/linux/sparc/lowlevellock.h (lll_unlock): Likewise.
(lll_robust_unlock): Likewise.
commit abff5f4160ad21dcc71841c5e92dac8db0b81cff
Author: Torvald Riegel <triegel@redhat.com>
Date: Tue Jul 14 21:58:34 2015 +0200
Do not violate mutex destruction requirements.
POSIX and C++11 require that a thread can destroy a mutex if no other
thread owns the mutex, is blocked on the mutex, or will try to acquire
it in the future. After destroying the mutex, it can reuse or unmap the
underlying memory. Thus, we must not access a mutex' memory after
releasing it. Currently, we can load the private flag after releasing
the mutex, which is fixed by this patch.
See https://sourceware.org/bugzilla/show_bug.cgi?id=13690 for more
background.
We need to call futex_wake on the lock after releasing it, however. This
is by design, and can lead to spurious wake-ups on unrelated futex words
(e.g., when the mutex memory is reused for another mutex). This behavior
is documented in the glibc-internal futex API and in recent drafts of the
Linux kernel's futex documentation (see the draft_futex branch of
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git).
diff --git a/nptl/pthread_mutex_unlock.c b/nptl/pthread_mutex_unlock.c
index 80939ba..42e4ec9 100644
--- a/nptl/pthread_mutex_unlock.c
+++ b/nptl/pthread_mutex_unlock.c
@@ -232,16 +232,18 @@ __pthread_mutex_unlock_full (pthread_mutex_t *mutex, int decr)
/* One less user. */
--mutex->__data.__nusers;
- /* Unlock. */
+ /* Unlock. Load all necessary mutex data before releasing the mutex
+ to not violate the mutex destruction requirements (see
+ lll_unlock). */
+ int robust = mutex->__data.__kind & PTHREAD_MUTEX_ROBUST_NORMAL_NP;
+ int private = (robust
+ ? PTHREAD_ROBUST_MUTEX_PSHARED (mutex)
+ : PTHREAD_MUTEX_PSHARED (mutex));
if ((mutex->__data.__lock & FUTEX_WAITERS) != 0
|| atomic_compare_and_exchange_bool_rel (&mutex->__data.__lock, 0,
THREAD_GETMEM (THREAD_SELF,
tid)))
{
- int robust = mutex->__data.__kind & PTHREAD_MUTEX_ROBUST_NORMAL_NP;
- int private = (robust
- ? PTHREAD_ROBUST_MUTEX_PSHARED (mutex)
- : PTHREAD_MUTEX_PSHARED (mutex));
INTERNAL_SYSCALL_DECL (__err);
INTERNAL_SYSCALL (futex, __err, 2, &mutex->__data.__lock,
__lll_private_flag (FUTEX_UNLOCK_PI, private));
diff --git a/sysdeps/nptl/lowlevellock.h b/sysdeps/nptl/lowlevellock.h
index 27f4142..d7eb282 100644
--- a/sysdeps/nptl/lowlevellock.h
+++ b/sysdeps/nptl/lowlevellock.h
@@ -191,14 +191,21 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
that's cast to void. */
/* Unconditionally set FUTEX to 0 (not acquired), releasing the lock. If FUTEX
was >1 (acquired, possibly with waiters), then wake any waiters. The waiter
- that acquires the lock will set FUTEX to >1. */
+ that acquires the lock will set FUTEX to >1.
+ Evaluate PRIVATE before releasing the lock so that we do not violate the
+ mutex destruction requirements. Specifically, we need to ensure that
+ another thread can destroy the mutex (and reuse its memory) once it
+ acquires the lock and when there will be no further lock acquisitions;
+ thus, we must not access the lock after releasing it, or those accesses
+ could be concurrent with mutex destruction or reuse of the memory. */
#define __lll_unlock(futex, private) \
((void) \
({ \
int *__futex = (futex); \
+ int *__private = (private); \
int __oldval = atomic_exchange_rel (__futex, 0); \
if (__glibc_unlikely (__oldval > 1)) \
- lll_futex_wake (__futex, 1, private); \
+ lll_futex_wake (__futex, 1, __private); \
}))
#define lll_unlock(futex, private) \
__lll_unlock (&(futex), private)
@@ -209,14 +216,17 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
that's cast to void. */
/* Unconditionally set FUTEX to 0 (not acquired), releasing the lock. If FUTEX
had FUTEX_WAITERS set then wake any waiters. The waiter that acquires the
- lock will set FUTEX_WAITERS. */
+ lock will set FUTEX_WAITERS.
+ Evaluate PRIVATE before releasing the lock so that we do not violate the
+ mutex destruction requirements (see __lll_unlock). */
#define __lll_robust_unlock(futex, private) \
((void) \
({ \
int *__futex = (futex); \
+ int *__private = (private); \
int __oldval = atomic_exchange_rel (__futex, 0); \
if (__glibc_unlikely (__oldval & FUTEX_WAITERS)) \
- lll_futex_wake (__futex, 1, private); \
+ lll_futex_wake (__futex, 1, __private); \
}))
#define lll_robust_unlock(futex, private) \
__lll_robust_unlock (&(futex), private)
diff --git a/sysdeps/unix/sysv/linux/sparc/lowlevellock.h b/sysdeps/unix/sysv/linux/sparc/lowlevellock.h
index 7608c01..9fa7337 100644
--- a/sysdeps/unix/sysv/linux/sparc/lowlevellock.h
+++ b/sysdeps/unix/sysv/linux/sparc/lowlevellock.h
@@ -126,17 +126,19 @@ __lll_robust_timedlock (int *futex, const struct timespec *abstime,
#define lll_unlock(lock, private) \
((void) ({ \
int *__futex = &(lock); \
+ int __private = (private); \
int __val = atomic_exchange_24_rel (__futex, 0); \
if (__glibc_unlikely (__val > 1)) \
- lll_futex_wake (__futex, 1, private); \
+ lll_futex_wake (__futex, 1, __private); \
}))
#define lll_robust_unlock(lock, private) \
((void) ({ \
int *__futex = &(lock); \
+ int __private = (private); \
int __val = atomic_exchange_rel (__futex, 0); \
if (__glibc_unlikely (__val & FUTEX_WAITERS)) \
- lll_futex_wake (__futex, 1, private); \
+ lll_futex_wake (__futex, 1, __private); \
}))
#define lll_islocked(futex) \