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Do not violate mutex destruction requirements.

Browse Source 
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).
This commit is contained in:
Torvald Riegel 2015-07-14 21:58:34 +02:00
parent 7962541a32
commit 389fdf78b2
5 changed files with 44 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
ChangeLog
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@ -1,3 +1,15 @@
2015-12-23 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.
* sysdeps/unix/sysv/linux/lowlevellock-futex.h (__lll_private_flag):
Prevent warnings in callers.
2015-12-23 Florian Weimer <fweimer@redhat.com>
* malloc/arena.c (list_lock): Update comment.

12
nptl/pthread_mutex_unlock.c
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@ -230,16 +230,18 @@ __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));

18
sysdeps/nptl/lowlevellock.h
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@ -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)

9
sysdeps/unix/sysv/linux/lowlevellock-futex.h
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@ -54,8 +54,13 @@
#if IS_IN (libc) || IS_IN (rtld)
/* In libc.so or ld.so all futexes are private. */
# ifdef __ASSUME_PRIVATE_FUTEX
# define __lll_private_flag(fl, private) \
((fl) | FUTEX_PRIVATE_FLAG)
# define __lll_private_flag(fl, private) \
({ \
/* Prevent warnings in callers of this macro. */ \
int __lll_private_flag_priv __attribute__ ((unused)); \
__lll_private_flag_priv = (private); \
((fl) | FUTEX_PRIVATE_FLAG); \
})
# else
# define __lll_private_flag(fl, private) \
((fl) | THREAD_GETMEM (THREAD_SELF, header.private_futex))

6
sysdeps/unix/sysv/linux/sparc/lowlevellock.h
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@ -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) \