diff mbox

[BZ,13690] Do not violate mutex destruction requirements.

Message ID 1436907056.19451.19.camel@localhost.localdomain
State Dropped
Headers show

Commit Message

Torvald Riegel July 14, 2015, 8:50 p.m. UTC
On Tue, 2015-07-14 at 13:39 -0700, Roland McGrath wrote:
> > 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?
> 
> You should always at least build-test your changes.

It does build on x86_64-linux, because those macros aren't used there ;)
I don't have a cross-build compiler ready, so I didn't try to build for
other archs.

> >  #define __lll_unlock(futex, private)                    \
> >    ((void)                                               \
> >     ({                                                   \
> >       int *__futex = (futex);                            \
> > +     int *__private = (private);                        \
> 
> Should be int, not int *.
> 
> >  #define __lll_robust_unlock(futex, private)             \
> >    ((void)                                               \
> >     ({                                                   \
> >       int *__futex = (futex);                            \
> > +     int *__private = (private);                        \
> 
> Same here.

Sorry.  Thanks for spotting these.  It seems I should also not try to
copy and paste late in the evening ;)

Updated patch attached.
diff mbox

Patch

commit f727387d874ab9d76835d1aa99b6caf47d3e26f7
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..7d41ef0 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) \