Commit 27e35715 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

rtmutex: Plug slow unlock race



When the rtmutex fast path is enabled the slow unlock function can
create the following situation:

spin_lock(foo->m->wait_lock);
foo->m->owner = NULL;
	    			rt_mutex_lock(foo->m); <-- fast path
				free = atomic_dec_and_test(foo->refcnt);
				rt_mutex_unlock(foo->m); <-- fast path
				if (free)
				   kfree(foo);

spin_unlock(foo->m->wait_lock); <--- Use after free.

Plug the race by changing the slow unlock to the following scheme:

     while (!rt_mutex_has_waiters(m)) {
     	    /* Clear the waiters bit in m->owner */
	    clear_rt_mutex_waiters(m);
      	    owner = rt_mutex_owner(m);
      	    spin_unlock(m->wait_lock);
      	    if (cmpxchg(m->owner, owner, 0) == owner)
      	       return;
      	    spin_lock(m->wait_lock);
     }

So in case of a new waiter incoming while the owner tries the slow
path unlock we have two situations:

 unlock(wait_lock);
					lock(wait_lock);
 cmpxchg(p, owner, 0) == owner
 	    	   			mark_rt_mutex_waiters(lock);
	 				acquire(lock);

Or:

 unlock(wait_lock);
					lock(wait_lock);
	 				mark_rt_mutex_waiters(lock);
 cmpxchg(p, owner, 0) != owner
					enqueue_waiter();
					unlock(wait_lock);
 lock(wait_lock);
 wakeup_next waiter();
 unlock(wait_lock);
					lock(wait_lock);
					acquire(lock);

If the fast path is disabled, then the simple

   m->owner = NULL;
   unlock(m->wait_lock);

is sufficient as all access to m->owner is serialized via
m->wait_lock;

Also document and clarify the wakeup_next_waiter function as suggested
by Oleg Nesterov.
Reported-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140611183852.937945560@linutronix.de


Cc: stable@vger.kernel.org
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 82084984
......@@ -83,6 +83,47 @@ static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
owner = *p;
} while (cmpxchg(p, owner, owner | RT_MUTEX_HAS_WAITERS) != owner);
}
/*
* Safe fastpath aware unlock:
* 1) Clear the waiters bit
* 2) Drop lock->wait_lock
* 3) Try to unlock the lock with cmpxchg
*/
static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
__releases(lock->wait_lock)
{
struct task_struct *owner = rt_mutex_owner(lock);
clear_rt_mutex_waiters(lock);
raw_spin_unlock(&lock->wait_lock);
/*
* If a new waiter comes in between the unlock and the cmpxchg
* we have two situations:
*
* unlock(wait_lock);
* lock(wait_lock);
* cmpxchg(p, owner, 0) == owner
* mark_rt_mutex_waiters(lock);
* acquire(lock);
* or:
*
* unlock(wait_lock);
* lock(wait_lock);
* mark_rt_mutex_waiters(lock);
*
* cmpxchg(p, owner, 0) != owner
* enqueue_waiter();
* unlock(wait_lock);
* lock(wait_lock);
* wake waiter();
* unlock(wait_lock);
* lock(wait_lock);
* acquire(lock);
*/
return rt_mutex_cmpxchg(lock, owner, NULL);
}
#else
# define rt_mutex_cmpxchg(l,c,n) (0)
static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
......@@ -90,6 +131,17 @@ static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
lock->owner = (struct task_struct *)
((unsigned long)lock->owner | RT_MUTEX_HAS_WAITERS);
}
/*
* Simple slow path only version: lock->owner is protected by lock->wait_lock.
*/
static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
__releases(lock->wait_lock)
{
lock->owner = NULL;
raw_spin_unlock(&lock->wait_lock);
return true;
}
#endif
static inline int
......@@ -650,7 +702,8 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
/*
* Wake up the next waiter on the lock.
*
* Remove the top waiter from the current tasks waiter list and wake it up.
* Remove the top waiter from the current tasks pi waiter list and
* wake it up.
*
* Called with lock->wait_lock held.
*/
......@@ -671,10 +724,23 @@ static void wakeup_next_waiter(struct rt_mutex *lock)
*/
rt_mutex_dequeue_pi(current, waiter);
rt_mutex_set_owner(lock, NULL);
/*
* As we are waking up the top waiter, and the waiter stays
* queued on the lock until it gets the lock, this lock
* obviously has waiters. Just set the bit here and this has
* the added benefit of forcing all new tasks into the
* slow path making sure no task of lower priority than
* the top waiter can steal this lock.
*/
lock->owner = (void *) RT_MUTEX_HAS_WAITERS;
raw_spin_unlock_irqrestore(&current->pi_lock, flags);
/*
* It's safe to dereference waiter as it cannot go away as
* long as we hold lock->wait_lock. The waiter task needs to
* acquire it in order to dequeue the waiter.
*/
wake_up_process(waiter->task);
}
......@@ -928,12 +994,49 @@ rt_mutex_slowunlock(struct rt_mutex *lock)
rt_mutex_deadlock_account_unlock(current);
if (!rt_mutex_has_waiters(lock)) {
lock->owner = NULL;
raw_spin_unlock(&lock->wait_lock);
return;
/*
* We must be careful here if the fast path is enabled. If we
* have no waiters queued we cannot set owner to NULL here
* because of:
*
* foo->lock->owner = NULL;
* rtmutex_lock(foo->lock); <- fast path
* free = atomic_dec_and_test(foo->refcnt);
* rtmutex_unlock(foo->lock); <- fast path
* if (free)
* kfree(foo);
* raw_spin_unlock(foo->lock->wait_lock);
*
* So for the fastpath enabled kernel:
*
* Nothing can set the waiters bit as long as we hold
* lock->wait_lock. So we do the following sequence:
*
* owner = rt_mutex_owner(lock);
* clear_rt_mutex_waiters(lock);
* raw_spin_unlock(&lock->wait_lock);
* if (cmpxchg(&lock->owner, owner, 0) == owner)
* return;
* goto retry;
*
* The fastpath disabled variant is simple as all access to
* lock->owner is serialized by lock->wait_lock:
*
* lock->owner = NULL;
* raw_spin_unlock(&lock->wait_lock);
*/
while (!rt_mutex_has_waiters(lock)) {
/* Drops lock->wait_lock ! */
if (unlock_rt_mutex_safe(lock) == true)
return;
/* Relock the rtmutex and try again */
raw_spin_lock(&lock->wait_lock);
}
/*
* The wakeup next waiter path does not suffer from the above
* race. See the comments there.
*/
wakeup_next_waiter(lock);
raw_spin_unlock(&lock->wait_lock);
......
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