This is a note to let you know that I've just added the patch titled
rtmutex: Plug slow unlock race
to the 3.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
rtmutex-plug-slow-unlock-race.patch
and it can be found in the queue-3.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 27e35715df54cbc4f2d044f681802ae30479e7fb Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date: Wed, 11 Jun 2014 18:44:04 +0000
Subject: rtmutex: Plug slow unlock race
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
commit 27e35715df54cbc4f2d044f681802ae30479e7fb upstream.
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: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20140611183852.937945560@xxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/rtmutex.c | 115 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 109 insertions(+), 6 deletions(-)
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -81,6 +81,47 @@ static inline void mark_rt_mutex_waiters
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)
@@ -88,6 +129,17 @@ static inline void mark_rt_mutex_waiters
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
/*
@@ -519,7 +571,8 @@ static int task_blocks_on_rt_mutex(struc
/*
* 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.
*/
@@ -540,10 +593,23 @@ static void wakeup_next_waiter(struct rt
*/
plist_del(&waiter->pi_list_entry, ¤t->pi_waiters);
- 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(¤t->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);
}
@@ -796,12 +862,49 @@ rt_mutex_slowunlock(struct rt_mutex *loc
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);
Patches currently in stable-queue which might be from tglx@xxxxxxxxxxxxx are
queue-3.4/rtmutex-fix-deadlock-detector-for-real.patch
queue-3.4/rtmutex-handle-deadlock-detection-smarter.patch
queue-3.4/rtmutex-plug-slow-unlock-race.patch
queue-3.4/rtmutex-detect-changes-in-the-pi-lock-chain.patch
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