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authorPeter Zijlstra <peterz@infradead.org>2017-03-22 11:35:54 +0100
committerThomas Gleixner <tglx@linutronix.de>2017-03-23 19:10:08 +0100
commit73d786bd043ebc855f349c81ea805f6b11cbf2aa (patch)
treec3bf955843c550cafc47c102d590f647794039fc /kernel/futex.c
parentbf92cf3a5100f5a0d5f9834787b130159397cb22 (diff)
downloadlinux-73d786bd043ebc855f349c81ea805f6b11cbf2aa.tar.bz2
futex: Rework inconsistent rt_mutex/futex_q state
There is a weird state in the futex_unlock_pi() path when it interleaves with a concurrent futex_lock_pi() at the point where it drops hb->lock. In this case, it can happen that the rt_mutex wait_list and the futex_q disagree on pending waiters, in particular rt_mutex will find no pending waiters where futex_q thinks there are. In this case the rt_mutex unlock code cannot assign an owner. The futex side fixup code has to cleanup the inconsistencies with quite a bunch of interesting corner cases. Simplify all this by changing wake_futex_pi() to return -EAGAIN when this situation occurs. This then gives the futex_lock_pi() code the opportunity to continue and the retried futex_unlock_pi() will now observe a coherent state. The only problem is that this breaks RT timeliness guarantees. That is, consider the following scenario: T1 and T2 are both pinned to CPU0. prio(T2) > prio(T1) CPU0 T1 lock_pi() queue_me() <- Waiter is visible preemption T2 unlock_pi() loops with -EAGAIN forever Which is undesirable for PI primitives. Future patches will rectify this. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: juri.lelli@arm.com Cc: bigeasy@linutronix.de Cc: xlpang@redhat.com Cc: rostedt@goodmis.org Cc: mathieu.desnoyers@efficios.com Cc: jdesfossez@efficios.com Cc: dvhart@infradead.org Cc: bristot@redhat.com Link: http://lkml.kernel.org/r/20170322104151.850383690@infradead.org Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'kernel/futex.c')
-rw-r--r--kernel/futex.c50
1 files changed, 14 insertions, 36 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 3b6dbeecd91b..51a248af1db9 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1404,12 +1404,19 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter
new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
/*
- * It is possible that the next waiter (the one that brought
- * top_waiter owner to the kernel) timed out and is no longer
- * waiting on the lock.
+ * When we interleave with futex_lock_pi() where it does
+ * rt_mutex_timed_futex_lock(), we might observe @this futex_q waiter,
+ * but the rt_mutex's wait_list can be empty (either still, or again,
+ * depending on which side we land).
+ *
+ * When this happens, give up our locks and try again, giving the
+ * futex_lock_pi() instance time to complete, either by waiting on the
+ * rtmutex or removing itself from the futex queue.
*/
- if (!new_owner)
- new_owner = top_waiter->task;
+ if (!new_owner) {
+ raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+ return -EAGAIN;
+ }
/*
* We pass it to the next owner. The WAITERS bit is always
@@ -2332,7 +2339,6 @@ static long futex_wait_restart(struct restart_block *restart);
*/
static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
{
- struct task_struct *owner;
int ret = 0;
if (locked) {
@@ -2346,43 +2352,15 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
}
/*
- * Catch the rare case, where the lock was released when we were on the
- * way back before we locked the hash bucket.
- */
- if (q->pi_state->owner == current) {
- /*
- * Try to get the rt_mutex now. This might fail as some other
- * task acquired the rt_mutex after we removed ourself from the
- * rt_mutex waiters list.
- */
- if (rt_mutex_futex_trylock(&q->pi_state->pi_mutex)) {
- locked = 1;
- goto out;
- }
-
- /*
- * pi_state is incorrect, some other task did a lock steal and
- * we returned due to timeout or signal without taking the
- * rt_mutex. Too late.
- */
- raw_spin_lock_irq(&q->pi_state->pi_mutex.wait_lock);
- owner = rt_mutex_owner(&q->pi_state->pi_mutex);
- if (!owner)
- owner = rt_mutex_next_owner(&q->pi_state->pi_mutex);
- raw_spin_unlock_irq(&q->pi_state->pi_mutex.wait_lock);
- ret = fixup_pi_state_owner(uaddr, q, owner);
- goto out;
- }
-
- /*
* Paranoia check. If we did not take the lock, then we should not be
* the owner of the rt_mutex.
*/
- if (rt_mutex_owner(&q->pi_state->pi_mutex) == current)
+ if (rt_mutex_owner(&q->pi_state->pi_mutex) == current) {
printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
"pi-state %p\n", ret,
q->pi_state->pi_mutex.owner,
q->pi_state->owner);
+ }
out:
return ret ? ret : locked;