Commit 73d786bd authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
Browse files

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: default avatarPeter 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent bf92cf3a

kernel/futex.c

+14 −36
Original line number Diff line number Diff line
@@ -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) {
@@ -2345,44 +2351,16 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked) 
		goto out;

	}



	/*

	 * 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;

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