From: Peter Zijlstra peterz@infradead.org
commit bebe5b514345f09be2c15e414d076b02ecb9cce8 upstream.
The problem with returning -EAGAIN when the waiter state mismatches is that it becomes very hard to proof a bounded execution time on the operation. And seeing that this is a RT operation, this is somewhat important.
While in practise; given the previous patch; it will be very unlikely to ever really take more than one or two rounds, proving so becomes rather hard.
However, now that modifying wait_list is done while holding both hb->lock and wait_lock, the scenario can be avoided entirely by acquiring wait_lock while still holding hb-lock. Doing a hand-over, without leaving a hole.
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/20170322104152.112378812@infradead.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Ben Hutchings ben@decadent.org.uk --- kernel/futex.c | 24 +++++++++++------------- 1 file changed, 11 insertions(+), 13 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c index c8ec4e7f3609..77cec33ea112 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1555,15 +1555,10 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_ WAKE_Q(wake_q); int ret = 0;
- raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); - if (!new_owner) { + if (WARN_ON_ONCE(!new_owner)) { /* - * Since we held neither hb->lock nor wait_lock when coming - * into this function, we could have raced with futex_lock_pi() - * such that 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). + * As per the comment in futex_unlock_pi() this should not happen. * * When this happens, give up our locks and try again, giving * the futex_lock_pi() instance time to complete, either by @@ -3018,15 +3013,18 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags) if (pi_state->owner != current) goto out_unlock;
+ get_pi_state(pi_state); /* - * Grab a reference on the pi_state and drop hb->lock. + * Since modifying the wait_list is done while holding both + * hb->lock and wait_lock, holding either is sufficient to + * observe it. * - * The reference ensures pi_state lives, dropping the hb->lock - * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to - * close the races against futex_lock_pi(), but in case of - * _any_ fail we'll abort and retry the whole deal. + * By taking wait_lock while still holding hb->lock, we ensure + * there is no point where we hold neither; and therefore + * wake_futex_pi() must observe a state consistent with what we + * observed. */ - get_pi_state(pi_state); + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); spin_unlock(&hb->lock);
ret = wake_futex_pi(uaddr, uval, pi_state);