diff options
author | Peter Zijlstra <peterz@infradead.org> | 2017-03-22 13:35:59 +0300 |
---|---|---|
committer | Thomas Gleixner <tglx@linutronix.de> | 2017-03-23 21:10:10 +0300 |
commit | bebe5b514345f09be2c15e414d076b02ecb9cce8 (patch) | |
tree | 6cc89f5210cc01abf5b0195bfd577e46d08bb8fd /kernel/futex.c | |
parent | cfafcd117da0216520568c195cb2f6cd1980c4bb (diff) | |
download | linux-bebe5b514345f09be2c15e414d076b02ecb9cce8.tar.xz |
futex: Futex_unlock_pi() determinism
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>
Diffstat (limited to 'kernel/futex.c')
-rw-r--r-- | kernel/futex.c | 24 |
1 files changed, 11 insertions, 13 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index eecce7bab86d..4cdc603b00c3 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1398,15 +1398,10 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_ DEFINE_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 @@ -2794,15 +2789,18 @@ retry: 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); |