Messages in this thread | | | Date | Wed, 21 Nov 2012 14:58:49 +0100 | From | Oleg Nesterov <> | Subject | Re: [REPOST-v2] sched: Prevent wakeup to enter critical section needlessly |
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On 11/21, Ivo Sieben wrote: > Hi > > 2012/11/19 Oleg Nesterov <oleg@redhat.com>: > > > > Because on a second thought I suspect this change is wrong. > > > > Just for example, please look at kauditd_thread(). It does > > > > set_current_state(TASK_INTERRUPTIBLE); > > > > add_wait_queue(&kauditd_wait, &wait); > > > > if (!CONDITION) // <-- LOAD > > schedule(); > > > > And the last LOAD can leak into the critical section protected by > > wait_queue_head_t->lock, and it can be reordered with list_add() > > inside this critical section. In this case we can race with wake_up() > > unless it takes the same lock. > > > > Oleg. > > > > I agree that I should solve my problem using the waitqueue_active() > function locally. I'll abandon this patch and fix it in the > tty_ldisc.c. > > But we try to understand your fault scenario: How can the LOAD leak > into the critical section? As far as we understand the spin_unlock() > function also contains a memory barrier ^^^^^^^^^^^^^^
Not really, in general unlock is a one-way barrier.
> to prevent such a reordering > from happening.
Please look at the comment above prepare_to_wait(), for example. Or look at wmb() in try_to_wake_up().
I guess this is not possible on x86, but in general
X; LOCK(); UNLOCK(); Y;
can be reordered as
LOCK(); Y; X; UNLOCK();
UNLOCK + LOCK is the full memory barrier.
Oleg.
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