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Message-ID: <20090118023211.GA14539@redhat.com>
Date: Sun, 18 Jan 2009 03:32:11 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Chris Mason <chris.mason@...cle.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Matthew Wilcox <matthew@....cx>,
Chuck Lever <cel@...i.umich.edu>,
Nick Piggin <nickpiggin@...oo.com.au>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3] wait: prevent waiter starvation in
__wait_on_bit_lock
On 01/18, Johannes Weiner wrote:
>
> On Sat, Jan 17, 2009 at 10:51:10PM +0100, Oleg Nesterov wrote:
> >
> > if ((ret = (*action)(q->key.flags))) {
> > __wake_up_bit(wq, q->key.flags, q->key.bit_nr);
> > // or just __wake_up(wq, TASK_NORMAL, 1, &q->key);
> > break;
> > }
> >
> > IOW, imho __wait_on_bit_lock() is buggy, not __lock_page_killable(),
> > no?
>
> I agree with you, already replied with a patch to linux-mm where Chris
> posted it originally.
>
> Peter noted that we have a spurious wake up in the case where A holds
> the page lock, B and C wait, B gets killed and does a wake up, then A
> unlocks and does a wake up. Your proposal has this problem too,
> right?
Yes sure. But I can't see how it is possible to avoid the false
wakeup for sure, please see below.
> @@ -182,8 +182,20 @@ __wait_on_bit_lock(wait_queue_head_t *wq
> do {
> prepare_to_wait_exclusive(wq, &q->wait, mode);
> if (test_bit(q->key.bit_nr, q->key.flags)) {
> - if ((ret = (*action)(q->key.flags)))
> + ret = action(q->key.flags);
> + if (ret) {
> + /*
> + * Contenders are woken exclusively. If
> + * we do not take the lock when woken up
> + * from an unlock, we have to make sure to
> + * wake the next waiter in line or noone
> + * will and shkle will wait forever.
> + */
> + if (!test_bit(q->key.bit_nr, q->key.flags))
> + __wake_up_bit(wq, q->key.flags,
Afaics, the spurious wake up is still possible if SIGKILL and
unlock_page() happen "at the same time".
__wait_on_bit_lock: unlock_page:
clear_bit_unlock()
test_bit() == T
__wake_up_bit() wake_up_page()
Note that sync_page_killable() returns with ->state == TASK_RUNNING,
__wake_up() will "ignore" us.
But, more importantly, I'm afraid we can also have the false negative,
this "if (!test_bit())" test lacks the barriers. This can't happen with
sync_page_killable() because it always calls schedule(). But let's
suppose we modify it to check signal_pending() first:
static int sync_page_killable(void *word)
{
if (fatal_signal_pending(current))
return -EINTR;
return sync_page(word);
}
It is still correct, but unless I missed something now __wait_on_bit_lock()
has problems again.
But don't get me wrong, I think you are right and it is better to minimize
the possibility of the false wakeup.
Oleg.
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