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Message-ID: <20081125173032.GA21539@redhat.com>
Date: Tue, 25 Nov 2008 18:30:32 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Tejun Heo <tj@...nel.org>
Cc: Eric Van Hensbergen <ericvh@...il.com>,
Ron Minnich <rminnich@...dia.gov>, Ingo Molnar <mingo@...e.hu>,
Christoph Hellwig <hch@...radead.org>,
Miklos Szeredi <mszeredi@...e.cz>,
Davide Libenzi <davidel@...ilserver.org>,
Brad Boyer <flar@...andria.com>,
Al Viro <viro@...iv.linux.org.uk>,
Roland McGrath <roland@...hat.com>,
Mauro Carvalho Chehab <mchehab@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org
Subject: Re: + poll-allow-f_op-poll-to-sleep-take-4.patch added to -mm tree
Hi!
Minor question about mbs, just trying to understand the patch...
> +static int pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
> +{
> + struct poll_wqueues *pwq = wait->private;
> + DECLARE_WAITQUEUE(dummy_wait, pwq->polling_task);
> +
> + set_mb(pwq->triggered, 1);
> +
> + /*
> + * Perform the default wake up operation using a dummy
> + * waitqueue.
> + *
> + * TODO: This is hacky but there currently is no interface to
> + * pass in @sync. @sync is scheduled to be removed and once
> + * that happens, wake_up_process() can be used directly.
> + */
> + return default_wake_function(&dummy_wait, mode, sync, key);
> +}
> +
> +int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
> + ktime_t *expires, unsigned long slack)
> +{
> + int rc = -EINTR;
> +
> + set_current_state(state);
> + if (!pwq->triggered)
> + rc = schedule_hrtimeout_range(expires, slack, HRTIMER_MODE_ABS);
> + __set_current_state(TASK_RUNNING);
So, why do we need this mb() in pollwake() ?
try_to_wake_up() has a full barrier semantics, note the wmb() before
task_rq_lock(). Since spin_lock() itself is STORE, the setting of
pwq->triggered can't be further re-ordered with the reading of p->state.
Or any other reason ?
> + /* clear triggered for the next iteration */
> + pwq->triggered = 0;
And don't we (in theory) actually need the mb() here instead?
Let's suppose do_poll() starts the next iteration, so we are doing
pwq->triggered = 0;
->poll(file)
if (!check_file(file))
return 0;
return POLLXXX;
We don't have any barriers in between (unless fget_light bumps
->f_count), so this can be reordered as
->poll(file)
if (!check_file(file))
return 0;
pwq->triggered = 0;
And, if pollwake() happens in between we can miss the event, no?
Oleg.
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