linux-kernel - Re: [PATCH 3/7] barrier: a scalable synchonisation barrier

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Date:	Thu, 1 Feb 2007 00:30:42 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Christoph Hellwig <hch@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] barrier: a scalable synchonisation barrier

On 02/01, Oleg Nesterov wrote:
> 
> +static inline void barrier_sync(struct barrier *b)
> +{
> +       might_sleep();
> +
> +       if (unlikely(atomic_read(&b->count))) {
> +               DEFINE_WAIT(wait);
> +               prepare_to_wait(&b->wait, &wait, TASK_UNINTERRUPTIBLE);
> +               while (atomic_read(&b->count))
> +                       schedule();
> +               finish_wait(&b->wait, &wait);
> +       }
> +}
> 
> This should be open-coded wait_event(), but wrong! With the scenario above this
> can hang forever! because the first wake_up removes the task from the &b->wait.

I am afraid I was not clear (as usual :). prepare_to_wait means autoremove_wake_function.
So, if barrier_unlock() wakes up the caller of barrier_sync(), it will be removed
from b->wait. If it goes to schedule() because another barrier_lock() incremented
b->count, we can't wake it via __wake_up(&b->wait, ...).

Oleg.

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