linux-kernel - Re: [take23 3/5] kevent: poll/select() notifications.

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Message-ID: <20061108084554.GD2447@2ka.mipt.ru>
Date:	Wed, 8 Nov 2006 11:45:54 +0300
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	David Miller <davem@...emloft.net>,
	Ulrich Drepper <drepper@...hat.com>,
	Andrew Morton <akpm@...l.org>, netdev <netdev@...r.kernel.org>,
	Zach Brown <zach.brown@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Chase Venters <chase.venters@...entec.com>,
	Johann Borck <johann.borck@...sedata.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeff Garzik <jeff@...zik.org>
Subject: Re: [take23 3/5] kevent: poll/select() notifications.

On Tue, Nov 07, 2006 at 02:53:33PM -0800, Davide Libenzi (davidel@...ilserver.org) wrote:
> On Tue, 7 Nov 2006, Evgeniy Polyakov wrote:
> 
> > +static int kevent_poll_wait_callback(wait_queue_t *wait,
> > +		unsigned mode, int sync, void *key)
> > +{
> > +	struct kevent_poll_wait_container *cont =
> > +		container_of(wait, struct kevent_poll_wait_container, wait);
> > +	struct kevent *k = cont->k;
> > +	struct file *file = k->st->origin;
> > +	u32 revents;
> > +
> > +	revents = file->f_op->poll(file, NULL);
> > +
> > +	kevent_storage_ready(k->st, NULL, revents);
> > +
> > +	return 0;
> > +}
> 
> Are you sure you can safely call file->f_op->poll() from inside a callback 
> based wakeup? The low level driver may be calling the wakeup with one of 
> its locks held, and during the file->f_op->poll may be trying to acquire 
> the same lock. I remember there was a discussion about this, and assuming 
> the above not true, made epoll code more complex (and slower, since an 
> extra O(R) loop was needed to fetch events).

Indeed, I have not paid too much attention to poll/select notifications in 
kevent actually. As far as I recall it should be called on behalf of process 
doing kevent_get_event(). I will check and fix if that is not correct.
Thanks Davide.
 
> - Davide
> 

-- 
	Evgeniy Polyakov
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