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

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Message-ID: <Pine.LNX.4.64.0611071449410.17731@alien.or.mcafeemobile.com>
Date:	Tue, 7 Nov 2006 14:53:33 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
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, 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).



- Davide


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