linux-kernel - Re: [PATCH] poll: allow f

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Date:	Fri, 28 Nov 2008 17:08:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Tejun Heo <htejun@...il.com>,
	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>,
	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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] poll: allow f_op->poll to sleep, take#6

On 11/27, Davide Libenzi wrote:
>
> On Thu, 27 Nov 2008, Tejun Heo wrote:
>
> > f_op->poll is the only vfs operation which is not allowed to sleep.  It's
> > because poll and select implementation used task state to synchronize
> > against wake ups, which doesn't have to be the case anymore as wait/wake
> > interface can now use custom wake up functions.  The non-sleep restriction
> > can be a bit tricky because ->poll is not called from an atomic context
> > and the result of accidentally sleeping in ->poll only shows up as
> > temporary busy looping when the timing is right or rather wrong.
> >
> > This patch converts poll/select to use custom wake up function and use
> > separate triggered variable to synchronize against wake up events.  The
> > only added overhead is an extra function call during wake up and
> > negligible.
> >
> > This patch removes the one non-sleep exception from vfs locking rules and
> > is beneficial to userland filesystem implementations like FUSE, 9p or
> > peculiar fs like spufs as it's very difficult for those to implement
> > non-sleeping poll method.
>
> Looks OK to me, although it'd be better if some more folks eye it, in
> order to avoid painful mistakes.

Just in case, I think the patch is correct too.

(v4l1-compat.c:poll_one() looks buggy, it doesn't check if poll_get_entry()
 fails, but this has nothing to do with this patch).

Oleg.

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