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Date: Thu, 27 Nov 2008 20:35:48 -0800 (PST)
From: Davide Libenzi <davidel@xmailserver.org>
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To: Tejun Heo <htejun@gmail.com>
cc: Oleg Nesterov <oleg@redhat.com>, Eric Van Hensbergen <ericvh@gmail.com>,
       Ron Minnich <rminnich@sandia.gov>, Ingo Molnar <mingo@elte.hu>,
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       Mauro Carvalho Chehab <mchehab@infradead.org>,
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       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] poll: allow f_op->poll to sleep, take#6
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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.
Did you test it al all in a live system?


- Davide