linux-kernel - Re: [RFC][PATCH v3 2/6] mm, directio: fix fork vs direct-io race (read(2) side IOW gup(write) side)

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Date:	Tue, 14 Apr 2009 21:48:25 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	Nick Piggin <nickpiggin@...oo.com.au>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, Hugh Dickins <hugh@...itas.com>,
	Zach Brown <zach.brown@...cle.com>,
	Andy Grover <andy.grover@...cle.com>
Subject: Re: [RFC][PATCH v3 2/6] mm, directio: fix fork vs direct-io race
	(read(2) side IOW gup(write) side)

On Tue, Apr 14, 2009 at 02:10:08PM -0400, Jeff Moyer wrote:
> Really?  I don't actually see that in the code, have I missed it?

Checking the spinlock version, when any writer is waiting, the
sem->wait_list won't empty and down_read will wait too. The wakeup is
FIFO with __rwsem_do_wake is doing a wake-one if first one in queue is
a down_write. So it looks ok to me. asm version should have an
equivalent logic.
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