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Date: Mon, 9 Apr 2001 22:43:53 -0700 (PDT)
From: Linus Torvalds <torvalds@transmeta.com>
To: Tachino Nobuhiro <tachino@open.nm.fujitsu.co.jp>
cc: Andrew Morton <andrewm@uow.edu.au>, Ben LaHaise <bcrl@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: rw_semaphores
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On Tue, 10 Apr 2001, Tachino Nobuhiro wrote:
>
>   I am not familiar with semaphore or x86, so this may not be correct,
> but if the following sequence is possible, the writer can call wake_up()
> before the reader calls add_wait_queue() and reader may sleep forever.
> Is it possible?

The ordering is certainly possible, but if it happens,
__down_read_failed() won't actually sleep, because it will notice that the
value is positive and just return immediately. So it will do some
unnecessary work (add itself to the wait-queue only to remove itself
immediately again), but it will do the right thing.

		Linus