linux-kernel - Re: [PATCH 11/12] rwsem: wake all readers when first waiter is a reader

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Message-ID: <CANN689HSaiMz=TRLFr6dXk0ndvaHJECL_XCHsV7Xh1+80U9K+Q@mail.gmail.com>
Date:	Thu, 14 Mar 2013 08:20:31 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Alex Shi <alex.shi@...el.com>, Ingo Molnar <mingo@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/12] rwsem: wake all readers when first waiter is a reader

On Thu, Mar 14, 2013 at 4:39 AM, Peter Hurley <peter@...leysoftware.com> wrote:
> On Thu, 2013-03-14 at 00:03 -0700, Michel Lespinasse wrote:
>> >   CPU 0                               |  CPU 1
>> >                                       |
>> >                                       | down_write()
>> >
>> > ... CPU 1 has the write lock for the semaphore.
>> >     Meanwhile, 1 or more down_read(s) are attempted and fail;
>> >     these are put on the wait list.
>>
>> I'm not sure of the relevance of these other down_read() calls -
>> please note that as these extra readers are put on the wait list,
>> their +read_bias adjustments are canceled so that the count value
>> ends up at write_bias + waiting_bias (for a total active count of 1)
>
> The relevance of the waiting readers is that when CPU 1 drops the writer
> ___and grabs the spin lock___, it then wakes up these already-waiting
> readers (CPU 0 is still parked in down_read_failed() waiting to acquire
> the spin lock).
>
> When CPU 1 wakes up these readers, the sem count goes > 0 and the
> waiting list is emptied. CPU 1 then drops the spin lock and leaves
> up_write().

Ah, correct. So the race you noticed is real, and the one I found is
just a more complicated way to hit it.

I'll add a fix in v2 of this patch series (probably later today).

BTW, you seem to have been paying pretty close attention, would you
mind giving appropriate Reviewed-by tags when I send v2 ?
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