linux-kernel - Re: [Patch v4] rwsem: fix rwsem_is

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Date:	Fri, 09 Oct 2009 17:02:30 +0800
From:	Amerigo Wang <amwang@...hat.com>
To:	David Howells <dhowells@...hat.com>
CC:	linux-kernel@...r.kernel.org, Ben Woodard <bwoodard@...l.gov>,
	akpm@...ux-foundation.org, Brian Behlendorf <behlendorf1@...l.gov>
Subject: Re: [Patch v4] rwsem: fix rwsem_is_locked() bugs

David Howells wrote:
> Amerigo Wang <amwang@...hat.com> wrote:
> 
>> rwsem_is_locked() tests ->activity without locks, so we should always
>> keep ->activity consistent. However, the code in __rwsem_do_wake()
>> breaks this rule, it updates ->activity after _all_ readers waken up,
>> this may give some reader a wrong ->activity value, thus cause
>> rwsem_is_locked() behaves wrong.
>>
>> Quote from Andrew:
>>
>> "
>> - we have one or more processes sleeping in down_read(), waiting for access.
>>
>> - we wake one or more processes up without altering ->activity
>>
>> - they start to run and they do rwsem_is_locked().  This incorrectly
>>   returns "false", because the waker process is still crunching away in
>>   __rwsem_do_wake().
>>
>> - the waker now alters ->activity, but it was too late.
>> "
>>
>> So we need get a spinlock to protect this. And rwsem_is_locked()
>> should not block, thus we use spin_trylock.
>>
>> Reported-by: Brian Behlendorf <behlendorf1@...l.gov>
>> Cc: Ben Woodard <bwoodard@...l.gov>
>> Cc: David Howells <dhowells@...hat.com>
>> Signed-off-by: WANG Cong <amwang@...hat.com>
> 
> I'd say the comment in __rwsem_do_wake() is unnecessary, but other than
> that...


The reason why I added it is to show that we have considered that
case already. :) If you have strong opinions to remove it, I can
update the patch.

> 
> Acked-by: David Howells <dhowells@...hat.com>


Thanks!
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