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

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Date:	Thu, 08 Oct 2009 11:45:34 +0100
From:	David Howells <dhowells@...hat.com>
To:	Amerigo Wang <amwang@...hat.com>
Cc:	dhowells@...hat.com, 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

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...

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