linux-kernel - Re: [patch] fs: aio fix rcu lookup

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Message-ID: <AANLkTiniSG3tfXx1iUtzf53sSsr=R6pmvmWrSV8FiYE9@mail.gmail.com>
Date:	Thu, 20 Jan 2011 07:18:53 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [patch] fs: aio fix rcu lookup

On Thu, Jan 20, 2011 at 6:46 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Jeff Moyer <jmoyer@...hat.com> writes:
>
>> Jan Kara <jack@...e.cz> writes:
>>
>>>  But there's the second race I describe making it possible
>>> for new IO to be created after io_destroy() has waited for all IO to
>>> finish...
>>
>> Can't that be solved by introducing memory barriers around the accesses
>> to ->dead?
>
> Upon further consideration, I don't think so.
>
> Given the options, I think adding the synchronize rcu to the io_destroy
> path is the best way forward.  You're already waiting for a bunch of
> queued I/O to finish, so there is no guarantee that you're going to
> finish that call quickly.

I think synchronize_rcu() is not something to sprinkle around outside
very slow paths. It can be done without synchronize_rcu.
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