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

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Message-ID: <x491v48d47d.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 19 Jan 2011 15:32:54 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Nick Piggin <npiggin@...il.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

Nick Piggin <npiggin@...il.com> writes:

> 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.

I'm not sure I understand what you're saying.  Do you mean to imply that
io_destroy is not a very slow path?  Because it is.  I prefer a solution
that doesn't re-architecht things in order to solve a theoretical issue
that's never been observed.

Cheers,
Jeff
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