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

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Message-ID: <AANLkTi=ChBc3+o_GcdKKn4HZykNkQ70mxPLu9HYDfeFj@mail.gmail.com>
Date:	Thu, 20 Jan 2011 07:45:42 +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 7:32 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> 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.

Even something that happens once per process lifetime, like in fork/exit
is not necessarily suitable for RCU. I don't know exactly how all programs
use io_destroy -- of the small number that do, probably an even smaller
number would care here. But I don't think it simplifies things enough to
use synchronize_rcu for it.
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