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

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Message-ID: <x49wrm0bo7g.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 19 Jan 2011 16:03:47 -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 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.

Now you've really lost me.  ;-)  Processes which utilize the in-kernel
aio interface typically create an ioctx at process startup, use that for
submitting all of their io, then destroy it on exit.  Think of a
database.  Every time you call io_submit, you're doing a lookup of the
ioctx.

> 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.

Above it sounded like you didn't think AIO should be using RCU at all.
Here it sounds like you are just against synchronize_rcu.  Which is it?
And if the latter, then please tell me in what cases you feel one would
be justified in calling synchronize_rcu.  For now, I simply disagree
with you.  As I said before, you're already potentially waiting for disk
I/O to complete.  It doesn't get much worse than that for latency.

Cheers,
Jeff
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