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

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Message-ID: <20110120200237.GC17752@linux.vnet.ibm.com>
Date:	Thu, 20 Jan 2011 12:02:37 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <npiggin@...il.com>
Cc:	Jeff Moyer <jmoyer@...hat.com>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] fs: aio fix rcu lookup

On Fri, Jan 21, 2011 at 05:31:53AM +1100, Nick Piggin wrote:
> On Thu, Jan 20, 2011 at 3:03 PM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > On Thu, Jan 20, 2011 at 08:20:00AM +1100, Nick Piggin wrote:
> >> On Thu, Jan 20, 2011 at 8:03 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> >> >> 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.
> >>
> >> synchronize_rcu of course, not RCU (typo).
> >
> > I think that Nick is suggesting that call_rcu() be used instead.
> > Perhaps also very sparing use of synchronize_rcu_expedited(), which
> > is faster than synchronize_rcu(), but which which uses more CPU time.
> 
> call_rcu() is the obvious alternative, yes.
> 
> Basically, once we give in to synchronize_rcu() we're basically giving
> up. That's certainly a very good tradeoff for something like filesystem
> unregistration or module unload, it buys big simplifications in real
> fastpaths. But I just don't think it should be taken lightly.

Makes sense to me!

BTW, on your earlier usage classification:

> I think synchronize_rcu should firstly not be used unless it gives a good
> simplification, or speedup in fastpath.
>
> When that is satified, then it is a question of exactly what kind of slow
> path it should be used in. I don't think it should be used in process-
> synchronous code (eg syscalls) except for error cases, resource
> exhaustion, management syscalls (like module unload).

I don't have any feedback either way on your guidance to where
synchronize_rcu() should be used, as I believe that it depends a lot
on the details of usage, and would vary from one part of the kernel
to another, and possibly also over time.

But I am very glad to see that you have been thinking about it and
that you are putting forth some clear guidelines!

							Thanx, Paul
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