Re: [PATCH] percpu-refcount: fix usage of this_cpu_ops (was Re: hanging aio process)

[Tejun Heo]

cc'ing Christoph.  Hey!

On Wed, Jun 04, 2014 at 03:58:24PM +0200, Sebastian Ott wrote:
> From 82295633cad58c7d6b9af4e470e3168ed43a6779 Mon Sep 17 00:00:00 2001
> From: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> Date: Wed, 4 Jun 2014 12:53:19 +0200
> Subject: [PATCH] percpu-refcount: fix usage of this_cpu_ops
> 
> The percpu-refcount infrastructure uses the underscore variants of
> this_cpu_ops in order to modify percpu reference counters.
> (e.g. __this_cpu_inc()).
> 
> However the underscore variants do not atomically update the percpu
> variable, instead they may be implemented using read-modify-write
> semantics (more than one instruction).  Therefore it is only safe to
> use the underscore variant if the context is always the same (process,
> softirq, or hardirq). Otherwise it is possible to lose updates.
> 
> This problem is something that Sebastian has seen within the aio
> subsystem which uses percpu refcounters both in process and softirq
> context leading to reference counts that never dropped to zeroes; even
> though the number of "get" and "put" calls matched.
> 
> Fix this by using the non-underscore this_cpu_ops variant which
> provides correct per cpu atomic semantics and fixes the corrupted
> reference counts.

Christoph, percpu-refcount misused __this_cpu_*() and subtly broke
s390 which uses the stock read-modify-write implementation.  It should
be possible to annotate __this_cpu_*() so that lockdep warns if it's
used from different contexts, right?  Hmm.... now that I think about
it, there's nothing to attach lockdep context to.  :(

Urgh... I really don't like the subtleties around __this_cpu_*().
It's too easy to get it wrong and fail to notice it.  :(

Thanks.

-- 
tejun
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