Messages in this thread | | | Date | Wed, 4 Jun 2014 12:41:06 -0400 | From | Tejun Heo <> | Subject | Re: [PATCH] percpu-refcount: fix usage of this_cpu_ops (was Re: hanging aio process) |
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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@de.ibm.com> > 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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