Messages in this thread | | | Date | Wed, 11 Sep 2013 08:40:57 -0700 | From | Anton Vorontsov <> | Subject | Re: [PATCH] vmpressure: fix divide-by-0 in vmpressure_work_fn |
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On Mon, Sep 09, 2013 at 01:08:47PM +0200, Michal Hocko wrote: > On Fri 06-09-13 22:59:16, Hugh Dickins wrote: > > Hit divide-by-0 in vmpressure_work_fn(): checking vmpr->scanned before > > taking the lock is not enough, we must check scanned afterwards too. > > As vmpressure_work_fn seems the be the only place where we set scanned > to 0 (except for the rare occasion when scanned overflows which > would be really surprising) then the only possible way would be two > vmpressure_work_fn racing over the same work item. system_wq is > !WQ_NON_REENTRANT so one work item might be processed by multiple > workers on different CPUs. This means that the vmpr->scanned check in > the beginning of vmpressure_work_fn is inherently racy. > > Hugh's patch fixes the issue obviously but doesn't it make more sense to > move the initial vmpr->scanned check under the lock instead? > > Anton, what was the initial motivation for the out of the lock > check? Does it really optimize anything?
Thanks a lot for the explanation.
Answering your question: the idea was to minimize the lock section, but the section is quite small anyway so I doubt that it makes any difference (during development I could not measure any effect of vmpressure() calls in my system, though the system itself was quite small).
I am happy with moving the check under the lock or moving the work into its own WQ_NON_REENTRANT queue.
Anton
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