Messages in this thread | | | Date | Wed, 8 Feb 2017 13:02:07 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: mm: deadlock between get_online_cpus/pcpu_alloc |
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On Wed, 8 Feb 2017, Michal Hocko wrote: > On Tue 07-02-17 23:25:17, Thomas Gleixner wrote: > > On Tue, 7 Feb 2017, Christoph Lameter wrote: > > > On Tue, 7 Feb 2017, Michal Hocko wrote: > > > > > > > I am always nervous when seeing hotplug locks being used in low level > > > > code. It has bitten us several times already and those deadlocks are > > > > quite hard to spot when reviewing the code and very rare to hit so they > > > > tend to live for a long time. > > > > > > Yep. Hotplug events are pretty significant. Using stop_machine_XXXX() etc > > > would be advisable and that would avoid the taking of locks and get rid of all the > > > ocmplexity, reduce the code size and make the overall system much more > > > reliable. > > > > Huch? stop_machine() is horrible and heavy weight. Don't go there, there > > must be simpler solutions than that. > > Absolutely agreed. We are in the page allocator path so using the > stop_machine* is just ridiculous. And, in fact, there is a much simpler > solution [1] > > [1] http://lkml.kernel.org/r/20170207201950.20482-1-mhocko@kernel.org
Well, yes. It's simple, but from an RT point of view I really don't like it as we have to fix it up again.
On RT we solved the problem of the page allocator differently which allows us to do drain_all_pages() from the caller CPU as a side effect. That's interesting not only for RT, it's also interesting for NOHZ FULL scenarios because you don't inflict the work on the other CPUs.
https://git.kernel.org/cgit/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=linux-4.9.y-rt-rebase&id=d577a017da694e29a06af057c517f2a7051eb305
That uses local locks (an RT speciality which compile away into preempt/irq disable/enable when RT is disabled).
Works like a charm :)
Thanks,
tglx
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