Re: [tip:timers/core] clockevents: Fix cpu_down() race for hrtimer based broadcasting

Preeti U Murthy
2015-04-03T10:39:13Z

On 04/02/2015 08:00 PM, tip-bot for Preeti U Murthy wrote:
> Commit-ID: 345527b1edce8df719e0884500c76832a18211c3
> Gitweb: http://git.kernel.org/tip/345527b1edce8df719e0884500c76832a18211c3
> Author: Preeti U Murthy <preeti@linux.vnet.ibm.com>
> AuthorDate: Mon, 30 Mar 2015 14:59:19 +0530
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Thu, 2 Apr 2015 14:25:39 +0200
>
> clockevents: Fix cpu_down() race for hrtimer based broadcasting
>
> It was found when doing a hotplug stress test on POWER, that the
> machine either hit softlockups or rcu_sched stall warnings. The
> issue was traced to commit:
>
> 7cba160ad789 ("powernv/cpuidle: Redesign idle states management")
>
> which exposed the cpu_down() race with hrtimer based broadcast mode:
>
> 5d1638acb9f6 ("tick: Introduce hrtimer based broadcast")
>
> The race is the following:
>
> Assume CPU1 is the CPU which holds the hrtimer broadcasting duty
> before it is taken down.
>
> CPU0 CPU1
>
> cpu_down() take_cpu_down()
> disable_interrupts()
>
> cpu_die()
>
> while (CPU1 != CPU_DEAD) {
> msleep(100);
> switch_to_idle();
> stop_cpu_timer();
> schedule_broadcast();
> }
>
> tick_cleanup_cpu_dead()
> take_over_broadcast()
>
> So after CPU1 disabled interrupts it cannot handle the broadcast
> hrtimer anymore, so CPU0 will be stuck forever.
>
> Fix this by explicitly taking over broadcast duty before cpu_die().
>
> This is a temporary workaround. What we really want is a callback
> in the clockevent device which allows us to do that from the dying
> CPU by pushing the hrtimer onto a different cpu. That might involve
> an IPI and is definitely more complex than this immediate fix.
>
> Changelog was picked up from:
>
> https://lkml.org/lkml/2015/2/16/213
>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Tested-by: Nicolas Pitre <nico@linaro.org>
> Signed-off-by: Preeti U. Murthy <preeti@linux.vnet.ibm.com>
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: mpe@ellerman.id.au
> Cc: nicolas.pitre@linaro.org
> Cc: peterz@infradead.org
> Cc: rjw@rjwysocki.net
> Fixes: http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html
> Link: http://lkml.kernel.org/r/20150330092410.24979.59887.stgit@preeti.in.ibm.com
> [ Merged it to the latest timer tree, renamed the callback, tidied up the changelog. ]
> Signed-off-by: Ingo Molnar <mingo@kernel.org>

Can this be marked for stable too please?

Regards
Preeti U Murthy