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SubjectRe: Query regarding synchronize_sched_expedited and resched_cpu
On Thu, Sep 21, 2017 at 09:00:48AM -0700, Paul E. McKenney wrote:
> commit c21c9b78182e35eb0e72ef4e3bba3054f26eaaea
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date: Mon Sep 18 08:54:40 2017 -0700
>
> sched: Make resched_cpu() unconditional
>
> The current implementation of synchronize_sched_expedited() incorrectly
> assumes that resched_cpu() is unconditional, which it is not. This means
> that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
> fails as follows (analysis by Neeraj Upadhyay):
>
> o CPU1 is waiting for expedited wait to complete:
>
> sync_rcu_exp_select_cpus
> rdp->exp_dynticks_snap & 0x1 // returns 1 for CPU5
> IPI sent to CPU5
>
> synchronize_sched_expedited_wait
> ret = swait_event_timeout(rsp->expedited_wq,
> sync_rcu_preempt_exp_done(rnp_root),
> jiffies_stall);
>
> expmask = 0x20, CPU 5 in idle path (in cpuidle_enter())
>
> o CPU5 handles IPI and fails to acquire rq lock.
>
> Handles IPI
> sync_sched_exp_handler
> resched_cpu
> returns while failing to try lock acquire rq->lock
> need_resched is not set
>
> o CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to
> idle (schedule() is not called).
>
> o CPU 1 reports RCU stall.


Inconsistent spacing after your bullet 'o', first two points have a
space the last two a tab or so.

> Given that resched_cpu() is now used only by RCU, this commit fixes the
> assumption by making resched_cpu() unconditional.

Other than that, yes looks _much_ better, thanks!

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Also, you might want to tag it for stable.

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