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SubjectRe: Query regarding synchronize_sched_expedited and resched_cpu
On Tue, Sep 19, 2017 at 08:31:26AM -0700, Paul E. McKenney wrote:
> So I have this one queued. Objections?

Changelog reads like its whitespace damaged.

> commit bc43e2e7e08134e6f403ac845edcf4f85668d803
> 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 , and CPU 5 is 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.
>
> Given that resched_cpu() is used only by RCU, this commit fixes the
> assumption by making resched_cpu() unconditional.
>
> Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Suggested-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index cab8c5ec128e..b2281971894c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -505,8 +505,7 @@ void resched_cpu(int cpu)
> struct rq *rq = cpu_rq(cpu);
> unsigned long flags;
>
> - if (!raw_spin_trylock_irqsave(&rq->lock, flags))
> - return;
> + raw_spin_lock_irqsave(&rq->lock, flags);
> resched_curr(rq);
> raw_spin_unlock_irqrestore(&rq->lock, flags);
> }
>

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