Re: Query regarding synchronize_sched_expedited and resched_cpu

[Peter Zijlstra]

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@xxxxxxxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxxx>
>     Suggested-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>     Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>     Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> 
> 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);
>  }
>