[lkml]   [2017]   [Sep]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
SubjectRe: Query regarding synchronize_sched_expedited and resched_cpu
On Thu, Sep 21, 2017 at 06:30:12PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 21, 2017 at 09:00:48AM -0700, Paul E. McKenney wrote:
> > commit c21c9b78182e35eb0e72ef4e3bba3054f26eaaea

[ . . . ]

> 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) <>
> Also, you might want to tag it for stable.

Like this, then?

Thanx, Paul


commit 62d94c97e96d5aa6a977a53dd007029df7a65586
Author: Paul E. McKenney <>
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:

rdp->exp_dynticks_snap & 0x1 // returns 1 for CPU5
IPI sent to CPU5

ret = swait_event_timeout(rsp->expedited_wq,

expmask = 0x20, CPU 5 in idle path (in cpuidle_enter())

o CPU5 handles IPI and fails to acquire rq lock.

Handles IPI
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 now used only by RCU, this commit fixes the
assumption by making resched_cpu() unconditional.

Reported-by: Neeraj Upadhyay <>
Suggested-by: Neeraj Upadhyay <>
Signed-off-by: Paul E. McKenney <>
Acked-by: Steven Rostedt (VMware) <>
Acked-by: Peter Zijlstra (Intel) <>

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);
raw_spin_unlock_irqrestore(&rq->lock, flags);
 \ /
  Last update: 2017-09-21 19:35    [W:0.106 / U:0.028 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site