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Subject[PATCH 3.18 03/67] sched: Make resched_cpu() unconditional
3.18-stable review patch.  If anyone has any objections, please let me know.


From: Paul E. McKenney <>

commit 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 upstream.

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) <>
Signed-off-by: Greg Kroah-Hartman <>

kernel/sched/core.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -632,8 +632,7 @@ 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);

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