This is a note to let you know that I've just added the patch titled
sched: Make resched_cpu() unconditional
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
sched-make-resched_cpu-unconditional.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Mon, 18 Sep 2017 08:54:40 -0700
Subject: sched: Make resched_cpu() unconditional
From: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
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:
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.
Given that resched_cpu() is now 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>
Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/core.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -600,8 +600,7 @@ 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);
}
Patches currently in stable-queue which might be from paulmck@xxxxxxxxxxxxxxxxxx are
queue-4.4/sched-make-resched_cpu-unconditional.patch