This is a note to let you know that I've just added the patch titled

sched: Make resched_cpu() unconditional

to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: sched-make-resched_cpu-unconditional.patch and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 Mon Sep 17 00:00:00 2001

From: "Paul E. McKenney" paulmck@linux.vnet.ibm.com Date: Mon, 18 Sep 2017 08:54:40 -0700 Subject: sched: Make resched_cpu() unconditional

From: Paul E. McKenney paulmck@linux.vnet.ibm.com

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@codeaurora.org Suggested-by: Neeraj Upadhyay neeraju@codeaurora.org Signed-off-by: Paul E. McKenney paulmck@linux.vnet.ibm.com Acked-by: Steven Rostedt (VMware) rostedt@goodmis.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

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

--- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -507,8 +507,7 @@ 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@linux.vnet.ibm.com are

queue-4.9/sched-make-resched_cpu-unconditional.patch