[PATCH] workqueue: Don't spin forever in worker_maybe_bind_and_lock

[PATCH] workqueue: Don't spin forever in worker_maybe_bind_and_lock

[Paul Mackerras]

On a 48-thread POWER7 box, I often see the system hang when offlining
processors.  What happens is that we get a rescuer thread trying to
move to some processor at the same time that a cpu offline operation
is happening for that processor, and we end up with one cpu spinning in
worker_maybe_bind_and_lock() and all of the rest of the online cpus
spinning inside the stop_machine code.  The rescuer thread is
continually calling set_cpus_allowed_ptr() which is continually
failing because the cpu it is trying to move to is no longer in the
cpu_active_mask.  The result is a deadlock.

This fixes worker_maybe_bind_and_lock so that it stops trying to move
to a cpu if that cpu is no longer in the cpu_active_mask, and instead
returns to its caller.  With this I no longer see the deadlocks when
offlining cpus.

Signed-off-by: Paul Mackerras <paulus@samba.org>
---
 kernel/workqueue.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 8859a41..12faf78 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1289,6 +1289,8 @@ __acquires(&gcwq->lock)
 		    cpumask_equal(&current->cpus_allowed,
 				  get_cpu_mask(gcwq->cpu)))
 			return true;
+		if (!cpumask_test_cpu(gcwq->cpu, cpu_active_mask))
+			return false;
 		spin_unlock_irq(&gcwq->lock);
 
 		/* CPU has come up in between, retry migration */
-- 
1.7.4.1

Re: [PATCH] workqueue: Don't spin forever in worker_maybe_bind_and_lock

[Tejun Heo]

On Wed, May 04, 2011 at 11:47:49AM +1000, Paul Mackerras wrote:
> On a 48-thread POWER7 box, I often see the system hang when offlining
> processors.  What happens is that we get a rescuer thread trying to
> move to some processor at the same time that a cpu offline operation
> is happening for that processor, and we end up with one cpu spinning in
> worker_maybe_bind_and_lock() and all of the rest of the online cpus
> spinning inside the stop_machine code.  The rescuer thread is
> continually calling set_cpus_allowed_ptr() which is continually
> failing because the cpu it is trying to move to is no longer in the
> cpu_active_mask.  The result is a deadlock.
> 
> This fixes worker_maybe_bind_and_lock so that it stops trying to move
> to a cpu if that cpu is no longer in the cpu_active_mask, and instead
> returns to its caller.  With this I no longer see the deadlocks when
> offlining cpus.
> 
> Signed-off-by: Paul Mackerras <paulus@samba.org>

Hmm.. fix for the problem has already been merged into mainline and
scheduled for -stable.  Can you please verify the following fixes the
problem?

Thank you.

>From 5035b20fa5cd146b66f5f89619c20a4177fb736d Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Fri, 29 Apr 2011 18:08:37 +0200
Subject: [PATCH] workqueue: fix deadlock in worker_maybe_bind_and_lock()

If a rescuer and stop_machine() bringing down a CPU race with each
other, they may deadlock on non-preemptive kernel.  The CPU won't
accept a new task, so the rescuer can't migrate to the target CPU,
while stop_machine() can't proceed because the rescuer is holding one
of the CPU retrying migration.  GCWQ_DISASSOCIATED is never cleared
and worker_maybe_bind_and_lock() retries indefinitely.

This problem can be reproduced semi reliably while the system is
entering suspend.

 http://thread.gmane.org/gmane.linux.kernel/1122051

A lot of kudos to Thilo-Alexander for reporting this tricky issue and
painstaking testing.

stable: This affects all kernels with cmwq, so all kernels since and
        including v2.6.36 need this fix.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Thilo-Alexander Ginkel <thilo@ginkel.com>
Tested-by: Thilo-Alexander Ginkel <thilo@ginkel.com>
Cc: stable@kernel.org
---
 kernel/workqueue.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 04ef830..e3378e8 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1291,8 +1291,14 @@ __acquires(&gcwq->lock)
 			return true;
 		spin_unlock_irq(&gcwq->lock);
 
-		/* CPU has come up inbetween, retry migration */
+		/*
+		 * We've raced with CPU hot[un]plug.  Give it a breather
+		 * and retry migration.  cond_resched() is required here;
+		 * otherwise, we might deadlock against cpu_stop trying to
+		 * bring down the CPU on non-preemptive kernel.
+		 */
 		cpu_relax();
+		cond_resched();
 	}
 }
 
-- 
1.7.1