Re: [PATCH] clock: Fix smp_processor_id() in preemptible bug

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Mar 09, 2017 at 10:37:32AM -0800, Paul E. McKenney wrote:
> On Thu, Mar 09, 2017 at 07:31:14AM -0800, Paul E. McKenney wrote:
> > On Thu, Mar 09, 2017 at 04:24:20PM +0100, Peter Zijlstra wrote:
> > > On Wed, Mar 08, 2017 at 01:53:06PM -0800, Paul E. McKenney wrote:
> > > > The v4.11-rc1 kernel emits the following splat in some configurations:
> > > > 
> > > > [   43.681891] BUG: using smp_processor_id() in preemptible [00000000] code: kworker/3:1/49
> > > > [   43.682511] caller is debug_smp_processor_id+0x17/0x20
> > > > [   43.682893] CPU: 0 PID: 49 Comm: kworker/3:1 Not tainted 4.11.0-rc1+ #1
> > > > [   43.683382] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> > > > [   43.683497] Workqueue: events __clear_sched_clock_stable
> > > > [   43.683497] Call Trace:
> > > > [   43.683497]  dump_stack+0x4f/0x69
> > > > [   43.683497]  check_preemption_disabled+0xd9/0xf0
> > > > [   43.683497]  debug_smp_processor_id+0x17/0x20
> > > > [   43.683497]  __clear_sched_clock_stable+0x11/0x60
> > > > [   43.683497]  process_one_work+0x146/0x430
> > > > [   43.683497]  worker_thread+0x126/0x490
> > > > [   43.683497]  kthread+0xfc/0x130
> > > > [   43.683497]  ? process_one_work+0x430/0x430
> > > > [   43.683497]  ? kthread_create_on_node+0x40/0x40
> > > > [   43.683497]  ? umh_complete+0x30/0x30
> > > > [   43.683497]  ? call_usermodehelper_exec_async+0x12a/0x130
> > > > [   43.683497]  ret_from_fork+0x29/0x40
> > > > [   43.689244] sched_clock: Marking unstable (43688244724, 179505618)<-(43867750342, 0)
> > > > 
> > > > This happens because workqueue handlers run with preemption enabled
> > > > by default and the new this_scd() function accesses per-CPU variables.
> > > > This commit therefore disables preemption across this call to this_scd()
> > > > and to the uses of the pointer that it returns.  Lightly tested
> > > > successfully on x86.
> > > 
> > > Does this also work?
> > 
> > Thank you!  I will give it a shot after the other tests complete.
> 
> And it does pass light testing.  I will hammer it harder this evening.
> 
> So please send a formal patch!

And the diffs below passed overnight testing, so:

Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

------------------------------------------------------------------------

commit e87df240b9e6a24635a1c8674f715554f27df6cc
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Thu Mar 9 07:51:25 2017 -0800

    EXPERIMENTAL: Peter Zijlstra hotplug fix
    
    Not-signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index a08795e21628..1560d5f961ef 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -172,8 +172,8 @@ void clear_sched_clock_stable(void)
 
 	smp_mb(); /* matches sched_clock_init_late() */
 
-	if (sched_clock_running == 2)
-		schedule_work(&sched_clock_work);
+	if (sched_clock_running == 2 && sched_clock_stable())
+		schedule_work_on(smp_processor_id(), &sched_clock_work);
 }
 
 void sched_clock_init_late(void)