On Wed, 30 Jul 2014, Lai Jiangshan wrote:
> I think the bug is here, it re-queues the per_cpu(vmstat_work, cpu) which is
> offline
> (after vmstat_cpuup_callback(CPU_DOWN_PREPARE). And cpu_stat_off is accessed
> without
> proper lock.
Ok. I guess we need to make the preemption check output more information
so that it tells us that an operation was performed on a processor that is
down?
> I suggest to use get_cpu_online() or a new cpu_stat_off_mutex to protect it.
If a processor is downed then cpu_stat_off bit should be cleared but also
the worker thread should not run.
> > case CPU_DOWN_PREPARE:
> > case CPU_DOWN_PREPARE_FROZEN:
> > - cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> > - per_cpu(vmstat_work, cpu).work.func = NULL;
> > + if (!cpumask_test_and_set_cpu(cpu, cpu_stat_off))
> > + cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
>
> It is suggest that cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu)) should
> be called unconditionally. And the cpu should be cleared from cpu_stat_off.
> (you set it, it is BUG according to vmstat_shepherd() and the semantics of the
> cpu_stat_off).
True.
Subject: vmstat ondemand: Fix online/offline races
Do not allow onlining/offlining while the shepherd task is checking
for vmstat threads.
On offlining a processor do the right thing cancelling the vmstat
worker thread if it exista and also exclude it from the shepherd
process checks.
Signed-off-by: Christoph Lameter <c...@linux.com>
Index: linux/mm/vmstat.c
===================================================================
--- linux.orig/mm/vmstat.c 2014-07-30 09:35:54.602662306 -0500
+++ linux/mm/vmstat.c 2014-07-30 09:43:07.109037043 -0500
@@ -1317,6 +1317,7 @@ static void vmstat_shepherd(struct work_
{
int cpu;
+ get_online_cpus();
/* Check processors whose vmstat worker threads have been disabled */
for_each_cpu(cpu, cpu_stat_off)
if (need_update(cpu) &&
@@ -1325,6 +1326,7 @@ static void vmstat_shepherd(struct work_
schedule_delayed_work_on(cpu, &per_cpu(vmstat_work,
cpu),
__round_jiffies_relative(sysctl_stat_interval,
cpu));
+ put_online_cpus();
schedule_delayed_work(&shepherd,
round_jiffies_relative(sysctl_stat_interval));
@@ -1380,8 +1382,8 @@ static int vmstat_cpuup_callback(struct
break;
case CPU_DOWN_PREPARE:
case CPU_DOWN_PREPARE_FROZEN:
- if (!cpumask_test_and_set_cpu(cpu, cpu_stat_off))
- cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
+ cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
+ cpumask_clear_cpu(cpu, cpu_stat_off);
break;
case CPU_DOWN_FAILED:
case CPU_DOWN_FAILED_FROZEN:
--
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