On Thu, Oct 23, 2014 at 01:40:32AM +0300, Yanko Kaneti wrote:
> On Wed-10/22/14-2014 15:33, Josh Boyer wrote:
> > On Wed, Oct 22, 2014 at 2:55 PM, Paul E. McKenney
> > <paul...@linux.vnet.ibm.com> wrote:
[ . . . ]
> > > Don't get me wrong -- the fact that this kthread appears to have
> > > blocked within rcu_barrier() for 120 seconds means that something is
> > > most definitely wrong here. I am surprised that there are no RCU CPU
> > > stall warnings, but perhaps the blockage is in the callback execution
> > > rather than grace-period completion. Or something is preventing this
> > > kthread from starting up after the wake-up callback executes. Or...
> > >
> > > Is this thing reproducible?
> >
> > I've added Yanko on CC, who reported the backtrace above and can
> > recreate it reliably. Apparently reverting the RCU merge commit
> > (d6dd50e) and rebuilding the latest after that does not show the
> > issue. I'll let Yanko explain more and answer any questions you have.
>
> - It is reproducible
> - I've done another build here to double check and its definitely the rcu
> merge
> that's causing it.
>
> Don't think I'll be able to dig deeper, but I can do testing if needed.
Please! Does the following patch help?
Thanx, Paul
------------------------------------------------------------------------
rcu: More on deadlock between CPU hotplug and expedited grace periods
Commit dd56af42bd82 (rcu: Eliminate deadlock between CPU hotplug and
expedited grace periods) was incomplete. Although it did eliminate
deadlocks involving synchronize_sched_expedited()'s acquisition of
cpu_hotplug.lock via get_online_cpus(), it did nothing about the similar
deadlock involving acquisition of this same lock via put_online_cpus().
This deadlock became apparent with testing involving hibernation.
This commit therefore changes put_online_cpus() acquisition of this lock
to be conditional, and increments a new cpu_hotplug.puts_pending field
in case of acquisition failure. Then cpu_hotplug_begin() checks for this
new field being non-zero, and applies any changes to cpu_hotplug.refcount.
Reported-by: Jiri Kosina <jkos...@suse.cz>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Tested-by: Jiri Kosina <jkos...@suse.cz>
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 356450f09c1f..90a3d017b90c 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -64,6 +64,8 @@ static struct {
* an ongoing cpu hotplug operation.
*/
int refcount;
+ /* And allows lockless put_online_cpus(). */
+ atomic_t puts_pending;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
@@ -113,7 +115,11 @@ void put_online_cpus(void)
{
if (cpu_hotplug.active_writer == current)
return;
- mutex_lock(&cpu_hotplug.lock);
+ if (!mutex_trylock(&cpu_hotplug.lock)) {
+ atomic_inc(&cpu_hotplug.puts_pending);
+ cpuhp_lock_release();
+ return;
+ }
if (WARN_ON(!cpu_hotplug.refcount))
cpu_hotplug.refcount++; /* try to fix things up */
@@ -155,6 +161,12 @@ void cpu_hotplug_begin(void)
cpuhp_lock_acquire();
for (;;) {
mutex_lock(&cpu_hotplug.lock);
+ if (atomic_read(&cpu_hotplug.puts_pending)) {
+ int delta;
+
+ delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
+ cpu_hotplug.refcount -= delta;
+ }
if (likely(!cpu_hotplug.refcount))
break;
__set_current_state(TASK_UNINTERRUPTIBLE);
--
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