Messages in this thread Patch in this message |  | | Date | Wed, 22 Oct 2014 16:24:21 -0700 | From | "Paul E. McKenney" <> | Subject | Re: localed stuck in recent 3.18 git in copy_net_ns? |
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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 > > <paulmck@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
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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 <jkosina@suse.cz> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Jiri Kosina <jkosina@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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