Re: [perf/x86] 8de4a00661: WARNING: CPU: 0 PID: 1 at kernel/locking/mutex-debug.c:80 debug_mutex_unlock+0x20c/0x2b3

[Thomas Gleixner]

On Tue, 12 Jul 2016, Thomas Gleixner wrote:
> On Wed, 13 Jul 2016, kernel test robot wrote:
> > [    1.863354] WARNING: CPU: 0 PID: 1 at kernel/locking/mutex-debug.c:80 debug_mutex_unlock+0x20c/0x2b3
> > [    1.877193] DEBUG_LOCKS_WARN_ON(lock->owner != current)
> > 
> > [    1.979431]  [<ffffffff8167508c>] mutex_unlock+0x9/0xb
> > [    1.979431]  [<ffffffff8167508c>] mutex_unlock+0x9/0xb
> > [    1.990691]  [<ffffffff8105a67f>] cpuhp_store_callbacks+0x5a/0x63
> 
> I have a hard time to figure out how that can happen:
> 
> static void cpuhp_store_callbacks(enum cpuhp_state state,
>                                   const char *name,
>                                   int (*startup)(unsigned int cpu),
>                                   int (*teardown)(unsigned int cpu))
> {
>         /* (Un)Install the callbacks for further cpu hotplug operations */
>         struct cpuhp_step *sp;
> 
>         mutex_lock(&cpuhp_state_mutex);
>  	sp = cpuhp_get_step(state);
> 	sp->startup = startup;
>         sp->teardown = teardown;
>         sp->name = name;
>         mutex_unlock(&cpuhp_state_mutex);
> }
> 
> Confused ....

And printing cpuhp_state_mutex.owner does not reduce the confusion level.

[    0.186490] WTF 1           (null)
>         mutex_lock(&cpuhp_state_mutex);
[    0.186848] WTF 2 ffff8800002a4000
>  	sp = cpuhp_get_step(state);
[    0.187174] WTF 3 ffff8800002a4000

and current is:

[    0.205749] CPU: 0 PID: 1 Comm: swapper 
[    0.207410] task: ffff8800002a4000

/me goes digging deeper