Re: [Patch] cpufreq: fix a deadlock during shutting down

[Cong Wang]

David Rientjes wrote:
> On Tue, 9 Feb 2010, Amerigo Wang wrote:
>
> > Benjamin reported that, the machine deadlocks right after printing the
> > following when doing a shutdown:
> >
> > halt/4071 is trying to acquire lock:
> >  (s_active){++++.+}, at: [<c0000000001ef868>] .sysfs_addrm_finish+0x58/0xc0
> >
> > but task is already holding lock:
> >  (&per_cpu(cpu_policy_rwsem, cpu)){+.+.+.}, at: [<c0000000004cd6ac>] .lock_policy_rwsem_write+0x84/0xf4
> >
> > which lock already depends on the new lock.
> >
> > the existing dependency chain (in reverse order) is:
> >
> > <nothing else ... machine deadlocked here>
> >
> >
> > This is because we are trying to kobject_put() a kobject while
> > we are holding cpu policy rwsem. So just move kobject_put()
> > down after releasing the rwsem.
> >
> > Totally untested.
> >
> > Reported-by: Xiaotian Feng <xtfeng@xxxxxxxxx>
> > Reported-by: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: WANG Cong <amwang@xxxxxxxxxx>
> > Cc: Dave Jones <davej@xxxxxxxxxx>
> > Cc: Thomas Renninger <trenn@xxxxxxx>
> > Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
> > Cc: Venkatesh Pallipadi <venkatesh.pallipadi@xxxxxxxxx>
> >
> > ---
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 67bc2ec..222b35f 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -1113,6 +1113,7 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
> >  	unsigned int cpu = sys_dev->id;
> >  	unsigned long flags;
> >  	struct cpufreq_policy *data;
> > +	struct kobject *kobj;
> >  #ifdef CONFIG_SMP
> >  	struct sys_device *cpu_sys_dev;
> >  	unsigned int j;
> > @@ -1192,7 +1193,7 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
> >  	if (cpufreq_driver->target)
> >  		__cpufreq_governor(data, CPUFREQ_GOV_STOP);
> >  
> > -	kobject_put(&data->kobj);
> > +	kobj = &data->kobj;
> >  
> >  	/* we need to make sure that the underlying kobj is actually
> >  	 * not referenced anymore by anybody before we proceed with
>
> NACK.
>
> If you read this comment, it says:
>
> 	/* we need to make sure that the underlying kobj is actually
> 	 * not referenced anymore by anybody before we proceed with
> 	 * unloading.
> 	 */
>
> That would suggest that the wait_for_completion(&data->kobj_unregister); 
> would never actually return if you're holding a reference to it in your 
> patch since it only completes when the last reference is dropped (the 
> ->release function is cpufreq_sysfs_release()).

Oh, my bad.

Then this case seems to be more complex... But anyway, this is _not_ a
bogus.

Thanks.
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