Re: [dm-devel] bug: using smp_processor_id() in preemptible code in rr_select_path()

[Benjamin Block <bblock@linux.vnet.ibm.com>]

On 12:06 Fri 05 Aug     , Mike Snitzer wrote:
> On Fri, Aug 05 2016 at 11:54am -0400,
> Jens Axboe <axboe@kernel.dk> wrote:
> 
> > On 08/05/2016 09:42 AM, Mike Snitzer wrote:
> > >On Fri, Aug 05 2016 at 11:33P -0400,
> > >Jens Axboe <axboe@kernel.dk> wrote:
> > >
> > >>On 08/05/2016 09:27 AM, Mike Snitzer wrote:
> > >>>On Wed, Aug 03 2016 at 11:35am -0400,
> > >>>Benjamin Block <bblock@linux.vnet.ibm.com> wrote:
> > >>>
> > >>>>Hej Mike,
> > >>>>
> > >>>>when running a debug-kernel today with several multipath-devices using
> > >>>>the round-robin path selector I noticed that the kernel throws these
> > >>>>warnings here:
> > >>>>
> > >>>>BUG: using smp_processor_id() in preemptible [00000000] code: kdmwork-252:0/881
> > >>>>caller is rr_select_path+0x36/0x108 [dm_round_robin]
> > >>>>CPU: 1 PID: 881 Comm: kdmwork-252:0 Not tainted 4.7.0-debug #4
> > >>>>      00000000617679b8 0000000061767a48 0000000000000002 0000000000000000
> > >>>>      0000000061767ae8 0000000061767a60 0000000061767a60 00000000001145d0
> > >>>>      0000000000000000 0000000000b962ae 0000000000bb291e 000000000000000b
> > >>>>      0000000061767aa8 0000000061767a48 0000000000000000 0000000000000000
> > >>>>      0700000000b962ae 00000000001145d0 0000000061767a48 0000000061767aa8
> > >>>>Call Trace:
> > >>>>([<00000000001144a2>] show_trace+0x8a/0xe0)
> > >>>>([<0000000000114586>] show_stack+0x8e/0xf0)
> > >>>>([<00000000006c7fdc>] dump_stack+0x9c/0xe0)
> > >>>>([<00000000006fbbc0>] check_preemption_disabled+0x108/0x130)
> > >>>>([<000003ff80268646>] rr_select_path+0x36/0x108 [dm_round_robin])
> > >>>>([<000003ff80259a42>] choose_path_in_pg+0x42/0xc8 [dm_multipath])
> > >>>>([<000003ff80259b62>] choose_pgpath+0x9a/0x1a0 [dm_multipath])
> > >>>>([<000003ff8025b51a>] __multipath_map.isra.5+0x72/0x228 [dm_multipath])
> > >>>>([<000003ff8025b75e>] multipath_map+0x3e/0x50 [dm_multipath])
> > >>>>([<000003ff80225eb6>] map_request+0x66/0x458 [dm_mod])
> > >>>>([<000003ff802262ec>] map_tio_request+0x44/0x70 [dm_mod])
> > >>>>([<000000000016835a>] kthread_worker_fn+0xf2/0x1d8)
> > >>>>([<00000000001681da>] kthread+0x112/0x120)
> > >>>>([<000000000098378a>] kernel_thread_starter+0x6/0xc)
> > >>>>([<0000000000983784>] kernel_thread_starter+0x0/0xc)
> > >>>>no locks held by kdmwork-252:0/881.
> > >>>>
[:snip:]
> > 
> > I always forget the details (if this confuses lockdep or not), but you
> > could potentially turn it into:
> > 
> > local_irq_save(flags);
> > x = this_cpu_ptr();
> > [...]
> > 
> > spin_lock(&s->lock);
> > [...]
> > 
> > instead.
> 
> Cool, I've coded up the patch (compile tested only).
> 
> Benjamin, any chance you could test this against your v4.7 kernel and
> report back?
> 
> Thanks,
> Mike
> 
> diff --git a/drivers/md/dm-round-robin.c b/drivers/md/dm-round-robin.c
> index 4ace1da..ed446f8 100644
> --- a/drivers/md/dm-round-robin.c
> +++ b/drivers/md/dm-round-robin.c
> @@ -210,14 +210,17 @@ static struct dm_path *rr_select_path(struct path_selector *ps, size_t nr_bytes)
>  	struct path_info *pi = NULL;
>  	struct dm_path *current_path = NULL;
> 
> +	local_irq_save(flags);
>  	current_path = *this_cpu_ptr(s->current_path);
>  	if (current_path) {
>  		percpu_counter_dec(&s->repeat_count);
> -		if (percpu_counter_read_positive(&s->repeat_count) > 0)
> +		if (percpu_counter_read_positive(&s->repeat_count) > 0) {
> +			local_irq_restore(flags);
>  			return current_path;
> +		}
>  	}
> 
> -	spin_lock_irqsave(&s->lock, flags);
> +	spin_lock(&s->lock);
>  	if (!list_empty(&s->valid_paths)) {
>  		pi = list_entry(s->valid_paths.next, struct path_info, list);
>  		list_move_tail(&pi->list, &s->valid_paths);
> @@ -225,7 +228,8 @@ static struct dm_path *rr_select_path(struct path_selector *ps, size_t nr_bytes)
>  		set_percpu_current_path(s, pi->path);
>  		current_path = pi->path;
>  	}
> -	spin_unlock_irqrestore(&s->lock, flags);
> +	spin_unlock(&s->lock);
> +	local_irq_restore(flags);
> 
>  	return current_path;
>  }
> 

Ok, this works as far as the warnings don't appear anymore. But while
applying the patch and thinking about it, why local_irq_save() and not
preempt_disable()? "Sounds" like this is the function you want, and I
also stumbled across this in Documentation/preempt-locking.txt:

  But keep in mind that 'irqs disabled' is a fundamentally unsafe way of
  disabling preemption - any spin_unlock() decreasing the preemption
  count to 0 might trigger a reschedule. 

The spinlock would do an other nested preempt_disable(), but those even
out.


                                                    Beste Gr��e / Best regards,
                                                      - Benjamin Block
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