On 08/08/2016 10:32 AM, Benjamin Block wrote:

On 12:06 Fri 05 Aug     , Mike Snitzer wrote:

quoted

On Fri, Aug 05 2016 at 11:54am -0400,
Jens Axboe [off-list ref] wrote:

quoted

On 08/05/2016 09:42 AM, Mike Snitzer wrote:

quoted

On Fri, Aug 05 2016 at 11:33P -0400,
Jens Axboe [off-list ref] wrote:

quoted

On 08/05/2016 09:27 AM, Mike Snitzer wrote:

quoted

On Wed, Aug 03 2016 at 11:35am -0400,
Benjamin Block [off-list ref] wrote:

quoted

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:]

quoted

quoted

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.

local_irq_save(), since we need to grab the lock irq safe very shortly
anyway. As long as they nest properly, the approach is fine, and it's
more efficient than first doing a preempt_disable(), then still needing
a irq safe spinlock.

-- 
Jens Axboe