Re: bug: using smp_processor_id() in preemptible code in rr_select_path()

[Mike Snitzer <snitzer@redhat.com>]

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.
> >>>>
> >>>>
> >>>>There are several more of these warnings, but all have the same
> >>>>stack-trace (this is on s390x, but this looks like its only common code)
> >>>>- sometimes the process-context is multipath.
> >>>>
> >>>>Looking at the changes in this function, it rather looks like this is
> >>>>caused by changes made in commit
> >>>>b0b477c7e0dd93f8916d106018ded1331b81bf61 (dm round robin: use percpu
> >>>>'repeat_count' and 'current_path').
> >>>>
> >>>>The kernel is a stock v4.7 with some debug options enabled (prominently
> >>>>DEBUG_PREEMPT). Need any more info?
> >>>
> >>>As you can see from commit b0b477c7e0dd9 the round-robin path selector
> >>>is now using percpu data (pointer) and a percpu_counter.
> >>>
> >>>I'm really not sure how else to access this percpu data.
> >>>
> >>>Cc'ing LKML, linux-block, Jens and hch to cast a wider net in the hopes
> >>>of getting an answer to how to fix this.
> >>
> >>From a quick look, looks like you are using this_cpu_ptr() without
> >>having preemption disabled.
> >
> >Right, that is what it looked like to me too.
> >
> >I'm just not sure on what the proper pattern is to fix this.
> >
> >I'll look closer though.
> 
> 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;
 }