Re: [dm-devel] bug: using smp_processor_id() in preemptible code in rr_select_path()
From: Jens Axboe
Date: Mon Aug 08 2016 - 12:39:34 EST
On 08/08/2016 10:32 AM, Benjamin Block wrote:
On 12:06 Fri 05 Aug , Mike Snitzer wrote:
On Fri, Aug 05 2016 at 11:54am -0400,
Jens Axboe <axboe@xxxxxxxxx> wrote:
On 08/05/2016 09:42 AM, Mike Snitzer wrote:
On Fri, Aug 05 2016 at 11:33P -0400,
Jens Axboe <axboe@xxxxxxxxx> wrote:
On 08/05/2016 09:27 AM, Mike Snitzer wrote:
On Wed, Aug 03 2016 at 11:35am -0400,
Benjamin Block <bblock@xxxxxxxxxxxxxxxxxx> 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.
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