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

[Jens Axboe]

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.
> > > >
> > > >
> > > > 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.

--
Jens Axboe