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

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Aug 05 2016 at 11:33P -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.