linux-kernel - Re: bug: using smp_processor_id() in preemptible code in rr_select

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Message-ID: <107dc81f-675d-8ae4-0ac2-99c248a23c60@kernel.dk>
Date:	Fri, 5 Aug 2016 09:54:35 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Benjamin Block <bblock@...ux.vnet.ibm.com>, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org, hch@....de,
	linux-block@...r.kernel.org
Subject: Re: bug: using smp_processor_id() in preemptible code in
 rr_select_path()

On 08/05/2016 09:42 AM, Mike Snitzer wrote:
> On Fri, Aug 05 2016 at 11:33P -0400,
> Jens Axboe <axboe@...nel.dk> wrote:
>
>> On 08/05/2016 09:27 AM, Mike Snitzer wrote:
>>> On Wed, Aug 03 2016 at 11:35am -0400,
>>> Benjamin Block <bblock@...ux.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.

-- 
Jens Axboe