[dm-devel] bug: using smp_processor_id() in preemptible code in rr_select_path()

[Benjamin Block]

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?


                                                    Beste Grüße / Best regards,
                                                      - Benjamin Block
-- 
Linux on z Systems Development         /         IBM Systems & Technology Group
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