Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Dmitry Vyukov]

On Sun, Jan 29, 2017 at 6:22 PM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 29.1.2017 13:44, Dmitry Vyukov wrote:
>> Hello,
>>
>> I've got the following deadlock report while running syzkaller fuzzer
>> on f37208bc3c9c2f811460ef264909dfbc7f605a60:
>>
>> [ INFO: possible circular locking dependency detected ]
>> 4.10.0-rc5-next-20170125 #1 Not tainted
>> -------------------------------------------------------
>> syz-executor3/14255 is trying to acquire lock:
>>  (cpu_hotplug.dep_map){++++++}, at: [<ffffffff814271c7>]
>> get_online_cpus+0x37/0x90 kernel/cpu.c:239
>>
>> but task is already holding lock:
>>  (pcpu_alloc_mutex){+.+.+.}, at: [<ffffffff81937fee>]
>> pcpu_alloc+0xbfe/0x1290 mm/percpu.c:897
>>
>> which lock already depends on the new lock.
>
> I suspect the dependency comes from recent changes in drain_all_pages(). They
> were later redone (for other reasons, but nice to have another validation) in
> the mmots patch [1], which AFAICS is not yet in mmotm and thus linux-next. Could
> you try if it helps?

It happened only once on linux-next, so I can't verify the fix. But I
will watch out for other occurrences.