Re: [PATCH v2] cpuset: fix a deadlock due to incomplete patching of cpusets_enabled()

[Dima Zavin]

On Fri, Jul 28, 2017 at 7:05 AM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 07/28/2017 11:30 AM, Peter Zijlstra wrote:
>> On Fri, Jul 28, 2017 at 09:45:16AM +0200, Vlastimil Babka wrote:
>>> [+CC PeterZ]
>>>
>>> On 07/27/2017 06:46 PM, Dima Zavin wrote:
>>>> In codepaths that use the begin/retry interface for reading
>>>> mems_allowed_seq with irqs disabled, there exists a race condition that
>>>> stalls the patch process after only modifying a subset of the
>>>> static_branch call sites.
>>>>
>>>> This problem manifested itself as a dead lock in the slub
>>>> allocator, inside get_any_partial. The loop reads
>>>> mems_allowed_seq value (via read_mems_allowed_begin),
>>>> performs the defrag operation, and then verifies the consistency
>>>> of mem_allowed via the read_mems_allowed_retry and the cookie
>>>> returned by xxx_begin. The issue here is that both begin and retry
>>>> first check if cpusets are enabled via cpusets_enabled() static branch.
>>>> This branch can be rewritted dynamically (via cpuset_inc) if a new
>>>> cpuset is created. The x86 jump label code fully synchronizes across
>>>> all CPUs for every entry it rewrites. If it rewrites only one of the
>>>> callsites (specifically the one in read_mems_allowed_retry) and then
>>>> waits for the smp_call_function(do_sync_core) to complete while a CPU is
>>>> inside the begin/retry section with IRQs off and the mems_allowed value
>>>> is changed, we can hang. This is because begin() will always return 0
>>>> (since it wasn't patched yet) while retry() will test the 0 against
>>>> the actual value of the seq counter.
>>>
>>> Hm I wonder if there are other static branch users potentially having
>>> similar problem. Then it would be best to fix this at static branch
>>> level. Any idea, Peter? An inelegant solution would be to have indicate
>>> static_branch_(un)likely() callsites ordering for the patching. I.e.
>>> here we would make sure that read_mems_allowed_begin() callsites are
>>> patched before read_mems_allowed_retry() when enabling the static key,
>>> and the opposite order when disabling the static key.
>>
>> I'm not aware of any other sure ordering requirements. But you can
>> manually create this order by using 2 static keys. Then flip them in the
>> desired order.
>
> Right, thanks for the suggestion. I think that would be preferable to
> complicating the cookie handling. Add a new key next to
> cpusets_enabled_key, let's say "cpusets_enabled_pre_key". Make
> read_mems_allowed_begin() check this key instead of cpusets_enabled().
> Change cpuset_inc/dec to inc/dec also this new key in the right order
> and that should be it. Dima, can you try that or should I?

Yeah, I like that approach much better. I'll re-spin a new version in a bit.

--Dima

>
> Thanks,
> Vlastimil
>
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