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

[Vlastimil Babka]

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?

Thanks,
Vlastimil

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