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

[Peter Zijlstra]

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.