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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
