On Fri, Jul 28, 2017 at 12:45 AM, Vlastimil Babka <vbabka@xxxxxxx> 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. > This was my main worry, that I'm just patching up one incarnation of this problem and other clients will eventually trip over this. >> The fix is to cache the value that's returned by cpusets_enabled() at the >> top of the loop, and only operate on the seqcount (both begin and retry) if >> it was true. > > Maybe we could just return e.g. -1 in read_mems_allowed_begin() when > cpusets are disabled, and test it in read_mems_allowed_retry() before > doing a proper seqcount retry check? Also I think you can still do the > cpusets_enabled() check in read_mems_allowed_retry() before the > was_enabled (or cookie == -1) test? Hmm, good point! If cpusets_enabled() is true, then we can still test against was_enabled and do the right thing (adds one extra branch in that case). When it's false, we still benefit from the static_branch fanciness. Thanks! Re setting the cookie to -1, I didn't really want to overload the cookie value but rather just make the state explicit so it's easier to grawk as this is all already subtle enough. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
