Messages in this thread | | | Date | Mon, 20 Nov 2023 14:18:24 +0900 | From | Masami Hiramatsu (Google) <> | Subject | Re: [PATCH v1] lib: objpool: fix head overrun on RK3588 SBC |
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On Tue, 14 Nov 2023 19:51:48 +0800 "wuqiang.matt" <wuqiang.matt@bytedance.com> wrote:
> objpool overrun stress with test_objpool on OrangePi5+ SBC triggered the > following kernel warnings: > > WARNING: CPU: 6 PID: 3115 at lib/objpool.c:168 objpool_push+0xc0/0x100 > > This message is from objpool.c:168: > > WARN_ON_ONCE(tail - head > pool->nr_objs); > > The overrun test case is to validate the case that pre-allocated objects > are insufficient: 8 objects are pre-allocated for each node and consumer > thread per node tries to grab 16 objects in a row. The testing system is > OrangePI 5+, with RK3588, a big.LITTLE SOC with 4x A76 and 4x A55. When > disabling either all 4 big or 4 little cores, the overrun tests run well, > and once with big and little cores mixed together, the overrun test would > always cause an overrun loop. It's likely the memory timing differences > of big and little cores cause this trouble. Here are the debugging data > of objpool_try_get_slot after try_cmpxchg_release: > > objpool_pop: cpu: 4/0 0:0 head: 278/279 tail:278 last:276/278 > > The local copies of 'head' and 'last' were 278 and 276, and reloading of > 'slot->head' and 'slot->last' got 279 and 278. After try_cmpxchg_release > 'slot->head' became 'head + 1', which is correct. But what's wrong here > is the stale value of 'last', and that stale value of 'last' finally led > the overrun of 'head'.
Ah, good catch! So even if the ring size is enough, the head/tail update value is not updated locally, it can cause the overrun!
> > Memory updating of 'last' and 'head' are performed in push() and pop() > independently, which could be the culprit leading this out of order > visibility of 'last' and 'head'. So for objpool_try_get_slot(), it's > not enough only checking the condition of 'head != slot', the implicit > condition 'last - head <= nr_objs' must also be explicitly asserted to > guarantee 'last' is always behind 'head' before the object retrieving.
Indeed. Thanks for the investigation!
> > This patch will check and try reloading of 'head' and 'last' to ensure > 'last' is behind 'head' at the time of object retrieving. Performance > testings show the average impact is about 0.1% for X86_64 and 1.12% for > ARM64. Here are the results: > > OS: Debian 10 X86_64, Linux 6.6rc > HW: XEON 8336C x 2, 64 cores/128 threads, DDR4 3200MT/s > 1T 2T 4T 8T 16T > native: 49543304 99277826 199017659 399070324 795185848 > objpool: 29909085 59865637 119692073 239750369 478005250 > objpool+: 29879313 59230743 119609856 239067773 478509029 > 32T 48T 64T 96T 128T > native: 1596927073 2390099988 2929397330 3183875848 3257546602 > objpool: 957553042 1435814086 1680872925 2043126796 2165424198 > objpool+: 956476281 1434491297 1666055740 2041556569 2157415622 > > OS: Debian 11 AARCH64, Linux 6.6rc > HW: Kunpeng-920 96 cores/2 sockets/4 NUMA nodes, DDR4 2933 MT/s > 1T 2T 4T 8T 16T > native: 30890508 60399915 123111980 242257008 494002946 > objpool: 14742531 28883047 57739948 115886644 232455421 > objpool+: 14107220 29032998 57286084 113730493 232232850 > 24T 32T 48T 64T 96T > native: 746406039 1000174750 1493236240 1998318364 2942911180 > objpool: 349164852 467284332 702296756 934459713 1387898285 > objpool+: 348388180 462750976 696606096 927865887 1368402195 >
OK, looks good to me.
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
And let me pick it.
> Signed-off-by: wuqiang.matt <wuqiang.matt@bytedance.com>
BTW, this is a real bugfix, so it should have Fixes tag :)
Fixes: b4edb8d2d464 ("lib: objpool added: ring-array based lockless MPMC")
Thank you!
> --- > lib/objpool.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/lib/objpool.c b/lib/objpool.c > index ce0087f64400..cfdc02420884 100644 > --- a/lib/objpool.c > +++ b/lib/objpool.c > @@ -201,6 +201,23 @@ static inline void *objpool_try_get_slot(struct objpool_head *pool, int cpu) > while (head != READ_ONCE(slot->last)) { > void *obj; > > + /* > + * data visibility of 'last' and 'head' could be out of > + * order since memory updating of 'last' and 'head' are > + * performed in push() and pop() independently > + * > + * before any retrieving attempts, pop() must guarantee > + * 'last' is behind 'head', that is to say, there must > + * be available objects in slot, which could be ensured > + * by condition 'last != head && last - head <= nr_objs' > + * that is equivalent to 'last - head - 1 < nr_objs' as > + * 'last' and 'head' are both unsigned int32 > + */ > + if (READ_ONCE(slot->last) - head - 1 >= pool->nr_objs) { > + head = READ_ONCE(slot->head); > + continue; > + } > + > /* obj must be retrieved before moving forward head */ > obj = READ_ONCE(slot->entries[head & slot->mask]); > > -- > 2.40.1 >
-- Masami Hiramatsu (Google) <mhiramat@kernel.org>
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