@@ -199,6 +199,23 @@ 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]);
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'. 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. 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 Fixes: b4edb8d2d464 ("lib: objpool added: ring-array based lockless MPMC") v1 -> v2: - Title updated since it's a common issue of objpool for big.LITTLE systems, verified on Rockchip RK3588 and Amlogic A311D - Fixes tag added, as reminded by Masami Hiramatsu Signed-off-by: wuqiang.matt <wuqiang.matt@bytedance.com> Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> --- lib/objpool.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+)