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Message-Id: <20231120141824.86bda7ae184baf331e3175d9@kernel.org>
Date: Mon, 20 Nov 2023 14:18:24 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: "wuqiang.matt" <wuqiang.matt@...edance.com>
Cc: linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
mattwu@....com
Subject: Re: [PATCH v1] lib: objpool: fix head overrun on RK3588 SBC
On Tue, 14 Nov 2023 19:51:48 +0800
"wuqiang.matt" <wuqiang.matt@...edance.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@...nel.org>
And let me pick it.
> Signed-off-by: wuqiang.matt <wuqiang.matt@...edance.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@...nel.org>
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