Comment 0 for bug 1799397

Hi, Christian

We've recently encountered a weird issue with Ubuntu 18.04 on the Skylake
server. I can always reproduce this crash and I could narrowed it down. I guess
it could be a GCC issue.

[1] How to reproduce
- ConnectX-4Lx/ConnectX-5 with mlx5 PMD in DPDK 18.02.1
- Ubuntu 18.04 on Intel Skylake server
- gcc (Ubuntu 7.3.0-16ubuntu3) 7.3.0
- Testpmd crashes when it starts to forward traffic. Easy to reproduce.
- Only happens on the Skylake server.
- DPDK 18.05 and later don't have such issue. git-bisect gives no clue.

This is because I enabled MEMPOOL_DEBUG and MLX5_DEBUG. As mempool/rte_memcpy is
inlined function, it should be affected. Now I can see the crash regardlessly -
18.02, 18.05 and 18.08.

[2] Failure point

The attached patch gives an insight of why it crashes. The following is the
result of the patch and the GDB commands.

In summary, rte_memcpy() doesn't work as expected. In __mempool_generic_put(),
there's rte_memcpy() to move the array of objects to the lcore cache. If I run
memcmp() right after rte_memcpy(dst, src, n), data in dst differs from data in
src. And it looks like some of data got shifted by a few bytes as you can see
below.

 [GDB command]
 $dst = 0x7ffff4e09ea8
 $src = 0x7fffce3fb970
 $n = 256
 x/32gx 0x7ffff4e09ea8
 x/32gx 0x7fffce3fb970
 testpmd: /home/mlnxtest/dpdk/build/include/rte_mempool.h:1140: __mempool_generic_put: Assertion `0' failed.

 Thread 4 "lcore-slave-1" received signal SIGABRT, Aborted.
 [Switching to Thread 0x7fffce3ff700 (LWP 69913)]
 (gdb) x/32gx 0x7ffff4e09ea8
 0x7ffff4e09ea8: 0x00007fffaac38ec0 0x00007fffaac38500
 0x7ffff4e09eb8: 0x00007fffaac37b40 0x00007fffaac37180
 0x7ffff4e09ec8: 0x850000007fffaac3 0x7b4000007fffaac3
 0x7ffff4e09ed8: 0x00007fffaac35440 0x00007fffaac34a80
 0x7ffff4e09ee8: 0xaac3850000007fff 0xaac37b4000007fff
 0x7ffff4e09ef8: 0x00007fffaac32d40 0x00007fffaac32380
 0x7ffff4e09f08: 0x7fffaac385000000 0x7fffaac37b400000
 0x7ffff4e09f18: 0x00007fffaac30640 0x00007fffaac2fc80
 0x7ffff4e09f28: 0x00007fffaac2f2c0 0x00007fffaac2e900
 0x7ffff4e09f38: 0x00007fffaac2df40 0x00007fffaac2d580
 0x7ffff4e09f48: 0x00007fffaac2cbc0 0x00007fffaac2c200
 0x7ffff4e09f58: 0x00007fffaac2b840 0x00007fffaac2ae80
 0x7ffff4e09f68: 0x00007fffaac2a4c0 0x00007fffaac29b00
 0x7ffff4e09f78: 0x00007fffaac29140 0x00007fffaac28780
 0x7ffff4e09f88: 0x00007fffaac27dc0 0x00007fffaac27400
 0x7ffff4e09f98: 0x00007fffaac26a40 0x00007fffaac26080
 (gdb) x/32gx 0x7fffce3fb970
 0x7fffce3fb970: 0x00007fffaac38ec0 0x00007fffaac38500
 0x7fffce3fb980: 0x00007fffaac37b40 0x00007fffaac37180
 0x7fffce3fb990: 0x00007fffaac367c0 0x00007fffaac35e00
 0x7fffce3fb9a0: 0x00007fffaac35440 0x00007fffaac34a80
 0x7fffce3fb9b0: 0x00007fffaac340c0 0x00007fffaac33700
 0x7fffce3fb9c0: 0x00007fffaac32d40 0x00007fffaac32380
 0x7fffce3fb9d0: 0x00007fffaac319c0 0x00007fffaac31000
 0x7fffce3fb9e0: 0x00007fffaac30640 0x00007fffaac2fc80
 0x7fffce3fb9f0: 0x00007fffaac2f2c0 0x00007fffaac2e900
 0x7fffce3fba00: 0x00007fffaac2df40 0x00007fffaac2d580
 0x7fffce3fba10: 0x00007fffaac2cbc0 0x00007fffaac2c200
 0x7fffce3fba20: 0x00007fffaac2b840 0x00007fffaac2ae80
 0x7fffce3fba30: 0x00007fffaac2a4c0 0x00007fffaac29b00
 0x7fffce3fba40: 0x00007fffaac29140 0x00007fffaac28780
 0x7fffce3fba50: 0x00007fffaac27dc0 0x00007fffaac27400
 0x7fffce3fba60: 0x00007fffaac26a40 0x00007fffaac26080

AFAIK, AVX512F support is disabled by default in DPDK as it is still
experimental (CONFIG_RTE_ENABLE_AVX512=n). But with gcc optimization, AVX2
version of rte_memcpy() seems to be optimized with 512b instructions. If I
disable it by adding EXTRA_CFLAGS="-mno-avx512f", then it works fine and doesn't
crash.

Do you have any idea regarding this issue or are you already aware of it?

Thanks,
Yongseok

$ git diff
diff --git a/config/common_base b/config/common_base
index ad03cf433..f512b5a88 100644
--- a/config/common_base
+++ b/config/common_base
@@ -275,8 +275,8 @@ CONFIG_RTE_LIBRTE_MLX4_TX_MP_CACHE=8
 #
 # Compile burst-oriented Mellanox ConnectX-4 & ConnectX-5 (MLX5) PMD
 #
-CONFIG_RTE_LIBRTE_MLX5_PMD=n
-CONFIG_RTE_LIBRTE_MLX5_DEBUG=n
+CONFIG_RTE_LIBRTE_MLX5_PMD=y
+CONFIG_RTE_LIBRTE_MLX5_DEBUG=y
 CONFIG_RTE_LIBRTE_MLX5_DLOPEN_DEPS=n
 CONFIG_RTE_LIBRTE_MLX5_TX_MP_CACHE=8

@@ -597,7 +597,7 @@ CONFIG_RTE_RING_USE_C11_MEM_MODEL=n
 #
 CONFIG_RTE_LIBRTE_MEMPOOL=y
 CONFIG_RTE_MEMPOOL_CACHE_MAX_SIZE=512
-CONFIG_RTE_LIBRTE_MEMPOOL_DEBUG=n
+CONFIG_RTE_LIBRTE_MEMPOOL_DEBUG=y

 #
 # Compile Mempool drivers
diff --git a/lib/librte_mempool/rte_mempool.h b/lib/librte_mempool/rte_mempool.h
index 8b1b7f7ed..9f48028d9 100644
--- a/lib/librte_mempool/rte_mempool.h
+++ b/lib/librte_mempool/rte_mempool.h
@@ -39,6 +39,7 @@
 #include <errno.h>
 #include <inttypes.h>
 #include <sys/queue.h>
+#include <assert.h>

 #include <rte_config.h>
 #include <rte_spinlock.h>
@@ -1123,6 +1124,22 @@ __mempool_generic_put(struct rte_mempool *mp, void * const *obj_table,
        /* Add elements back into the cache */
        rte_memcpy(&cache_objs[0], obj_table, sizeof(void *) * n);

+ if(memcmp(&cache_objs[0], obj_table, sizeof(void *) * n)) {
+ printf("[GDB command] \n"
+ "$dst = %p\n"
+ "$src = %p\n"
+ "$n = %ld\n"
+ "x/%ldgx %p\n"
+ "x/%ldgx %p\n",
+ (void *)&cache_objs[0],
+ (const void *)obj_table,
+ sizeof(void *) * n,
+ sizeof(void *) * n / 8, (void *)&cache_objs[0],
+ sizeof(void *) * n / 8, (const void *)obj_table
+ );
+ assert(0);
+ }
+
        cache->len += n;

        if (cache->len >= cache->flushthresh) {