[dpdk-dev] [PATCH 2/2] ethdev: fix the race condition for fp ops reset

[Bing Zhao]

In the function "eth_dev_fp_ops_reset", a structure assignment
operation is used to reset one queue's callback functions, etc., but
it is not thread safe.

The structure assignment is not atomic, a lot of instructions will
be generated. Right now, since not all the fields are needed, the
fields in the "dummy_ops" which is not set explicitly will be 0s
based on the specification and compiler behavior. In order to make
"fpo" has the same content with "dummy_ops", some clearing to 0
operation is needed.

By checking the object instructions (e.g. with GCC 4.8.5)
   0x0000000000a58317 <+35>:	mov    %rsi,%rdi
   0x0000000000a5831a <+38>:	mov    %rdx,%rcx
=> 0x0000000000a5831d <+41>:	rep stos %rax,%es:(%rdi)
   0x0000000000a58320 <+44>:	mov    -0x38(%rsp),%rax
   0x0000000000a58325 <+49>:	lea    -0xe0(%rip),%rdx
        // # 0xa5824c <dummy_eth_rx_burst>

It shows that "rep stos" will clear the "fpo" structure before
assigning new values.

In the other thread, if some data path Tx / Rx functions are still
running, there is a risk to get 0 instead of the correct dummy
content.
  1. qd = p->rxq.data[queue_id]
  2. (void **)&p->rxq.clbk[queue_id]
"data" and "clbk" may be observed with NULL (0) in other threads.
Even it is temporary, the accessing to a NULL pointer will cause a
crash. Using "memcpy" could get rid of this.

Fixes: c87d435a4d79 ("ethdev: copy fast-path API into separate structure")
Cc: konstantin.ananyev at intel.com

Signed-off-by: Bing Zhao <bingz at nvidia.com>
---
 lib/ethdev/ethdev_private.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/ethdev/ethdev_private.c b/lib/ethdev/ethdev_private.c
index fbc3df91ad..cda9a6e228 100644
--- a/lib/ethdev/ethdev_private.c
+++ b/lib/ethdev/ethdev_private.c
@@ -206,7 +206,7 @@ eth_dev_fp_ops_reset(struct rte_eth_fp_ops *fpo)
 		.txq = {.data = dummy_data, .clbk = dummy_data,},
 	};
 
-	*fpo = dummy_ops;
+	rte_memcpy(fpo, &dummy_ops, sizeof(struct rte_eth_fp_ops));
 }
 
 void
-- 
2.27.0