Re: [PATCH net] net/smc: fix NULL sndbuf_desc in smc_cdc_tx_handler()

[Wenjia Zhang <wenjia@linux.ibm.com>]

On 06.03.23 17:38, Alexander H Duyck wrote:
> On Mon, 2023-03-06 at 11:36 +0800, D. Wythe wrote:
>> From: "D. Wythe" <alibuda@linux.alibaba.com>
>>
>> When performing a stress test on SMC-R by rmmod mlx5_ib driver
>> during the wrk/nginx test, we found that there is a probability
>> of triggering a panic while terminating all link groups.
>>
>> This issue dues to the race between smc_smcr_terminate_all()
>> and smc_buf_create().
>>
>> 			smc_smcr_terminate_all
>>
>> smc_buf_create
>> /* init */
>> conn->sndbuf_desc = NULL;
>> ...
>>
>> 			__smc_lgr_terminate
>> 				smc_conn_kill
>> 					smc_close_abort
>> 						smc_cdc_get_slot_and_msg_send
>>
>> 			__softirqentry_text_start
>> 				smc_wr_tx_process_cqe
>> 					smc_cdc_tx_handler
>> 						READ(conn->sndbuf_desc->len);
>> 						/* panic dues to NULL sndbuf_desc */
>>
>> conn->sndbuf_desc = xxx;
>>
>> This patch tries to fix the issue by always to check the sndbuf_desc
>> before send any cdc msg, to make sure that no null pointer is
>> seen during cqe processing.
>>
>> Fixes: 0b29ec643613 ("net/smc: immediate termination for SMCR link groups")
>> Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
> 
> Looking at the code for __smc_buf_create it seems like you might have
> more issues hiding in the code. From what I can tell smc_buf_get_slot
> can only return a pointer or NULL but it is getting checked for being
> being a PTR_ERR or IS_ERR in several spots that are likely all dead
> code.
> 
This smc_buf_get_slot() is used to get a reusable slot, which is 
originally assigned by smcr_new_buf_create() or smcd_new_buf_create() 
depending on the device being used. In 
smcr_new_buf_create()/smcd_new_buf_create(), the pointer values of the 
return codes are converted from integer values.

>> ---
>>   net/smc/smc_cdc.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/net/smc/smc_cdc.c b/net/smc/smc_cdc.c
>> index 53f63bf..2f0e2ee 100644
>> --- a/net/smc/smc_cdc.c
>> +++ b/net/smc/smc_cdc.c
>> @@ -114,6 +114,9 @@ int smc_cdc_msg_send(struct smc_connection *conn,
>>   	union smc_host_cursor cfed;
>>   	int rc;
>>   
>> +	if (unlikely(!READ_ONCE(conn->sndbuf_desc)))
>> +		return -EINVAL;
>> +
> 
> This return value doesn't seem right to me. Rather than en EINVAL
> should this be something like a ENOBUFS just to make it easier to debug
> when this issue is encountered?
I agree.
> 
>>   	smc_cdc_add_pending_send(conn, pend);
>>   
>>   	conn->tx_cdc_seq++;
> 
>