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

[Tony Lu <tonylu@linux.alibaba.com>]

On Mon, Mar 06, 2023 at 08:38:52AM -0800, 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.
> 
> > ---
> >  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 with you. It is reasonable to use ENOBUFS here.

Thanks.

> >  	smc_cdc_add_pending_send(conn, pend);
> >  
> >  	conn->tx_cdc_seq++;
>