Re: [PATCH V2 3/4] nvme: tcp: complete non-IO requests atomically

[Ming Lei <ming.lei@redhat.com>]

On Wed, Oct 21, 2020 at 10:20:11AM +0800, Chao Leng wrote:
> 
> 
> On 2020/10/21 9:22, Ming Lei wrote:
> > On Tue, Oct 20, 2020 at 05:04:29PM +0800, Chao Leng wrote:
> > > 
> > > 
> > > On 2020/10/20 16:53, Ming Lei wrote:
> > > > During controller's CONNECTING state, admin/fabric/connect requests
> > > > are submitted for recovery controller, and we allow to abort this request
> > > > directly in time out handler for not blocking setup procedure.
> > > > 
> > > > So timout vs. normal completion race exists on these requests since
> > > > admin/fabirc/connect queues won't be shutdown before handling timeout
> > > > during CONNECTING state.
> > > > 
> > > > Add atomic completion for requests from connect/fabric/admin queue for
> > > > avoiding the race.
> > > > 
> > > > CC: Chao Leng <lengchao@huawei.com>
> > > > Cc: Sagi Grimberg <sagi@grimberg.me>
> > > > Reported-by: Yi Zhang <yi.zhang@redhat.com>
> > > > Tested-by: Yi Zhang <yi.zhang@redhat.com>
> > > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > > ---
> > > >    drivers/nvme/host/tcp.c | 40 +++++++++++++++++++++++++++++++++++++---
> > > >    1 file changed, 37 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/drivers/nvme/host/tcp.c b/drivers/nvme/host/tcp.c
> > > > index d6a3e1487354..7e85bd4a8d1b 100644
> > > > --- a/drivers/nvme/host/tcp.c
> > > > +++ b/drivers/nvme/host/tcp.c
> > > > @@ -30,6 +30,8 @@ static int so_priority;
> > > >    module_param(so_priority, int, 0644);
> > > >    MODULE_PARM_DESC(so_priority, "nvme tcp socket optimize priority");
> > > > +#define REQ_STATE_COMPLETE     0
> > > > +
> > > >    enum nvme_tcp_send_state {
> > > >    	NVME_TCP_SEND_CMD_PDU = 0,
> > > >    	NVME_TCP_SEND_H2C_PDU,
> > > > @@ -56,6 +58,8 @@ struct nvme_tcp_request {
> > > >    	size_t			offset;
> > > >    	size_t			data_sent;
> > > >    	enum nvme_tcp_send_state state;
> > > > +
> > > > +	unsigned long		comp_state;
> > > I do not think adding state is a good idea.
> > > It is similar to rq->state.
> > > In the teardown process, after quiesced queues delete the timer and
> > > cancel the timeout work maybe a better option.
> > > I will send the patch later.
> > > The patch is already tested with roce more than one week.
> > 
> > Actually there isn't race between timeout and teardown, and patch 1 and patch
> > 2 are enough to fix the issue reported by Yi.
> > 
> > It is just that rq->state is updated to IDLE in its. complete(), so
> > either one of code paths may think that this rq isn't completed, and
> > patch 2 has addressed this issue.
> > 
> > In short, teardown lock is enough to cover the race.
> The race may cause abnormals:
> 1. Reported by Yi Zhang <yi.zhang@redhat.com>
> detail: https://lore.kernel.org/linux-nvme/1934331639.3314730.1602152202454.JavaMail.zimbra@redhat.com/
> 2. BUG_ON in blk_mq_requeue_request
> Because error recovery and time out may repeated completion request.
> First error recovery cancel request in tear down process, the request
> will be retried in completion, rq->state will be changed to IDEL.

Right.

> And then time out will complete the request again, and samely retry
> the request, BUG_ON will happen in blk_mq_requeue_request.

With patch2 in this patchset, timeout handler won't complete the request any
more.

> 3. abnormal link disconnection
> Firt error recovery cancel all request, reconnect success, the request
> will be restarted. And then time out will complete the request again,
> the queue will be stoped in nvme_rdma(tcp)_complete_timed_out,
> Abnormal link diconnection will happen. The condition(time out process
> is delayed long time by some reason such as hardware interrupt) is need.
> So the probability is low.

OK, the timeout handler may just get chance to run after recovery is
done, and it can be fixed by calling nvme_sync_queues() after
updating to CONNECTING or before updating to LIVE together with patch 1 & 2.

> teardown_lock just serialize the race. and add checkint the rq->state can avoid
> the 1 and 2 scenario, but 3 scenario can not be fixed.

I didn't think of scenario 3, which seems not triggered in Yi's test.


thanks,
Ming


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