Re: [PATCH 2/3] nvme-fabrics: fix kernel crash while shutting down controller

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

On Tue, Oct 29, 2024 at 06:10:00PM +0530, Nilay Shroff wrote:
> 
> 
> On 10/29/24 12:53, Ming Lei wrote:
> > On Sun, Oct 27, 2024 at 10:32:05PM +0530, Nilay Shroff wrote:
> >> The nvme keep-alive operation, which executes at a periodic interval,
> >> could potentially sneak in while shutting down a fabric controller.
> >> This may lead to a race between the fabric controller admin queue
> >> destroy code path (invoked while shutting down controller) and hw/hctx
> >> queue dispatcher called from the nvme keep-alive async request queuing
> >> operation. This race could lead to the kernel crash shown below:
> >>
> >> Call Trace:
> >>     autoremove_wake_function+0x0/0xbc (unreliable)
> >>     __blk_mq_sched_dispatch_requests+0x114/0x24c
> >>     blk_mq_sched_dispatch_requests+0x44/0x84
> >>     blk_mq_run_hw_queue+0x140/0x220
> >>     nvme_keep_alive_work+0xc8/0x19c [nvme_core]
> >>     process_one_work+0x200/0x4e0
> >>     worker_thread+0x340/0x504
> >>     kthread+0x138/0x140
> >>     start_kernel_thread+0x14/0x18
> >>
> >> While shutting down fabric controller, if nvme keep-alive request sneaks
> >> in then it would be flushed off. The nvme_keep_alive_end_io function is
> >> then invoked to handle the end of the keep-alive operation which
> >> decrements the admin->q_usage_counter and assuming this is the last/only
> >> request in the admin queue then the admin->q_usage_counter becomes zero.
> >> If that happens then blk-mq destroy queue operation (blk_mq_destroy_
> >> queue()) which could be potentially running simultaneously on another
> >> cpu (as this is the controller shutdown code path) would forward
> >> progress and deletes the admin queue. So, now from this point onward
> >> we are not supposed to access the admin queue resources. However the
> >> issue here's that the nvme keep-alive thread running hw/hctx queue
> >> dispatch operation hasn't yet finished its work and so it could still
> >> potentially access the admin queue resource while the admin queue had
> >> been already deleted and that causes the above crash.
> >>
> >> The above kernel crash is regression caused due to changes implemented
> >> in commit a54a93d0e359 ("nvme: move stopping keep-alive into
> >> nvme_uninit_ctrl()"). Ideally we should stop keep-alive at the very
> >> beggining of the controller shutdown code path so that it wouldn't
> >> sneak in during the shutdown operation. However we removed the keep
> >> alive stop operation from the beginning of the controller shutdown
> >> code path in commit a54a93d0e359 ("nvme: move stopping keep-alive into
> >> nvme_uninit_ctrl()") and that now created the possibility of keep-alive
> >> sneaking in and interfering with the shutdown operation and causing
> >> observed kernel crash. So to fix this crash, now we're adding back the
> >> keep-alive stop operation at very beginning of the fabric controller
> >> shutdown code path so that the actual controller shutdown opeation only
> >> begins after it's ensured that keep-alive operation is not in-flight and
> >> also it can't be scheduled in future.
> >>
> >> Fixes: a54a93d0e359 ("nvme: move stopping keep-alive into nvme_uninit_ctrl()")
> >> Link: https://lore.kernel.org/all/196f4013-3bbf-43ff-98b4-9cb2a96c20c2@grimberg.me/#t
> >> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> >> ---
> >>  drivers/nvme/host/core.c | 5 +++++
> >>  1 file changed, 5 insertions(+)
> >>
> >> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> >> index 5016f69e9a15..865c00ea19e3 100644
> >> --- a/drivers/nvme/host/core.c
> >> +++ b/drivers/nvme/host/core.c
> >> @@ -4648,6 +4648,11 @@ void nvme_stop_ctrl(struct nvme_ctrl *ctrl)
> >>  {
> >>  	nvme_mpath_stop(ctrl);
> >>  	nvme_auth_stop(ctrl);
> >> +	/*
> >> +	 * the transport driver may be terminating the admin tagset a little
> >> +	 * later on, so we cannot have the keep-alive work running
> >> +	 */
> >> +	nvme_stop_keep_alive(ctrl);
> >>  	nvme_stop_failfast_work(ctrl);
> >>  	flush_work(&ctrl->async_event_work);
> >>  	cancel_work_sync(&ctrl->fw_act_work);
> > 
> > The change looks fine.
> > 
> > IMO the `nvme_stop_keep_alive` in nvme_uninit_ctrl() may be moved to
> > entry of nvme_remove_admin_tag_set(), then this one in nvme_stop_ctrl()
> > can be saved?
> > 
> Yes that should work however IMO, stopping keep-alive at very beginning of
> shutdown operation would make sense because delaying the stopping of keep-alive 
> would not be useful anyways once we start the controller shutdown. It may
> sneak in unnecessarily while we shutdown controller and later we will have to 
> flush it off.
> 
> And yes, as you mentioned, in this case we would save one call site but 
> looking at the code we have few other call sites already present where we
> call nvme_stop_keep_alive().

If it works, I'd suggest to move nvme_stop_keep_alive() from
nvme_uninit_ctrl() into nvme_remove_admin_tag_set() because:

1) it isn't correct to do it in nvme_uninit_ctrl()

2) stop keep alive in nvme_remove_admin_tag_set() covers everything

3) most of nvme_stop_keep_alive() can be removed in failure path


Thanks,
Ming