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

[Nilay Shroff]

On 10/30/24 07:50, Ming Lei wrote:
> 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 at 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
> 

Moving nvme_stop_keep_alive() from nvme_uninit_ctrl() to nvme_remove_admin_
tag_set() works, I tested it with blktest nvme/037. However, I am afraid 
that we may not be able to remove most of nvme_stop_keep_alive() from the
failure/recovery code path. The reason being, in most of the failure/recovery
code path we don't invoke nvme_remove_admin_tag_set(). 

For instance, in case of nvme_tcp_error_recovery_work(), the nvme_remove_
admin_tag_set() is not invoked and we explicitly call nvme_stop_keep_alive(). 
The same is true for error recovery case of nvme rdma.

Another example when nvme_tcp_setup_ctrl fails, we don't call nvme_remove_
admin_tag_set() but we explicitly call the nvme_stop_keep_alive(). 

Third example while resetting tcp/rdam ctrl, we don't call nvme_remove_admin_
tag_set() but we explicitly call the nvme_stop_keep_alive().

In your first point above, you mentioned that calling nvme_stop_keep_alive()
from nvme_uninit_ctrl() isn't correct, would you please explain why is it so, 
considering fact that we call nvme_start_keep_alive() from nvme_init_ctrl_
finish()?

Thanks,
--Nilay