[PATCH 3/4] nvmet: fix hang in nvmet_ns_disable()

[Chaitanya Kulkarni]

On 1/3/23 02:03, Taehee Yoo wrote:
> nvme target namespace is enabled or disabled by nvmet_ns_enable() or
> nvmet_ns_disable().
> The subsys->lock is used to disallow to use namespace data while
> nvmet_ns_enable() or nvmet_ns_disable() are working.
> The ns->enabled boolean variable prevents using namespace data in wrong
> state such as uninitialized state.
> 
> nvmet_ns_disable() acquires ns->lock and set ns->enabled false.
> Then, it releases ns->lock for a while to wait ns->disable_done completion.
> At this point, nvmet_ns_enable() can be worked concurrently and it calls
> percpu_ref_init().
> So, ns->disable_done will never be completed.
> Therefore hang would occur at this point.
> 
>     CPU0                                     CPU1
>     nvmet_ns_disable();
>     mutex_lock(&subsys->lock);               nvmet_ns_enable();
>                                              mutex_lock(&subsys->lock);
>     ns->enabled = false;
>     mutex_unlock(&subsys->lock);
>                                              percpu_ref_init();
>     wait_for_completion(&ns->disable_done);  <-- infinite wait
> 
>     mutex_lock(&subsys->lock);
>     mutex_unlock(&subsys->lock);
> 
> INFO: task bash:926 blocked for more than 30 seconds.
>        Tainted: G        W          6.1.0+ #17
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:bash            state:D stack:27200 pid:926   ppid:911
> flags:0x00004000
> Call Trace:
>   <TASK>
>   __schedule+0xafc/0x2930
>   ? io_schedule_timeout+0x160/0x160
>   ? _raw_spin_unlock_irq+0x24/0x50
>   ? __wait_for_common+0x39b/0x5c0
>   ? usleep_range_state+0x190/0x190
>   schedule+0x130/0x230
>   schedule_timeout+0x18a/0x240
>   ? usleep_range_state+0x190/0x190
>   ? rcu_read_lock_sched_held+0x12/0x80
>   ? lock_downgrade+0x700/0x700
>   ? do_raw_spin_trylock+0xb5/0x180
>   ? lock_contended+0xdf0/0xdf0
>   ? _raw_spin_unlock_irq+0x24/0x50
>   ? trace_hardirqs_on+0x3c/0x190
>   __wait_for_common+0x1ca/0x5c0
>   ? usleep_range_state+0x190/0x190
>   ? bit_wait_io+0xf0/0xf0
>   ? _raw_spin_unlock_irqrestore+0x59/0x70
>   nvmet_ns_disable+0x288/0x490
>   ? nvmet_ns_enable+0x970/0x970
>   ? lockdep_hardirqs_on_prepare+0x410/0x410
>   ? rcu_read_lock_sched_held+0x12/0x80
>   ? configfs_write_iter+0x1df/0x480
>   ? nvmet_ns_revalidate_size_store+0x220/0x220
>   nvmet_ns_enable_store+0x85/0xe0
> [ ... ]
> 
> Fixes: a07b4970f464 ("nvmet: add a generic NVMe target")
> Signed-off-by: Taehee Yoo <ap420073 at gmail.com>

[...]

> diff --git a/drivers/nvme/target/nvmet.h b/drivers/nvme/target/nvmet.h
> index 89bedfcd974c..e609787577c6 100644
> --- a/drivers/nvme/target/nvmet.h
> +++ b/drivers/nvme/target/nvmet.h
> @@ -56,6 +56,12 @@
>   #define IPO_IATTR_CONNECT_SQE(x)	\
>   	(cpu_to_le32(offsetof(struct nvmf_connect_command, x)))
>   
> +enum nvmet_ns_state {
> +	NVMET_NS_ENABLED,
> +	NVMET_NS_DISABLING,
> +	NVMET_NS_DISABLED
> +};
> +

The patch looks good to me, but I'm wondering if there is a way
we can do this without adding new enable disable states ?

-ck