[PATCH] blk-mq: avoid to hang in the cpuhp offline handler

[Christoph Hellwig]

On Tue, Sep 20, 2022 at 10:17:24AM +0800, Ming Lei wrote:
> For avoiding to trigger io timeout when one hctx becomes inactive, we
> drain IOs when all CPUs of one hctx are offline. However, driver's
> timeout handler may require cpus_read_lock, such as nvme-pci,
> pci_alloc_irq_vectors_affinity() is called in nvme-pci reset context,
> and irq_build_affinity_masks() needs cpus_read_lock().
> 
> Meantime when blk-mq's cpuhp offline handler is called, cpus_write_lock
> is held, so deadlock is caused.
> 
> Fixes the issue by breaking the wait loop if enough long time elapses,
> and these in-flight not drained IO still can be handled by timeout
> handler.

I'm not sure that this actually is a good idea on its own, and it kinda
defeats the cpu hotplug processing.

So if I understand your log above correctly the problem is that
we have commands that would time out, and we exacalate to a
controller reset that is racing with the CPU unplug, or if not can
you explain the scenario a little more?

> Cc: linux-nvme at lists.infradead.org
> Reported-by: Yi Zhang <yi.zhang at redhat.com>
> Fixes: bf0beec0607d ("blk-mq: drain I/O when all CPUs in a hctx are offline")
> Signed-off-by: Ming Lei <ming.lei at redhat.com>
> ---
>  block/blk-mq.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c96c8c4f751b..4585985b8537 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3301,6 +3301,7 @@ static inline bool blk_mq_last_cpu_in_hctx(unsigned int cpu,
>  	return true;
>  }
>  
> +#define BLK_MQ_MAX_OFFLINE_WAIT_MSECS 3000
>  static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
>  {
>  	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
> @@ -3326,8 +3327,13 @@ static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
>  	 * frozen and there are no requests.
>  	 */
>  	if (percpu_ref_tryget(&hctx->queue->q_usage_counter)) {
> -		while (blk_mq_hctx_has_requests(hctx))
> +		unsigned int wait_ms = 0;
> +
> +		while (blk_mq_hctx_has_requests(hctx) && wait_ms <
> +				BLK_MQ_MAX_OFFLINE_WAIT_MSECS) {
>  			msleep(5);
> +			wait_ms += 5;
> +		}
>  		percpu_ref_put(&hctx->queue->q_usage_counter);
>  	}
>  
> -- 
> 2.31.1
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