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SubjectRe: [PATCH 2/2] nvme-fc: Wait with a timeout for queue to freeze
On Mon, Jul 05, 2021 at 06:34:00PM +0200, Daniel Wagner wrote:
> On Tue, Jun 29, 2021 at 09:39:30AM +0800, Ming Lei wrote:
> > Can you investigate a bit on why there is the hang? FC shouldn't use
> > managed IRQ, so the interrupt won't be shutdown.
>
> So far, I was not able to figure out why this hangs. In my test setup I
> don't have to do any I/O, I just toggle the remote port.
>
> grep busy /sys/kernel/debug/block/*/hctx*/tags | grep -v busy=0
>
> and this seems to confirm, no I/O in flight.

What is the output of the following command after the hang is triggered?

(cd /sys/kernel/debug/block/nvme0n1 && find . -type f -exec grep -aH . {} \;)

Suppose the hang disk is nvme0n1.

>
> So I started to look at the q_usage_counter. The obvious observational
> is that counter is not 0. The least bit is set, thus we are in atomic
> mode.
>
> (gdb) p/x *((struct request_queue*)0xffff8ac992fbef20)->q_usage_counter->data
> $10 = {
> count = {
> counter = 0x8000000000000001
> },
> release = 0xffffffffa02e78b0,
> confirm_switch = 0x0,
> force_atomic = 0x0,
> allow_reinit = 0x1,
> rcu = {
> next = 0x0,
> func = 0x0
> },
> ref = 0xffff8ac992fbef30
> }
>
> I am a bit confused about the percpu-refcount API. My naive
> interpretation is that when we are in atomic mode percpu_ref_is_zero()
> can't be used. But this seems rather strange. I must miss something.

No, percpu_ref_is_zero() is fine to be called in atomic mode.


Thanks,
Ming

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