linux-kernel - Re: [PATCH 2/2] nvme-fc: Wait with a timeout for queue to freeze

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Date:   Mon, 5 Jul 2021 18:34:00 +0200
From:   Daniel Wagner <dwagner@...e.de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        James Smart <james.smart@...adcom.com>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Sagi Grimberg <sagi@...mberg.me>
Subject: Re: [PATCH 2/2] nvme-fc: Wait with a timeout for queue to freeze

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.

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.