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    SubjectRe: [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.

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