linux-kernel - Re: [PATCH] nvme-pci: use NOWAIT flag for nvme_set_host

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Date:   Mon, 29 Jan 2018 13:17:16 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Jianchao Wang <jianchao.w.wang@...cle.com>, axboe@...com,
        hch@....de, linux-nvme@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme-pci: use NOWAIT flag for nvme_set_host_mem

On Mon, Jan 29, 2018 at 09:55:41PM +0200, Sagi Grimberg wrote:
> > Thanks for the fix. It looks like we still have a problem, though.
> > Commands submitted with the "shutdown_lock" held need to be able to make
> > forward progress without relying on a completion, but this one could
> > block indefinitely.
> 
> Can you explain to me why is the shutdown_lock needed to synchronize
> nvme_dev_disable? More concretely, how is nvme_dev_disable different
> from other places where we rely on the ctrl state to serialize stuff?
> 
> The only reason I see would be to protect against completion-after-abort
> scenario but I think the block layer should protect against it (checks
> if the request timeout timer fired).

We can probably find a way to use the state machine for this. Disabling
the controller pre-dates the state machine, and the mutex is there to
protect against two actors shutting the controller down at the same
time, like a hot removal at the same time as a timeout handling reset.