linux-kernel - Re: [PATCH 0/4] Rework NVMe abort handling

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Message-ID: <20180719142355.GA18800@lst.de>
Date:   Thu, 19 Jul 2018 16:23:55 +0200
From:   Christoph Hellwig <hch@....de>
To:     Johannes Thumshirn <jthumshirn@...e.de>
Cc:     Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <keith.busch@...el.com>,
        James Smart <james.smart@...adcom.com>,
        Hannes Reinecke <hare@...e.de>, Ewan Milne <emilne@...hat.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] Rework NVMe abort handling

On Thu, Jul 19, 2018 at 04:10:25PM +0200, Johannes Thumshirn wrote:
> The problem I'm trying to solve here is really just single commands
> timing out because of i.e. a bad switch in between which causes frame
> loss somewhere.

And that is exactly the case where NVMe abort does not actually work
in any sensible way.

Remember that while NVMe guarantes ordered delivery inside a given
queue it does not guarantee anything between multiple queues.

So now you have your buggy FC setup where an I/O command times out
because your switch delayed it for two hours due to a firmware bug.

After 30 seconds we send an abort over the admin queue, which happens
to pass through just fine.  The controller will tell you: no command
found as it has never seen it.

No with the the code following what we have in PCIe that just means
we'll eventually controller reset after the I/O command times out
the second time as we still won't have seen a completion for it.

If you incorrectly just continue and resend the command we'll actually
get the command sent twice and thus a potential bug once the original
command just gets sent along.