linux-kernel - Re: [PATCH] NVMe: do not touch sq door bell if nvmeq has been suspended

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Date:	Tue, 2 Feb 2016 19:20:01 +0200
From:	Sagi Grimberg <sagig@....mellanox.co.il>
To:	Keith Busch <keith.busch@...el.com>
Cc:	Wenbo Wang <wenbo.wang@...blaze.com>, Jens Axboe <axboe@...com>,
	Wenbo Wang <mail_weber_wang@....com>,
	"Wenwei.Tao" <wenwei.tao@...blaze.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>
Subject: Re: [PATCH] NVMe: do not touch sq door bell if nvmeq has been
 suspended


> We free the transfer buffers when a command is cancelled. The controller,
> however, may still own the command and may try to write to them. We
> have to fence the controller off from being able to do that, so we can't
> cancel inflight commands while the PCI device is still bus master enabled.
>
> In a perfect world, we could trust in disabling with NVMe registers,
> but sometimes we can't rely on that.

OK, I wasn't aware that we cannot rely on that.

So it looks like we cannot change the ordering. So this leaves us with
the need to guarantee that no queue_rq is inflight before we unmap.