linux-kernel - Re: [PATCH 2/9] IB: add a proper completion queue abstraction

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Date:	Mon, 23 Nov 2015 15:18:06 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	Christoph Hellwig <hch@....de>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	"sagig@....mellanox.co.il" <sagig@....mellanox.co.il>,
	"axboe@...com" <axboe@...com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] IB: add a proper completion queue abstraction

On Mon, Nov 23, 2015 at 01:54:05PM -0800, Bart Van Assche wrote:

> Not really ... Please have a look at the SRP initiator source code. What the
> SRP initiator does is to poll the send queue before sending a new
> SCSI

I see that. What I don't see is how SRP handles things when the
sendq fills up, ie the case where __srp_get_tx_iu() == NULL. It looks
like the driver starts to panic and generates printks. I can't tell if
it can survive that, but it doesn't look very good..

It would be a lot better if this wasn't allowed to happen, the polling
loop can run until a sendq becomes available, and never return null
from __srp_get_tx_iu().

Ie, __srp_get_tx_iu should look more like

   ib_poll_cq_until(..., !list_empty(&ch->free_tx));

Which would be a fairly sane core API for this direct usage.. Ideally
the core code would sleep if possible and not just spin. Maybe also
protect it with a timer.

> command to the target system starts. I think this approach could also be
> used in other ULP drivers if the send queue poll frequency is such that no
> send queue overflow occurs.

Yes, I agree, but it has to be done properly :)

Jason
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