linux-kernel - Re: [PATCH 5/7] blk-mq: initialize request on allocation

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Message-ID: <20140417145705.GA14787@lst.de>
Date:	Thu, 17 Apr 2014 16:57:05 +0200
From:	Christoph Hellwig <hch@....de>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	Matias Bjorling <m@...rling.me>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux SCSI List <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH 5/7] blk-mq: initialize request on allocation

On Thu, Apr 17, 2014 at 10:54:23PM +0800, Ming Lei wrote:
> On Mon, Apr 14, 2014 at 4:30 PM, Christoph Hellwig <hch@....de> wrote:
> > If we want to share tag and request allocation between queues we cannot
> > initialize the request at init/free time, but need to initialize it
> > at allocation time as it might get used for different queues over its
> > lifetime.
> 
> Could you explain the use pattern? Looks you mean there are
> still users of the tag/req even after it is freed, that looks a bit
> weird since the tag/req can still be reallocated in another path
> after it is freed.

No difference in use pattern.  But blk_rq_init initializes the rq->q field,
and a request might get reused for a different queue.
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