linux-kernel - Re: blk-mq flush fix

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Message-ID: <20131028193024.GA21108@infradead.org>
Date:	Mon, 28 Oct 2013 12:30:24 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Alexander Gordeev <agordeev@...hat.com>,
	Tejun Heo <tj@...nel.org>,
	Nicholas Bellinger <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org
Subject: Re: blk-mq flush fix

On Mon, Oct 28, 2013 at 10:59:32AM -0600, Jens Axboe wrote:
> > The problem with it is that it will pass a tag number to the low level
> > driver which it doesn't expect.  In case the tags are used 1:1 as
> > hardware tags that would lead to nasy bugs.
> > 
> > At vefy least we'd need to mess with ->tag for this special request.
> 
> Yes indeed. Actually the more I think about it, the better I like just
> using the normal tagging infrastructure and punting to a reserved tag
> for the flush. Just needs adding of the check whether it actually needs
> it or not.

That issue happens when using reserved tags as-is: e.g. the device
supports 32 hardware tag, first reserved one is 33, something the
hardware can't handle.

So either way we'll have to copy over the tag from the original request.

I don't think I can come up with something sane for that ASAP, so I'd
suggest you actually do take my patch for now, and we'll sort something
out less in a hurry.

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