linux-kernel - Re: [PATCH] Fix race with shared tag queue maps

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Date:	Fri, 14 Sep 2007 09:47:31 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	orgis@...ld.uni-potsdam.de, arekm@...en.pl, ed.lin@...mise.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	James.Bottomley@...elEye.com
Subject: Re: [PATCH] Fix race with shared tag queue maps

On Thu, Sep 13 2007, Linus Torvalds wrote:
> 
> 
> On Thu, 13 Sep 2007, Jens Axboe wrote:
> > 
> > My bad, I think I added the smp_mb__before_clear_bit() when it was
> > __test_and_set_bit() like in the first hunk.
> 
> Ahh, that wouldn't work at all. The "__test_and_set_bit()" thing isn't 
> atomic at all, and no amount of memory barriers around it would help 
> (you'd need to use real locking, but at that point the memory barriers are 
> pointless anyway).

Hence the change, it looks like an oversight from when we didn't allow
sharing of tag maps (then the queue lock provided adequate protection).

> > +	/*
> > +	 * Ensure ordering between ->tag_index[tag] clear and tag clear
> > +	 */
> > +	smp_mb__after_clear_bit();
> 
> You still left this one. But never mind - I already edited your original 
> patch and it's in my tree with both of those things removed.

I took at look at the committed patch and it looks fine, thanks Linus.

-- 
Jens Axboe

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