linux-kernel - Re: [GIT PULL] locking changes for v4.4

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Message-ID: <20151104041644.GO29027@linux.vnet.ibm.com>
Date:	Tue, 3 Nov 2015 20:16:44 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] locking changes for v4.4

On Tue, Nov 03, 2015 at 05:30:29PM -0800, Linus Torvalds wrote:
> On Tue, Nov 3, 2015 at 3:58 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I think I'll pull this, but then just make a separate commit to remove
> > all the bogus games with "control" dependencies that seem to have no
> > basis is reality.
> 
> So the attached is what I committed in my tree. It took much longer to
> try to write the rationale than it took to actually remove the
> atomic_read_ctrl() functions, and even so I'm not sure how good that
> commit message is. But at least it tries to explain what's going on.
> 
> Note the final part of the rationale:
> 
>     I may have to eat my words at some point, but in the absense of clear
>     proof that alpha actually needs this, or indeed even an explanation of
>     how alpha could _possibly_ need it, I do not believe these functions are
>     called for.
> 
>     And if it turns out that alpha really _does_ need a barrier for this
>     case, that barrier still should not be "smp_read_barrier_depends()".
>     We'd have to make up some new speciality barrier just for alpha, along
>     with the documentation for why it really is necessary.

For whatever it is worth, the patch looks good to me.  The reasons I
could imagine why we might want to mark control dependencies are things
like documentation and tooling, but given that we currently only have a
very small number of them, it is hard to argue that this is of immediate
concern, if it is ever of concern.

							Thanx, Paul

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