linux-kernel - Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field

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Message-ID: <20160519134331.GM3528@linux.vnet.ibm.com>
Date:	Thu, 19 May 2016 06:43:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Waiman Long <Waiman.Long@....com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>, kcc@...gle.com,
	dvyukov@...gle.com, dhowells@...hat.com
Subject: Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner
 field

On Thu, May 19, 2016 at 11:00:13AM +0200, Peter Zijlstra wrote:
> On Wed, May 18, 2016 at 10:26:06AM -0700, Paul E. McKenney wrote:
> > On Wed, May 18, 2016 at 01:05:55PM +0200, Peter Zijlstra wrote:
> 
> > > Alternatively, could we try and talk to our GCC friends to make sure GCC
> > > doesn't tear loads/stores irrespective of what the C language spec
> > > allows?
> > 
> > Interestingly enough, they used to make that guarantee, but removed it
> > when C11 showed up.
> 
> Did someone tell them this was a regression and have them fix it? They
> can't just change things like this.

I did, informally.  I was told that the atomics were to replace them.
I have been bugging them about volatile ever since, given that some
people would dearly like to eliminate volatile from the language.  (I
believe I am making good progress on preventing this, with a lot of help
more recently.)

> > Me, I would feel better explicitly telling the compiler what I needed.
> > It is all too easy for bugs to slip in otherwise, especially when the
> > gcc guys are adding exciting new optimizations.
> 
> GCC guys (as opposed to the language guys) should be far more amenable
> to our needs, and I don't think they want to break the kernel any more
> than we do.

Some are, some aren't.  We should of course cherish the ones who would
like to avoid breaking the kernel.

							Thanx, Paul