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

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Message-ID: <20160518172606.GU3528@linux.vnet.ibm.com>
Date:	Wed, 18 May 2016 10:26:06 -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 Wed, May 18, 2016 at 01:05:55PM +0200, Peter Zijlstra wrote:
> On Tue, May 17, 2016 at 12:46:07PM -0700, Paul E. McKenney wrote:
> > Actually, if you show a case where this makes a visible system-wide
> > difference, you could create a set of primitives for #1 below.  Have
> > a compiler version check, and if it is an old compiler, map them to
> > READ_ONCE() and WRITE_ONCE(), otherwise as follows, though preferably
> > with better names:
> > 
> > #define READ_NOTEAR(x) __atomic_load_n(&(x), __ATOMIC_RELAXED)
> > #define WRITE_NOTEAR(x, v) __atomic_store_n(&(x), (v), __ATOMIC_RELAXED)
> > 
> > The ambiguity between "no tear" and "not ear" should help motivate a
> > better choice of name.
> 
> 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.

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.

							Thanx, Paul