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

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Date:	Wed, 18 May 2016 10:28:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Waiman Long <waiman.long@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Peter Hurley <peter@...leysoftware.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 11:56:39AM -0400, Waiman Long wrote:
> On 05/18/2016 07:05 AM, 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?
> 
> Maybe the GCC guys can define a tag which can be set in the variable
> or structure field declarations that those variables or field have
> to be read from or written to atomically. This can allow critical
> data that are used by multiple CPUs to be handled correctly while
> allowing compiler the freedom to do what it sees fit for the less
> critical data. This approach is also easier than looking for all the
> places where the data items are accessed and modifying them.

Having such a tag on gcc's internal data structures is what I want,
regardless of exactly how that tag gets there.

The sorts of things that I am especially concerned about are cases
where there is a union or other type-punning involved, and gcc has
the pieces of the desired location already in registers.  Of course,
these pieces came from separate accesses.  We really don't need this
kind of thing tripping us up!

							Thanx, Paul