linux-kernel - Re: [PATCH tip/core/rcu 01/14] rcu: Protect rcu_boost() lockless accesses with ACCESS

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Message-ID: <20150109134954.GO5280@linux.vnet.ibm.com>
Date:	Fri, 9 Jan 2015 05:49:54 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 01/14] rcu: Protect rcu_boost() lockless
 accesses with ACCESS_ONCE()

On Thu, Jan 08, 2015 at 10:41:54PM -0800, Davidlohr Bueso wrote:
> On Thu, 2015-01-08 at 07:22 -0800, Paul E. McKenney wrote:
> > > Didn't we just obsolete ACCESS_ONCE with that {READ,WRITE}_ONCE stuff?
> > 
> > Indeed we did!  But that was after I did this commit back on October 29th.
> > 
> > I am planning a bulk change to READ_ONCE() and ASSIGN_ONCE() either as
> > the last patch for 3.20 or as the first one for 3.21.  Probably as the
> > first for 3.21 to minimize rebasing hassles with any needed 3.20 fixes.
> 
> That reminds me, I think the new conversion for stores will most likely
> introduce silly arg bugs:
> 
> -       ACCESS_ONCE(a) = b;
> +       ASSIGN_ONCE(b, a);

I was planning to do mine by hand for this sort of reason.

Or are you thinking of something more subtle than the case where
"b" is an unparenthesized comma-separated expression?

							Thanx, Paul

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