linux-kernel - Re: Linux-kernel examples for LKMM recipes

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Date:   Wed, 18 Oct 2017 15:57:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will.deacon@....com>, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Linux-kernel examples for LKMM recipes

On Wed, Oct 18, 2017 at 05:18:37PM -0400, Alan Stern wrote:
> On Wed, 18 Oct 2017, Paul E. McKenney wrote:
> 
> > > Well, you could explicitly mention that in the multi-thread case, this
> > > means all accesses to the shared variable had better use READ_ONCE() or
> > > WRITE_ONCE().
> > 
> > Like this?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > 	d.	If there are multiple CPUs, accesses to shared variables
> > 		should use READ_ONCE() and WRITE_ONCE() or stronger
> > 		to prevent load/store tearing, load/store fusing, and
> > 		invented loads and stores.  There are exceptions to
> > 		this rule, for example:
> > 
> > 		i.	When there is no possibility of a given
> > 			shared variable being updated, for example,
> > 			while holding the update-side lock, reads
> > 			from that variable need not use READ_ONCE().
> > 
> > 		ii.	When there is no possibility of a given shared
> > 			variable being either read or updated, for
> > 			example, when running during early boot, reads
> > 			from that variable need not use READ_ONCE() and
> > 			writes to that variable need not use WRITE_ONCE().
> 
> Yeah, except that you mean being read or updated by another thread.

Good point, added that qualifier.

							Thanx, Paul