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

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Date:   Wed, 18 Oct 2017 17:18:37 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, 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.

Alan