Re: Linux-kernel examples for LKMM recipes

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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