On Mon, Nov 14, 2016 at 11:57:46AM -0500, Pranith Kumar wrote:
> Hi Paul,
>
> On Mon, Nov 14, 2016 at 11:47 AM, Paul E. McKenney
> <paul...@linux.vnet.ibm.com> wrote:
> > Recent memory-model work deduces the relationships of RCU read-side
> > critical sections and grace periods based on the relationships of
> > accesses within a critical section and accesses preceding and following
> > the grace period. This commit therefore adds this viewpoint.
> >
> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > ---
> > .../RCU/Design/Requirements/Requirements.html | 25
> > +++++++++++++++++++++-
> > 1 file changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/RCU/Design/Requirements/Requirements.html
> > b/Documentation/RCU/Design/Requirements/Requirements.html
> > index a4d3838130e4..81b40cb83435 100644
> > --- a/Documentation/RCU/Design/Requirements/Requirements.html
> > +++ b/Documentation/RCU/Design/Requirements/Requirements.html
> > @@ -547,7 +547,7 @@ The <tt>rcu_access_pointer()</tt> on line 6 is
> > similar to
> > It could reuse a value formerly fetched from this same pointer.
> > It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
> > manner, resulting in <i>load tearing</i>, in turn resulting a
> > bytewise
> > - mash-up of two distince pointer values.
> > + mash-up of two distinct pointer values.
> > It might even use value-speculation optimizations, where it makes
> > a wrong guess, but by the time it gets around to checking the
> > value, an update has changed the pointer to match the wrong guess.
> > @@ -659,6 +659,29 @@ systems with more than one CPU:
> > In other words, a given instance of <tt>synchronize_rcu()</tt>
> > can avoid waiting on a given RCU read-side critical section only
> > if it can prove that <tt>synchronize_rcu()</tt> started first.
> > +
> > + <p>
> > + A related question is “When <tt>rcu_read_lock()</tt>
> > + doesn't generate any code, why does it matter how it relates
> > + to a grace period?”
> > + The answer if that it is not the relationship of
>
> s/if/is?
Good catch, fixed!
Thanx, Paul
> > + <tt>rcu_read_lock()</tt> itself that is important, but rather
> > + the relationship of the code within the enclosed RCU read-side
> > + critical section to the code preceding and following the
> > + grace period.
> > + If we take this viewpoint, then a given RCU read-side critical
> > + section begins before a given grace period when some access
> > + preceding the grace period observes the effect of some access
> > + within the critical section, in which case none of the accesses
> > + within the critical section may observe the effects of any
> > + access following the grace period.
> > +
> > + <p>
> > + As of late 2016, mathematical models of RCU take this
> > + viewpoint, for example, see slides 62 and 63
> > + of the
> > + <a
> > href="http://www2.rdrop.com/users/paulmck/scalability/paper/LinuxMM.2016.10.04c.LCE.pdf";>2016
> > LinuxCon EU</a>
> > + presentation.
> > </font></td></tr>
> > <tr><td> </td></tr>
> > </table>
> > --
> > 2.5.2
> >
>
>
>
> --
> Pranith
>
