On Wed, Mar 10, 2021 at 07:43:40AM +0900, Akira Yokosawa wrote:
> On Tue, 9 Mar 2021 11:46:38 -0800, Paul E. McKenney wrote:
> > On Wed, Mar 10, 2021 at 12:32:30AM +0900, Akira Yokosawa wrote:
> >> Hi Paul,
> >>
> >> I'm reviewing Section 9.5.
> >> I find Figures 9.11 -- 9.13 are somewhat hard to grasp.
> >>
> >> In those figures, P0() is called "RCU Reader" due to the
> >> RCU read-side critical section.
> >>
> >> However, it updates x and y, and you are talking about
> >> final values of x and y.
> >>
> >> At first glance, it looked as though P0() would read from
> >> x and y, and get values 1 or 2.
> >>
> >> Some clarification would help new readers of Edition 2,
> >> I suppose.
> >
> > How about as shown below? (Not particularly human-readable given that
> > I modified the figures as well.)
>
> So, I was totally confused by those figures!
> P0() is literally a reader from the beginning. Oh well...
>
> Updated text and figures would not have confused me.
Very good, thank you!
Unfortunately, it is perfectly legal to have writes in RCU read-side
critical sections. I have added a QQ on this, shown below.
This is not just RCU! Please see section 5.4.6 ("Applying Exact Limit
Counters") for an example with writes in reader-writer locking read-side
critical sections. Names can be slippery things. ;-)
Thanx, Paul
------------------------------------------------------------------------
commit 35fbf113d662365d5f8717e273af55838038f52e
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date: Tue Mar 9 13:44:55 2021 -0800
defer/rcufundamental: Add QQ about writes in RCU readers
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
diff --git a/defer/rcufundamental.tex b/defer/rcufundamental.tex
index d8ca411..f2e643f 100644
--- a/defer/rcufundamental.tex
+++ b/defer/rcufundamental.tex
@@ -312,6 +312,29 @@ Section~\ref{sec:formal:Axiomatic Approaches and RCU}.
In addition, RCU's ordering properties are examined in much
greater detail in Section~\ref{sec:memorder:RCU}.
+\QuickQuiz{
+ What would happen if \co{P0()}'s accesses in
+ \crefrange{fig:defer:RCU Reader and Later Grace Period}{fig:defer:RCU Reader Within Grace Period}
+ were stores?
+}\QuickQuizAnswer{
+ The exact same ordering rules would apply, that is,
+ (1)~If any part of \co{P0()}'s RCU read-side critical section
+ preceded the beginning of \co{P1()}'s grace period, all of
+ \co{P0()}'s RCU read-side critical section would precede the
+ end of \co{P1()}'s grace period, and
+ (2)~If any part of \co{P0()}'s RCU read-side critical section
+ followed the end of \co{P1()}'s grace period, all of \co{P0()}'s
+ RCU read-side critical section would follow the beginning of
+ \co{P1()}'s grace period.
+
+ It might seem strange to have RCU read-side critical sections
+ containing writes, but RCU is just fine with this. This
+ capability is used frequently in the Linux kernel, for example,
+ acquiring a lock on or reference to a data structure. Acquiring
+ either a lock or a reference results in a write to memory,
+ and it is OK to do these within an RCU read-side critical section.
+}\QuickQuizEnd
+
Although RCU's wait-for-readers capability really is sometimes used to
order the assignment of values to variables as shown in
\crefrange{fig:defer:RCU Reader and Later Grace Period}
