On Sun, Apr 09, 2023 at 09:11:58AM +0900, Akira Yokosawa wrote:
> Hi,
>
> On Sat, 8 Apr 2023 02:04:24 +0800, Alan Huang wrote:
> > Hi Paul and Akira,
> >
> > This is the patch v3, I forgot to add a version tag…
> >
> > And I think it may be better to add a question in a subsequent path.
>
> So I'm looking at Quick Quizzes on count_lim_sig.c.
> It looks to me Quick Quiz 5.50 lost its context due to the
> removal of smp_mb() and the use of smp_store_release().
>
> A band-aide fix would be to change the quiz to:
>
> In Listing 5.18's function flush_local_count_sig(), why
> are there READ_ONCE(), WRITE_ONCE(), and smp_store_release()
> wrappers around the uses of the theft per-thread variable?
>
> , and to adjust the line count in its Answer.
>
> However, smp_store_release() is by no means a simple wrapper.
> So I'm wondering if it is worthwhile to keep this quiz.
>
> Paul, how about replacing the quiz with the new quiz asking:
>
> In Listing 5.18, doesn't flush_local_count_sig() need
> a stronger memory barrier or two?
>
> , and picking Alan's reasoning in the answer?
How about something like this?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/count/count.tex b/count/count.tex
index 2aade053..ccc2e839 100644
--- a/count/count.tex
+++ b/count/count.tex
@@ -2441,28 +2441,11 @@ the fastpath happens before this change of \co{theft} to READY\@.
\QuickQuiz{
In \cref{lst:count:Signal-Theft Limit Counter Value-Migration Functions}'s
- function \co{flush_local_count_sig()}, why are there
- \co{READ_ONCE()} and \co{WRITE_ONCE()} wrappers around
- the uses of the
- \co{theft} per-thread variable?
+ doesn't \co{flush_local_count_sig()} need stronger memory barriers?
}\QuickQuizAnswer{
- \begin{fcvref}[ln:count:count_lim_sig:migration:flush_sig]
- The first one (on \clnref{check:REQ}) can be argued to be unnecessary.
- The last two (\clnref{set:ACK,set:READY}) are important.
- If these are removed, the compiler would be within its rights
- to rewrite \clnrefrange{set:ACK}{set:READY} as follows:
- \end{fcvref}
-
-\begin{VerbatimN}[firstnumber=14]
-theft = THEFT_READY;
-if (counting) {
- theft = THEFT_ACK;
-}
-\end{VerbatimN}
-
- This would be fatal, as the slowpath might see the transient
- value of \co{THEFT_READY}, and start stealing before the
- corresponding thread was ready.
+ No, that \co{smp_store_release()} suffices because this code
+ communicates only with \co{flush_local_count()}, and there is
+ no need for store-to-load ordering.
}\QuickQuizEnd
\begin{fcvref}[ln:count:count_lim_sig:migration:flush]
