>From f56134cbf4de7748ed7f361f7630b1e7a22c2822 Mon Sep 17 00:00:00 2001
From: Akira Yokosawa <akiyks@xxxxxxxxx>
Date: Sun, 13 Jan 2019 20:04:01 +0900
Subject: [PATCH 5/6] datastruct/hash: Add Quick Quiz on READ_ONCE/WRITE_ONCE in hash_resize.c
The answer is borrowed from the change log of Paul's commit
1aac0c703482 ("datastruct/hash: Remove extraneous barrier from
hashtab_resize()")
Signed-off-by: Akira Yokosawa <akiyks@xxxxxxxxx>
---
datastruct/datastruct.tex | 32 +++++++++++++++++++++++++-------
1 file changed, 25 insertions(+), 7 deletions(-)
diff --git a/datastruct/datastruct.tex b/datastruct/datastruct.tex
index 6d8350a..c78e5c5 100644
--- a/datastruct/datastruct.tex
+++ b/datastruct/datastruct.tex
@@ -1115,15 +1115,15 @@ or \co{NULL} when the search failed.
\QuickQuiz{}
The \co{hashtab_lookup()} function in
Listing~\ref{lst:datastruct:Resizable Hash-Table Access Functions}
- searches only the current hash bucket.
- Doesn't this mean that readers might miss a newly added
- element?
+ does not take account of concurrent resize operations.
+ Doesn't this mean that readers might miss an element
+ added while resizing is progressing?
\QuickQuizAnswer{
No.
- As will be described soon,
- The \co{hashtab_add()} and \co{hashtab_del()} functions
- keep updating the current hash table while resizing is
- progressing.
+ As we will see soon,
+ the \co{hashtab_add()} and \co{hashtab_del()} functions
+ keep the current hash table up-to-date until a resize operation
+ completes.
} \QuickQuizEnd
\begin{lineref}[ln:datastruct:hash_resize:access:add]
@@ -1292,6 +1292,24 @@ line~\lnref{free} frees
the old hash table, and finally line~\lnref{ret_success} returns success.
\end{lineref}
+\QuickQuiz{}
+ \begin{lineref}[ln:datastruct:hash_resize:resize]
+ Why is there a \co{WRITE_ONCE()} on line~\lnref{update_resize}
+ in Listing~\ref{lst:datastruct:Resizable Hash-Table Resizing}?
+ \end{lineref}
+\QuickQuizAnswer{
+ \begin{lineref}[ln:datastruct:hash_resize:lock_unlock_mod]
+ Together with the \co{READ_ONCE()}
+ on line~\lnref{l:ifresized} in \co{hashtab_lock_mod()}
+ of Listing~\ref{lst:datastruct:Resizable Hash-Table Update-Side
+ Concurrency Control},
+ it tells the compiler that the non-initialization accesses
+ to \co{->ht_resize_cur} must remain because reads
+ from \co{->ht_resize_cur} really can race with writes,
+ just not in a way to change the ``if'' conditions.
+ \end{lineref}
+} \QuickQuizEnd
+
\subsection{Resizable Hash Table Discussion}
\label{sec:datastruct:Resizable Hash Table Discussion}
--
2.7.4
