On Wed, Oct 03, 2018 at 12:24:17AM +0900, Akira Yokosawa wrote:
> On 2018/10/03 00:16:12 +0900, Akira Yokosawa wrote:
> > From a9c276c3da87ed327d003a70d60777f41999b4fc Mon Sep 17 00:00:00 2001
> > From: Akira Yokosawa <akiyks@xxxxxxxxx>
> > Date: Wed, 3 Oct 2018 00:08:49 +0900
> > Subject: [PATCH 2/2] count: Adjust type of variable 'counter' with code snippet
> >
> > In the actual code of count_stat.c, the per-thread variable
> > "counter" has the type "unsigned long".
Looks good, so I applied and pushed both patches, thank you!
> And the other description on Listing 5.3 in the text needs some reworking
> to reflect the changes in the snippet. Paul, can you have a look into
> this?
How does the patch below look?
Thanx, Paul
------------------------------------------------------------------------
commit cc0409cfb0581924b05744a15fa3ebd5c1e298af
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
Date: Tue Oct 2 11:19:33 2018 -0700
count: Update code description and QQ based on {READ,WRITE}_ONCE()
Reported-by: Akira Yokosawa <akiyks@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
diff --git a/count/count.tex b/count/count.tex
index 9f9fd0f64838..c36e7d826a7b 100644
--- a/count/count.tex
+++ b/count/count.tex
@@ -495,23 +495,22 @@ show a function that increments the counters, using the
thread's element of the \co{counter} array.
Because this element is modified only by the corresponding thread,
non-atomic increment suffices.
-
-Lines~\lnref{read:b}-\lnref{read:e}
-show a function that reads out the aggregate value of the counter,
-using the \co{for_each_thread()} primitive to iterate over the list of
-currently running threads, and using the \co{per_thread()} primitive
-to fetch the specified thread's counter.
-Because the hardware can fetch and store a properly aligned \co{unsigned long}
-atomically, and because \GCC\ is kind enough to make use of this capability,
-normal loads suffice, and no special atomic instructions are required.
-\end{lineref}
+However, this code uses \co{WRITE_ONCE()} to prevent destructive compiler
+optimizations.
+For but one example, the compiler is within its rights to use a
+to-be-stored-to location as temporary storage, thus writing what
+would be for all intents and purposes garbage to that location
+just before doing the desired store.
+This could of course be rather confusing to anything attempting to
+read out the count.
+The use of \co{WRITE_ONCE()} prevents this optimization and others besides.
\QuickQuiz{}
- What other choice does \GCC\ have, anyway???
+ What other nasty optimizations could \GCC\ apply?
\QuickQuizAnswer{
- According to the C standard, the effects of fetching a variable
- that might be concurrently modified by some other thread are
- undefined.
+ According to the C standard, the effects of doing a normal store
+ to a variable that might be concurrently loaded by some other
+ thread are undefined.
It turns out that the C standard really has no other choice,
given that C must support (for example) eight-bit architectures
which are incapable of atomically loading a \co{long}.
@@ -540,8 +539,10 @@ normal loads suffice, and no special atomic instructions are required.
It is therefore reasonable to expect that any Linux-kernel
adoption of C11 atomics will be incremental at best.
- So why not just use plain old C-language assignment statements
- to access shared variables?
+ But if the code is doing loads and stores that the underlying
+ hardware can implement with single instructions, why not just
+ use plain old C-language assignment statements to access shared
+ variables?
The problem is that the compiler is within its rights to assume
that no other thread is modifying the variable being accessed.
Given a plain old C-language load, the compiler would therefore
@@ -581,6 +582,22 @@ normal loads suffice, and no special atomic instructions are required.
coding standards.
} \QuickQuizEnd
+Lines~\lnref{read:b}-\lnref{read:e}
+show a function that reads out the aggregate value of the counter,
+using the \co{for_each_thread()} primitive to iterate over the list of
+currently running threads, and using the \co{per_thread()} primitive
+to fetch the specified thread's counter.
+This code also uses \co{READ_ONCE()} to ensure that the compiler doesn't
+optimize these loads into oblivion.
+For but one example, a pair of consecutive calls to \co{read_count()}
+might be inlined, and an intrepid optimizer might notice that the same
+locations were being summed and thus incorrectly conclude that it would
+be simply wonderful to sum them once and use the resulting value twice.
+This sort of optimization might be rather frustrating to people expecting
+later \co{read_count()} calls to return larger values.
+The use of \co{READ_ONCE()} prevents this optimization and others besides.
+\end{lineref}
+
\QuickQuiz{}
How does the per-thread \co{counter} variable in
Listing~\ref{lst:count:Array-Based Per-Thread Statistical Counters}
