>From 19fb8b2915e5d918a6c1e98f265ab89bd373fea7 Mon Sep 17 00:00:00 2001
From: Akira Yokosawa <akiyks@xxxxxxxxx>
Date: Sat, 6 Oct 2018 23:47:44 +0900
Subject: [PATCH 3/5] count: Tweak counttorture.h to avoid segfault
As has been mentioned in comment to count_read(), count_tstat
sometimes fails. The cause is the use of count_read() in
counttorture.h after wait_all_threads() returns. Because __thread
variables are freed when owning threads exit, count_read() would
access nonexistent variables.
As a workaround, add #ifndef KEEP_GCC_THREAD_LOCAL in
counttorture.h and define _wait_all_thread(),
_count_unregister_thread(), and final_wait_all_thread() as macros
which change behavior when KEEP_GCC_THREAD_LOCAL is defined.
They permit counttorture's controlling thread to participate in
the thread group to be kept alive.
Signed-off-by: Akira Yokosawa <akiyks@xxxxxxxxx>
---
CodeSamples/count/count_tstat.c | 3 ++-
CodeSamples/count/counttorture.h | 21 ++++++++++++++++++---
count/count.tex | 4 ++--
3 files changed, 22 insertions(+), 6 deletions(-)
diff --git a/CodeSamples/count/count_tstat.c b/CodeSamples/count/count_tstat.c
index 4b99415..d30ee48 100644
--- a/CodeSamples/count/count_tstat.c
+++ b/CodeSamples/count/count_tstat.c
@@ -34,7 +34,7 @@ static __inline__ void inc_count(void)
}
static __inline__ unsigned long read_count(void)
- /* known failure with counttorture! */
+ /* need to tweak counttorture! */
{
int t;
unsigned long sum = 0;
@@ -69,4 +69,5 @@ void count_cleanup(void)
}
#define NEED_REGISTER_THREAD
+#define KEEP_GCC_THREAD_LOCAL
#include "counttorture.h"
diff --git a/CodeSamples/count/counttorture.h b/CodeSamples/count/counttorture.h
index bdfc7d4..60d1d11 100644
--- a/CodeSamples/count/counttorture.h
+++ b/CodeSamples/count/counttorture.h
@@ -64,6 +64,20 @@ int goflag __attribute__((__aligned__(CACHE_LINE_SIZE))) = GOFLAG_INIT;
#define count_unregister_thread(n) do ; while (0)
#endif /* #ifndef NEED_REGISTER_THREAD */
+#ifndef KEEP_GCC_THREAD_LOCAL
+#define _wait_all_threads() wait_all_threads()
+#define _count_unregister_thread(n) count_unregister_thread(n)
+#define final_wait_all_threads()
+#else /* #ifndef KEEP_GCC_THREAD_LOCAL */
+#define _wait_all_threads() { \
+ while (READ_ONCE(finalthreadcount) < nthreadsexpected) \
+ poll(NULL, 0, 1);}
+#define _count_unregister_thread(n) count_unregister_thread(n + 1)
+#define final_wait_all_threads() { \
+ WRITE_ONCE(finalthreadcount, nthreadsexpected + 1); \
+ wait_all_threads();}
+#endif /* #ifndef KEEP_GCC_THREAD_LOCAL */
+
unsigned long garbage = 0; /* disable compiler optimizations. */
/*
@@ -90,7 +104,7 @@ void *count_read_perf_test(void *arg)
n_reads_local += COUNT_READ_RUN;
}
__get_thread_var(n_reads_pt) += n_reads_local;
- count_unregister_thread(nthreadsexpected);
+ _count_unregister_thread(nthreadsexpected);
garbage += j;
return (NULL);
@@ -113,7 +127,7 @@ void *count_update_perf_test(void *arg)
n_updates_local += COUNT_UPDATE_RUN;
}
__get_thread_var(n_updates_pt) += n_updates_local;
- count_unregister_thread(nthreadsexpected);
+ _count_unregister_thread(nthreadsexpected);
return NULL;
}
@@ -139,7 +153,7 @@ void perftestrun(int nthreads, int nreaders, int nupdaters)
smp_mb();
goflag = GOFLAG_STOP;
smp_mb();
- wait_all_threads();
+ _wait_all_threads();
count_cleanup();
for_each_thread(t) {
n_reads += per_thread(n_reads_pt, t);
@@ -159,6 +173,7 @@ void perftestrun(int nthreads, int nreaders, int nupdaters)
(double)n_reads),
((duration * 1000*1000.*(double)nupdaters) /
(double)n_updates));
+ final_wait_all_threads();
exit(EXIT_SUCCESS);
}
diff --git a/count/count.tex b/count/count.tex
index a4e9c7f..b008a19 100644
--- a/count/count.tex
+++ b/count/count.tex
@@ -1127,9 +1127,9 @@ variables vanish when that thread exits.
Listing~\ref{lst:count:Per-Thread Statistical Counters With Lockless Summation}
(\path{count_tstat.c}).
Analysis of this code is left as an exercise to the reader,
- however, please note that it does not fit well into the
+ however, please note that it requires tweaks in the
\path{counttorture.h} counter-evaluation scheme.
- (Why not?)
+ (Hint: See \co{#ifndef KEEP_GCC_THREAD_LOCAL}.)
Chapter~\ref{chp:Deferred Processing} will introduce
synchronization mechanisms that handle this situation in a much
more graceful manner.
--
2.7.4
