[PATCH] pi_stress: Add memory barrier to resolve crash

["John B. Wyatt IV" <jwyatt@redhat.com>]

This patch is also an error report seen on RHEL 9 and Fedora 40.
pi_stress crashes if you run this near full cores with one core
(2 threads) free for housekeeping. The crash usually happens at
around 2 hours of pi_stress running. This issue was reproduced
on a RHEL server with 2 sockets of 10 cores/20 threads (total 40
threads) and a Fedora server with 2 sockets of 56 cores/112
threads (total 226 threads).

The pthread_barrier_wait should guarantee that only one
thread at a time interacts with the variables below that
if block.

GCC -O2 optimization rearranges the two increments above the wait
function calls. This causes a race issue found by Helgrind.
You can prove this by commenting out the memory barrier
and compiling with `-O0`. The crash does not happen with -O0.
Thank you to Valentin Schneider <vschneid@redhat.com> for
suggesting about -O2.

Add a memory barrier to force GCC to increment the variables
below the pthread calls. The memory barrier prevented any crashes
for several 24 hours tests. This function depends on C11.

Reported-by: Valgrind's Helgrind tool

Signed-off-by: "John B. Wyatt IV" <jwyatt@redhat.com>
Signed-off-by: "John B. Wyatt IV" <sageofredondo@gmail.com>
---
 src/pi_tests/pi_stress.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/src/pi_tests/pi_stress.c b/src/pi_tests/pi_stress.c
index 371e906..37023a1 100644
--- a/src/pi_tests/pi_stress.c
+++ b/src/pi_tests/pi_stress.c
@@ -44,6 +44,7 @@
 #include <stdint.h>
 #include <inttypes.h>
 #include <limits.h>
+#include <stdatomic.h>
 
 #include "rt-sched.h"
 #include "rt-utils.h"
@@ -952,12 +953,31 @@ void *high_priority(void *arg)
 			    ("high_priority[%d]: pthread_barrier_wait(finish): %x", p->id, status);
 			return NULL;
 		}
+
+
+		/**
+		 * The pthread_barrier_wait should guarantee that only one
+		 * thread at a time interacts with the variables below that
+		 * if block.
+		 *
+		 * GCC -O2 rearranges the two increments above the wait
+		 * function calls causing a race issue if you run this
+		 * near full cores with one core (2 threads) free for
+		 * housekeeping. This causes a crash at around 2 hour of
+		 * running. You can prove this by commenting out the barrier
+		 * and compiling with `-O0`. The crash does not show with
+		 * -O0.
+		 *
+		 * Add a memory barrier to force GCC to increment the variables
+		 * below the pthread calls. This funcion depends on C11.
+		 **/
+		atomic_thread_fence(memory_order_seq_cst);
+
 		/* update the group stats */
 		p->total++;
 
 		/* update the watchdog counter */
 		p->watchdog++;
-
 	}
 	set_shutdown_flag();
 
-- 
2.47.0