linux-kernel - [PATCH 1/2] selftests/mm: replace atomic_bool with pthread_barrier

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Message-ID: <20241003211716.371786-2-edliaw@google.com>
Date: Thu,  3 Oct 2024 21:17:10 +0000
From: Edward Liaw <edliaw@...gle.com>
To: linux-kselftest@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Shuah Khan <shuah@...nel.org>, Lokesh Gidra <lokeshgidra@...gle.com>, 
	Edward Liaw <edliaw@...gle.com>, Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com, linux-mm@...ck.org
Subject: [PATCH 1/2] selftests/mm: replace atomic_bool with pthread_barrier_t

Swaps synchronization primitive with pthread_barrier, so that
stdatomic.h does not need to be included.

The synchronization is needed on Android ARM64; we see a deadlock with
pthread_create when the parent thread races forward before the child has
a chance to start doing work.

Fixes: 8c864371b2a1 ("selftests/mm: fix ARM related issue with fork after
pthread_create")
Signed-off-by: Edward Liaw <edliaw@...gle.com>
---
 tools/testing/selftests/mm/uffd-common.c     |  5 +++--
 tools/testing/selftests/mm/uffd-common.h     |  3 +--
 tools/testing/selftests/mm/uffd-unit-tests.c | 14 ++++++++------
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/mm/uffd-common.c b/tools/testing/selftests/mm/uffd-common.c
index 717539eddf98..852e7281026e 100644
--- a/tools/testing/selftests/mm/uffd-common.c
+++ b/tools/testing/selftests/mm/uffd-common.c
@@ -18,7 +18,7 @@ bool test_uffdio_wp = true;
 unsigned long long *count_verify;
 uffd_test_ops_t *uffd_test_ops;
 uffd_test_case_ops_t *uffd_test_case_ops;
-atomic_bool ready_for_fork;
+pthread_barrier_t ready_for_fork;
 
 static int uffd_mem_fd_create(off_t mem_size, bool hugetlb)
 {
@@ -519,7 +519,8 @@ void *uffd_poll_thread(void *arg)
 	pollfd[1].fd = pipefd[cpu*2];
 	pollfd[1].events = POLLIN;
 
-	ready_for_fork = true;
+	/* Ready for parent thread to fork */
+	pthread_barrier_wait(&ready_for_fork);
 
 	for (;;) {
 		ret = poll(pollfd, 2, -1);
diff --git a/tools/testing/selftests/mm/uffd-common.h b/tools/testing/selftests/mm/uffd-common.h
index a70ae10b5f62..3e6228d8e0dc 100644
--- a/tools/testing/selftests/mm/uffd-common.h
+++ b/tools/testing/selftests/mm/uffd-common.h
@@ -33,7 +33,6 @@
 #include <inttypes.h>
 #include <stdint.h>
 #include <sys/random.h>
-#include <stdatomic.h>
 
 #include "../kselftest.h"
 #include "vm_util.h"
@@ -105,7 +104,7 @@ extern bool map_shared;
 extern bool test_uffdio_wp;
 extern unsigned long long *count_verify;
 extern volatile bool test_uffdio_copy_eexist;
-extern atomic_bool ready_for_fork;
+extern pthread_barrier_t ready_for_fork;
 
 extern uffd_test_ops_t anon_uffd_test_ops;
 extern uffd_test_ops_t shmem_uffd_test_ops;
diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index b3d21eed203d..3db2296ac631 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -774,7 +774,7 @@ static void uffd_sigbus_test_common(bool wp)
 	char c;
 	struct uffd_args args = { 0 };
 
-	ready_for_fork = false;
+	pthread_barrier_init(&ready_for_fork, NULL, 2);
 
 	fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
 
@@ -791,8 +791,9 @@ static void uffd_sigbus_test_common(bool wp)
 	if (pthread_create(&uffd_mon, NULL, uffd_poll_thread, &args))
 		err("uffd_poll_thread create");
 
-	while (!ready_for_fork)
-		; /* Wait for the poll_thread to start executing before forking */
+	/* Wait for child thread to start before forking */
+	pthread_barrier_wait(&ready_for_fork);
+	pthread_barrier_destroy(&ready_for_fork);
 
 	pid = fork();
 	if (pid < 0)
@@ -833,7 +834,7 @@ static void uffd_events_test_common(bool wp)
 	char c;
 	struct uffd_args args = { 0 };
 
-	ready_for_fork = false;
+	pthread_barrier_init(&ready_for_fork, NULL, 2);
 
 	fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
 	if (uffd_register(uffd, area_dst, nr_pages * page_size,
@@ -844,8 +845,9 @@ static void uffd_events_test_common(bool wp)
 	if (pthread_create(&uffd_mon, NULL, uffd_poll_thread, &args))
 		err("uffd_poll_thread create");
 
-	while (!ready_for_fork)
-		; /* Wait for the poll_thread to start executing before forking */
+	/* Wait for child thread to start before forking */
+	pthread_barrier_wait(&ready_for_fork);
+	pthread_barrier_destroy(&ready_for_fork);
 
 	pid = fork();
 	if (pid < 0)
-- 
2.46.1.824.gd892dcdcdd-goog