selftests/mm: replace atomic_bool with pthread_barrier_t

Patch series "selftests/mm: fix deadlock after pthread_create".

On Android arm, pthread_create followed by a fork caused a deadlock in the
case where the fork required work to be completed by the created thread.

Update the synchronization primitive to use pthread_barrier instead of
atomic_bool.

Apply the same fix to the wp-fork-with-event test.


This patch (of 2):

Swap 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.

Link: https://lkml.kernel.org/r/20241003211716.371786-1-edliaw@google.com
Link: https://lkml.kernel.org/r/20241003211716.371786-2-edliaw@google.com
Fixes: cff2945 ("selftests/mm: extend and rename uffd pagemap test")
Signed-off-by: Edward Liaw <edliaw@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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);

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;

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)