selftests/mm: fix ARM related issue with fork after pthread_create

Following issue was observed while running the uffd-unit-tests selftest
on ARM devices. On x86_64 no issues were detected:

pthread_create followed by fork caused deadlock in certain cases wherein
fork required some work to be completed by the created thread.  Used
synchronization to ensure that created thread's start function has started
before invoking fork.

[edliaw@google.com: refactored to use atomic_bool]
Link: https://lkml.kernel.org/r/20240325194100.775052-1-edliaw@google.com
Fixes: 760aee0b ("selftests/mm: add tests for RO pinning vs fork()")
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Signed-off-by: Edward Liaw <edliaw@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/uffd-common.c

@@ -18,6 +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;
 
 static int uffd_mem_fd_create(off_t mem_size, bool hugetlb)
 {
@@ -518,6 +519,8 @@ void *uffd_poll_thread(void *arg)
 	pollfd[1].fd = pipefd[cpu*2];
 	pollfd[1].events = POLLIN;
 
+	ready_for_fork = true;
+
 	for (;;) {
 		ret = poll(pollfd, 2, -1);
 		if (ret <= 0) {

tools/testing/selftests/mm/uffd-common.h

@@ -32,6 +32,7 @@
 #include <inttypes.h>
 #include <stdint.h>
 #include <sys/random.h>
+#include <stdatomic.h>
 
 #include "../kselftest.h"
 #include "vm_util.h"
@@ -103,6 +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 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

@@ -775,6 +775,8 @@ static void uffd_sigbus_test_common(bool wp)
 	char c;
 	struct uffd_args args = { 0 };
 
+	ready_for_fork = false;
+
 	fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
 
 	if (uffd_register(uffd, area_dst, nr_pages * page_size,
@@ -790,6 +792,9 @@ 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 */
+
 	pid = fork();
 	if (pid < 0)
 		err("fork");
@@ -829,6 +834,8 @@ static void uffd_events_test_common(bool wp)
 	char c;
 	struct uffd_args args = { 0 };
 
+	ready_for_fork = false;
+
 	fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
 	if (uffd_register(uffd, area_dst, nr_pages * page_size,
 			  true, wp, false))
@@ -838,6 +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 */
+
 	pid = fork();
 	if (pid < 0)
 		err("fork");