[PATCH v2] uffd-unit-tests: Fix ARM related issue with fork after pthread_create

[Edward Liaw <edliaw@google.com>]

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.

Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
[edliaw: Refactored to use atomic_bool]
Signed-off-by: Edward Liaw <edliaw@google.com>
---
 tools/testing/selftests/mm/uffd-common.c     |  3 +++
 tools/testing/selftests/mm/uffd-common.h     |  2 ++
 tools/testing/selftests/mm/uffd-unit-tests.c | 10 ++++++++++
 3 files changed, 15 insertions(+)

diff --git a/tools/testing/selftests/mm/uffd-common.c b/tools/testing/selftests/mm/uffd-common.c
index b0ac0ec2356d..7ad6ba660c7d 100644
--- a/tools/testing/selftests/mm/uffd-common.c
+++ b/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) {
diff --git a/tools/testing/selftests/mm/uffd-common.h b/tools/testing/selftests/mm/uffd-common.h
index cb055282c89c..cc5629c3d2aa 100644
--- a/tools/testing/selftests/mm/uffd-common.h
+++ b/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;
diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index 2b9f8cc52639..4a48dc617c6b 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/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");
-- 
2.44.0.396.g6e790dbe36-goog