[PATCH 5/7] selftests/mm: Replace uffd_read_mutex with a semaphore

[Peter Xu <peterx@xxxxxxxxxx>]

Each uffd read threads unlocks the read mutex first, probably just to make
sure the thread is reaching a stage where pthread_cancel() can always work
before the main thread moves on.

However keeping the mutex locked always and unlock in the thread is a bit
hacky.  Replacing it with a semaphore which should be much clearer, where
the main thread will wait() and the thread will just post().  Move it to
uffd-common.* to be reused later.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 tools/testing/selftests/mm/uffd-common.c | 1 +
 tools/testing/selftests/mm/uffd-common.h | 2 ++
 tools/testing/selftests/mm/uffd-stress.c | 8 +++-----
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/mm/uffd-common.c b/tools/testing/selftests/mm/uffd-common.c
index 02b89860e193..aded06cab285 100644
--- a/tools/testing/selftests/mm/uffd-common.c
+++ b/tools/testing/selftests/mm/uffd-common.c
@@ -17,6 +17,7 @@ bool map_shared;
 bool test_uffdio_wp = true;
 unsigned long long *count_verify;
 uffd_test_ops_t *uffd_test_ops;
+sem_t uffd_read_sem;
 
 static int uffd_mem_fd_create(off_t mem_size, bool hugetlb)
 {
diff --git a/tools/testing/selftests/mm/uffd-common.h b/tools/testing/selftests/mm/uffd-common.h
index 7c4fa964c3b0..521523baded1 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 <semaphore.h>
 
 #include "../kselftest.h"
 #include "vm_util.h"
@@ -97,6 +98,7 @@ extern bool map_shared;
 extern bool test_uffdio_wp;
 extern unsigned long long *count_verify;
 extern volatile bool test_uffdio_copy_eexist;
+extern sem_t uffd_read_sem;
 
 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-stress.c b/tools/testing/selftests/mm/uffd-stress.c
index 469e0476af26..7219f55ae794 100644
--- a/tools/testing/selftests/mm/uffd-stress.c
+++ b/tools/testing/selftests/mm/uffd-stress.c
@@ -125,14 +125,12 @@ static int copy_page_retry(int ufd, unsigned long offset)
 	return __copy_page(ufd, offset, true, test_uffdio_wp);
 }
 
-pthread_mutex_t uffd_read_mutex = PTHREAD_MUTEX_INITIALIZER;
-
 static void *uffd_read_thread(void *arg)
 {
 	struct uffd_args *args = (struct uffd_args *)arg;
 	struct uffd_msg msg;
 
-	pthread_mutex_unlock(&uffd_read_mutex);
+	sem_post(&uffd_read_sem);
 	/* from here cancellation is ok */
 
 	for (;;) {
@@ -196,7 +194,7 @@ static int stress(struct uffd_args *args)
 					   uffd_read_thread,
 					   (void *)&args[cpu]))
 				return 1;
-			pthread_mutex_lock(&uffd_read_mutex);
+			sem_wait(&uffd_read_sem);
 		}
 		if (pthread_create(&background_threads[cpu], &attr,
 				   background_thread, (void *)cpu))
@@ -258,7 +256,7 @@ static int userfaultfd_stress(void)
 	zeropage = area;
 	bzero(zeropage, page_size);
 
-	pthread_mutex_lock(&uffd_read_mutex);
+	sem_init(&uffd_read_sem, 0, 0);
 
 	pthread_attr_init(&attr);
 	pthread_attr_setstacksize(&attr, 16*1024*1024);
-- 
2.41.0