[PATCH 0/2] selftests/mm: fix deadlock after pthread_create

[PATCH 0/2] selftests/mm: fix deadlock after pthread_create

[Edward Liaw]

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.

Updated the synchronization primitive to use pthread_barrier instead of
atomic_bool.

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

Edward Liaw (2):
  selftests/mm: replace atomic_bool with pthread_barrier_t
  selftests/mm: fix deadlock for fork after pthread_create on ARM

 tools/testing/selftests/mm/uffd-common.c     |  5 +++--
 tools/testing/selftests/mm/uffd-common.h     |  3 +--
 tools/testing/selftests/mm/uffd-unit-tests.c | 21 ++++++++++++++------
 3 files changed, 19 insertions(+), 10 deletions(-)

--
2.46.1.824.gd892dcdcdd-goog

[PATCH 1/2] selftests/mm: replace atomic_bool with pthread_barrier_t

[Edward Liaw]

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@google.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

[PATCH 2/2] selftests/mm: fix deadlock for fork after pthread_create on ARM

[Edward Liaw]

On Android with arm, there is some synchronization needed to avoid a
deadlock when forking after pthread_create.

Fixes: cff294582798 ("selftests/mm: extend and rename uffd pagemap test")
Signed-off-by: Edward Liaw <edliaw@google.com>
---
 tools/testing/selftests/mm/uffd-unit-tests.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index 3db2296ac631..c8a3b1c7edff 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -241,6 +241,9 @@ static void *fork_event_consumer(void *data)
 	fork_event_args *args = data;
 	struct uffd_msg msg = { 0 };
 
+	/* Ready for parent thread to fork */
+	pthread_barrier_wait(&ready_for_fork);
+
 	/* Read until a full msg received */
 	while (uffd_read_msg(args->parent_uffd, &msg));
 
@@ -308,8 +311,12 @@ static int pagemap_test_fork(int uffd, bool with_event, bool test_pin)
 
 	/* Prepare a thread to resolve EVENT_FORK */
 	if (with_event) {
+		pthread_barrier_init(&ready_for_fork, NULL, 2);
 		if (pthread_create(&thread, NULL, fork_event_consumer, &args))
 			err("pthread_create()");
+		/* Wait for child thread to start before forking */
+		pthread_barrier_wait(&ready_for_fork);
+		pthread_barrier_destroy(&ready_for_fork);
 	}
 
 	child = fork();
-- 
2.46.1.824.gd892dcdcdd-goog

Re: [PATCH 0/2] selftests/mm: fix deadlock after pthread_create

[Andrew Morton]

On Thu,  3 Oct 2024 21:17:09 +0000 Edward Liaw <edliaw@google.com> wrote:

> 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.
> 
> Updated the synchronization primitive to use pthread_barrier instead of
> atomic_bool.
> 
> Applied the same fix to the wp-fork-with-event test.
> 
> Edward Liaw (2):
>   selftests/mm: replace atomic_bool with pthread_barrier_t
>   selftests/mm: fix deadlock for fork after pthread_create on ARM

These fixes have different Fixes: targets, which might cause
backporting issues - some kernels might end up with one patch and not
the other.  Was this intended?  Is it OK?

Re: [PATCH 1/2] selftests/mm: replace atomic_bool with pthread_barrier_t

[Muhammad Usama Anjum]

On 10/4/24 2:17 AM, Edward Liaw wrote:
> 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@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.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)

-- 
BR,
Muhammad Usama Anjum

Re: [PATCH 1/2] selftests/mm: replace atomic_bool with pthread_barrier_t

[Ryan Roberts]

On 03/10/2024 22:17, Edward Liaw wrote:
> 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@google.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);

Hi Edward,

This change is causing uffd-unit-tests to hang on arm64 when running the "move on anon" test. It's happening because this wait is never returning for this case. And that happens because ready_for_fork was never initialized for this test. It looks like there are other places where a thread is created for uffd_poll_thread() where ready_for_fork is not initialized too.

I added this change and it solves the problem, although it's pretty hacky.

This is blocking our arm64 testing on linux-next so would appreciate either a quick fix or removing the change until a fix is ready.

Thanks,
Ryan

----8<-----
diff --git a/tools/testing/selftests/mm/uffd-common.c b/tools/testing/selftests/mm/uffd-common.c
index 852e7281026e..a05eb705be02 100644
--- a/tools/testing/selftests/mm/uffd-common.c
+++ b/tools/testing/selftests/mm/uffd-common.c
@@ -19,6 +19,7 @@ unsigned long long *count_verify;
 uffd_test_ops_t *uffd_test_ops;
 uffd_test_case_ops_t *uffd_test_case_ops;
 pthread_barrier_t ready_for_fork;
+bool wait_ready_for_fork;
 
 static int uffd_mem_fd_create(off_t mem_size, bool hugetlb)
 {
@@ -520,7 +521,8 @@ void *uffd_poll_thread(void *arg)
        pollfd[1].events = POLLIN;
 
        /* Ready for parent thread to fork */
-       pthread_barrier_wait(&ready_for_fork);
+       if (wait_ready_for_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 3e6228d8e0dc..e94329a39b34 100644
--- a/tools/testing/selftests/mm/uffd-common.h
+++ b/tools/testing/selftests/mm/uffd-common.h
@@ -105,6 +105,7 @@ extern bool test_uffdio_wp;
 extern unsigned long long *count_verify;
 extern volatile bool test_uffdio_copy_eexist;
 extern pthread_barrier_t ready_for_fork;
+extern bool wait_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 3db2296ac631..d1fc4cd6a948 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -775,6 +775,7 @@ static void uffd_sigbus_test_common(bool wp)
        struct uffd_args args = { 0 };
 
        pthread_barrier_init(&ready_for_fork, NULL, 2);
+       wait_ready_for_fork = true;
 
        fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
 
@@ -794,6 +795,7 @@ static void uffd_sigbus_test_common(bool wp)
        /* Wait for child thread to start before forking */
        pthread_barrier_wait(&ready_for_fork);
        pthread_barrier_destroy(&ready_for_fork);
+       wait_ready_for_fork = false;
 
        pid = fork();
        if (pid < 0)
@@ -835,6 +837,7 @@ static void uffd_events_test_common(bool wp)
        struct uffd_args args = { 0 };
 
        pthread_barrier_init(&ready_for_fork, NULL, 2);
+       wait_ready_for_fork = true;
 
        fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
        if (uffd_register(uffd, area_dst, nr_pages * page_size,
@@ -848,6 +851,7 @@ static void uffd_events_test_common(bool wp)
        /* Wait for child thread to start before forking */
        pthread_barrier_wait(&ready_for_fork);
        pthread_barrier_destroy(&ready_for_fork);
+       wait_ready_for_fork = false;
 
        pid = fork();
        if (pid < 0)
----8<-----


>  
>  	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)

Re: [PATCH 1/2] selftests/mm: replace atomic_bool with pthread_barrier_t

[Edward Liaw]

On Fri, Oct 18, 2024 at 7:37 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 03/10/2024 22:17, Edward Liaw wrote:
> > 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@google.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);
>
> Hi Edward,
>
> This change is causing uffd-unit-tests to hang on arm64 when running the "move on anon" test. It's happening because this wait is never returning for this case. And that happens because ready_for_fork was never initialized for this test. It looks like there are other places where a thread is created for uffd_poll_thread() where ready_for_fork is not initialized too.
>
> I added this change and it solves the problem, although it's pretty hacky.
>
> This is blocking our arm64 testing on linux-next so would appreciate either a quick fix or removing the change until a fix is ready.

Ah, I should not have changed it to a pthread_barrier, since that
depends on the parent creating the barrier; I'm sending a revert and
subsequent fix.

>
> Thanks,
> Ryan
>
> ----8<-----
> diff --git a/tools/testing/selftests/mm/uffd-common.c b/tools/testing/selftests/mm/uffd-common.c
> index 852e7281026e..a05eb705be02 100644
> --- a/tools/testing/selftests/mm/uffd-common.c
> +++ b/tools/testing/selftests/mm/uffd-common.c
> @@ -19,6 +19,7 @@ unsigned long long *count_verify;
>  uffd_test_ops_t *uffd_test_ops;
>  uffd_test_case_ops_t *uffd_test_case_ops;
>  pthread_barrier_t ready_for_fork;
> +bool wait_ready_for_fork;
>
>  static int uffd_mem_fd_create(off_t mem_size, bool hugetlb)
>  {
> @@ -520,7 +521,8 @@ void *uffd_poll_thread(void *arg)
>         pollfd[1].events = POLLIN;
>
>         /* Ready for parent thread to fork */
> -       pthread_barrier_wait(&ready_for_fork);
> +       if (wait_ready_for_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 3e6228d8e0dc..e94329a39b34 100644
> --- a/tools/testing/selftests/mm/uffd-common.h
> +++ b/tools/testing/selftests/mm/uffd-common.h
> @@ -105,6 +105,7 @@ extern bool test_uffdio_wp;
>  extern unsigned long long *count_verify;
>  extern volatile bool test_uffdio_copy_eexist;
>  extern pthread_barrier_t ready_for_fork;
> +extern bool wait_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 3db2296ac631..d1fc4cd6a948 100644
> --- a/tools/testing/selftests/mm/uffd-unit-tests.c
> +++ b/tools/testing/selftests/mm/uffd-unit-tests.c
> @@ -775,6 +775,7 @@ static void uffd_sigbus_test_common(bool wp)
>         struct uffd_args args = { 0 };
>
>         pthread_barrier_init(&ready_for_fork, NULL, 2);
> +       wait_ready_for_fork = true;
>
>         fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
>
> @@ -794,6 +795,7 @@ static void uffd_sigbus_test_common(bool wp)
>         /* Wait for child thread to start before forking */
>         pthread_barrier_wait(&ready_for_fork);
>         pthread_barrier_destroy(&ready_for_fork);
> +       wait_ready_for_fork = false;
>
>         pid = fork();
>         if (pid < 0)
> @@ -835,6 +837,7 @@ static void uffd_events_test_common(bool wp)
>         struct uffd_args args = { 0 };
>
>         pthread_barrier_init(&ready_for_fork, NULL, 2);
> +       wait_ready_for_fork = true;
>
>         fcntl(uffd, F_SETFL, uffd_flags | O_NONBLOCK);
>         if (uffd_register(uffd, area_dst, nr_pages * page_size,
> @@ -848,6 +851,7 @@ static void uffd_events_test_common(bool wp)
>         /* Wait for child thread to start before forking */
>         pthread_barrier_wait(&ready_for_fork);
>         pthread_barrier_destroy(&ready_for_fork);
> +       wait_ready_for_fork = false;
>
>         pid = fork();
>         if (pid < 0)
> ----8<-----
>
>
> >
> >       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)
>