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

[Ryan Roberts <ryan.roberts@arm.com>]

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)