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Message-ID: <b17802cb-5dcd-4161-b192-89abd55c1228@arm.com>
Date: Fri, 18 Oct 2024 15:37:22 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Edward Liaw <edliaw@...gle.com>, linux-kselftest@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
Lokesh Gidra <lokeshgidra@...gle.com>, Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com,
linux-mm@...ck.org, Aishwarya TCV <Aishwarya.TCV@....com>
Subject: Re: [PATCH 1/2] selftests/mm: replace atomic_bool with
pthread_barrier_t
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@...gle.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)
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