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Message-ID: <CAG4es9X3r2kCY+Fp+CnpRyX8ChhdhaWMOjT=KePBcFCAQHEbeQ@mail.gmail.com>
Date: Fri, 18 Oct 2024 10:17:46 -0700
From: Edward Liaw <edliaw@...gle.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: 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>,
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 Fri, Oct 18, 2024 at 7:37 AM Ryan Roberts <ryan.roberts@....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@...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.
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)
>
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