Fixes a race between parent and child threads in futex_requeue.

Similar to commit fbf4dec70277 ("selftests/futex: Order calls to futex_lock_pi"), which fixed a flake in futex_lock_pi due to racing between the parent and child threads.

The same issue can occur in the futex_requeue test, because it expects waiterfn to make progress to futex_wait before the parent starts to requeue. This is mitigated by the parent sleeping for WAKE_WAIT_US, but it still fails occasionally. This can be reproduced by adding a sleep in the waiterfn before futex_wait:

TAP version 13 1..2 not ok 1 futex_requeue simple returned: 0 not ok 2 futex_requeue simple returned: 0 not ok 3 futex_requeue many returned: 0 not ok 4 futex_requeue many returned: 0

This issue can be resolved by checking whether the child thread is in a sleeping state. This is because when the child thread goes to sleep, it indicates that it is waiting for the futex lock.

Fixes: 7cb5dd8e2c8c ("selftests: futex: Add futex compare requeue test") Signed-off-by: Yuwen Chen ywen.chen@foxmail.com Co-developed-by: Edward Liaw edliaw@google.com --- .../selftests/futex/functional/Makefile | 2 +- .../futex/functional/futex_requeue.c | 58 ++++++++++++++++--- 2 files changed, 52 insertions(+), 8 deletions(-)

diff --git a/tools/testing/selftests/futex/functional/Makefile b/tools/testing/selftests/futex/functional/Makefile index 490ace1f017e8..8589917a4b126 100644 --- a/tools/testing/selftests/futex/functional/Makefile +++ b/tools/testing/selftests/futex/functional/Makefile @@ -2,7 +2,7 @@ PKG_CONFIG ?= pkg-config LIBNUMA_TEST = $(shell sh -c "$(PKG_CONFIG) numa --atleast-version 2.0.16 > /dev/null 2>&1 && echo SUFFICIENT || echo NO")

-INCLUDES := -I../include -I../../ $(KHDR_INCLUDES) +INCLUDES := -I../include -I../../ $(KHDR_INCLUDES) -I../../../../include CFLAGS := $(CFLAGS) -g -O2 -Wall -pthread -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64 $(INCLUDES) $(KHDR_INCLUDES) -DLIBNUMA_VER_$(LIBNUMA_TEST)=1 LDLIBS := -lpthread -lrt -lnuma

diff --git a/tools/testing/selftests/futex/functional/futex_requeue.c b/tools/testing/selftests/futex/functional/futex_requeue.c index 7a22458c7fc96..994295fac6972 100644 --- a/tools/testing/selftests/futex/functional/futex_requeue.c +++ b/tools/testing/selftests/futex/functional/futex_requeue.c @@ -7,6 +7,7 @@

#include <pthread.h> #include <limits.h> +#include <linux/compiler.h>

#include "futextest.h" #include "../../kselftest_harness.h" @@ -15,6 +16,7 @@ #define WAKE_WAIT_US 10000

volatile futex_t *f1; +static pthread_barrier_t barrier;

void *waiterfn(void *arg) { @@ -23,28 +25,59 @@ void *waiterfn(void *arg) to.tv_sec = 0; to.tv_nsec = timeout_ns;

+ WRITE_ONCE(*((pid_t *)arg), gettid()); + pthread_barrier_wait(&barrier); + if (futex_wait(f1, *f1, &to, 0)) printf("waiter failed errno %d\n", errno);

return NULL; }

+static int get_thread_state(pid_t pid) +{ + FILE *fp; + char buf[80], tag[80]; + char val = 0; + + snprintf(buf, sizeof(buf), "/proc/%d/status", pid); + fp = fopen(buf, "r"); + if (!fp) + return -1; + + while (fgets(buf, sizeof(buf), fp)) + if (fscanf(fp, "%s %c\n", tag, &val) == 2 && !strcmp(tag, "State:")) + break; + + fclose(fp); + return val; +} + TEST(requeue_single) { volatile futex_t _f1 = 0; volatile futex_t f2 = 0; pthread_t waiter; - int res; + pid_t tids; + int res, state;

f1 = &_f1; + pthread_barrier_init(&barrier, NULL, 2);

/* * Requeue a waiter from f1 to f2, and wake f2. */ - if (pthread_create(&waiter, NULL, waiterfn, NULL)) + if (pthread_create(&waiter, NULL, waiterfn, &tids)) ksft_exit_fail_msg("pthread_create failed\n");

- usleep(WAKE_WAIT_US); + pthread_barrier_wait(&barrier); + pthread_barrier_destroy(&barrier); + while ((state = get_thread_state(READ_ONCE(tids))) != 'S') { + usleep(WAKE_WAIT_US); + + if (state < 0) + break; + }

ksft_print_dbg_msg("Requeuing 1 futex from f1 to f2\n"); res = futex_cmp_requeue(f1, 0, &f2, 0, 1, 0); @@ -71,7 +104,8 @@ TEST(requeue_multiple) volatile futex_t _f1 = 0; volatile futex_t f2 = 0; pthread_t waiter[10]; - int res, i; + pid_t tids[10]; + int res, i, state;

f1 = &_f1;

@@ -80,11 +114,21 @@ TEST(requeue_multiple) * At futex_wake, wake INT_MAX (should be exactly 7). */ for (i = 0; i < 10; i++) { - if (pthread_create(&waiter[i], NULL, waiterfn, NULL)) + pthread_barrier_init(&barrier, NULL, 2); + + if (pthread_create(&waiter[i], NULL, waiterfn, &tids[i])) ksft_exit_fail_msg("pthread_create failed\n"); - }

- usleep(WAKE_WAIT_US); + pthread_barrier_wait(&barrier); + pthread_barrier_destroy(&barrier); + + while ((state = get_thread_state(READ_ONCE(tids[i]))) != 'S') { + usleep(WAKE_WAIT_US); + + if (state < 0) + break; + } + }

ksft_print_dbg_msg("Waking 3 futexes at f1 and requeuing 7 futexes from f1 to f2\n"); res = futex_cmp_requeue(f1, 0, &f2, 3, 7, 0);

In the requeue_single function, the variable "waits" only uses one element. There is no need to use an array.

Signed-off-by: Yuwen Chen ywen.chen@foxmail.com --- tools/testing/selftests/futex/functional/futex_requeue.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/futex/functional/futex_requeue.c b/tools/testing/selftests/futex/functional/futex_requeue.c index 69e2555b60399..1807465de2144 100644 --- a/tools/testing/selftests/futex/functional/futex_requeue.c +++ b/tools/testing/selftests/futex/functional/futex_requeue.c @@ -33,7 +33,7 @@ TEST(requeue_single) { volatile futex_t _f1 = 0; volatile futex_t f2 = 0; - pthread_t waiter[10]; + pthread_t waiter; int res;

f1 = &_f1; @@ -41,7 +41,7 @@ TEST(requeue_single) /* * Requeue a waiter from f1 to f2, and wake f2. */ - if (pthread_create(&waiter[0], NULL, waiterfn, NULL)) + if (pthread_create(&waiter, NULL, waiterfn, NULL)) ksft_exit_fail_msg("pthread_create failed\n");

usleep(WAKE_WAIT_US);