[PATCH v3] nptl: Fix __futex_clocklock64 return error check [BZ #26964]

[Adhemerval Zanella]

On 30/11/2020 18:37, Lucas A. M. Magalhaes wrote:
> The earlier implementation of this, __lll_clocklock, calls lll_clockwait
> that doesn't return the futex syscall error codes.  It always tries again
> if that fails.
> 
> However in the current implementation, when the futex returns EAGAIN,
> __futex_clocklock64 will also return EGAIN, even if the futex is taken.
> 
> This patch fixes the EAGAIN issue and also adds a check for EINTR.  As
> futex syscall can return EINTR if the thread is interrupted by a signal.
> In this case I'm assuming the function should continue trying to lock as
> there is no mention to about it on POSIX.  Also add a test for both
> scenarios.

LGTM with the change in comment below (there is no need to send a v4).

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  nptl/Makefile                         |   2 +-
>  nptl/tst-pthread-timedlock-lockloop.c | 138 ++++++++++++++++++++++++++
>  sysdeps/nptl/futex-internal.h         |   7 +-
>  3 files changed, 142 insertions(+), 5 deletions(-)
>  create mode 100644 nptl/tst-pthread-timedlock-lockloop.c
> 
> diff --git a/nptl/Makefile b/nptl/Makefile
> index a48426a396..42e30a4ce1 100644
> --- a/nptl/Makefile
> +++ b/nptl/Makefile
> @@ -298,7 +298,7 @@ tests = tst-attr2 tst-attr3 tst-default-attr \
>  	tst-thread-affinity-sched \
>  	tst-pthread-defaultattr-free \
>  	tst-pthread-attr-sigmask \
> -
> +	tst-pthread-timedlock-lockloop
>  
>  tests-container =  tst-pthread-getattr
>  
> diff --git a/nptl/tst-pthread-timedlock-lockloop.c b/nptl/tst-pthread-timedlock-lockloop.c
> new file mode 100644
> index 0000000000..91635b27d4
> --- /dev/null
> +++ b/nptl/tst-pthread-timedlock-lockloop.c
> @@ -0,0 +1,138 @@
> +/* Make sure pthread_mutex_timedlock doesn't return spurious error codes.
> +
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <errno.h>
> +#include <pthread.h>
> +#include <signal.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <support/check.h>
> +#include <support/timespec.h>
> +#include <support/xsignal.h>
> +#include <support/xthread.h>
> +#include <support/xtime.h>
> +#include <time.h>
> +#include <unistd.h>
> +
> +#define NANO_PER_SEC 1000000000LL
> +#define TIMEOUT (NANO_PER_SEC / 1000LL)
> +#define NUM_THREADS 50
> +#define RETEST_TIMES 100
> +
> +static pthread_mutex_t mutex;
> +static int runs;
> +static clockid_t clockid;
> +
> +static void
> +signal_handler (int sig_num)
> +{
> +  TEST_COMPARE (sig_num, SIGUSR1);
> +}
> +
> +/* Call pthread_mutex_timedlock()/pthread_mutex_unlock() repetitively, hoping
> +   that one of them returns EAGAIN or EINTR unexpectedly.  */
> +static void *
> +worker_timedlock (void *arg)
> +{
> +  for (unsigned int run = 0; run < runs; run++)
> +    {
> +      struct timespec abs_time = timespec_add (xclock_now (CLOCK_REALTIME),
> +					       make_timespec (0, 1000000));
> +
> +      int ret = pthread_mutex_timedlock (&mutex, &abs_time);
> +
> +      if (ret == 0)
> +	xpthread_mutex_unlock (&mutex);
> +
> +      TEST_VERIFY_EXIT (ret == 0 || ret == ETIMEDOUT);
> +    }
> +  return NULL;
> +}
> +
> +static void *
> +worker_clocklock (void *arg)
> +{
> +  for (unsigned int run = 0; run < runs; run++)
> +    {
> +      struct timespec time =
> +	timespec_add (xclock_now (clockid), make_timespec (0, 1000000));
> +
> +      int ret = pthread_mutex_clocklock (&mutex, clockid, &time);
> +
> +      if (ret == 0)
> +	xpthread_mutex_unlock (&mutex);
> +
> +      TEST_VERIFY_EXIT (ret == 0 || ret == ETIMEDOUT);
> +    }
> +  return NULL;
> +}
> +
> +static int
> +run_test_set (void *(*worker) (void *))
> +{
> +  pthread_t workers[NUM_THREADS];
> +
> +  /* Run the checks to catch an EAGAIN.  */
> +  /* As there is no way to ensure the error condition, just run the test many
> +     times hoping to catch the error.  */

I think the comment should be more clear, since we are not really expecting an
error here.  I suggest:

  /* Check if default pthread_mutex_{timed,clock}lock with valid arguments 
     returns either 0 or ETIMEDOUT.  Since there is no easy way to force 
     the error condition, the test creates multiple threads which in turn
     issues pthread_mutex_timedlock multiple times.  */

> +  runs = 100;
> +  for (int run = 0; run < RETEST_TIMES; run++)
> +    {
> +      for (int i = 0; i < NUM_THREADS; i++)
> +	workers[i] = xpthread_create (NULL, worker, NULL);
> +      for (int i = 0; i < NUM_THREADS; i++)
> +	xpthread_join (workers[i]);
> +    }
> +
> +  /* Run the test to check if we catch an EINTR.  */
> +  /* As there is no way to ensure the error condition, just run the test many
> +     times hoping to catch the error.  */

  /* The idea is similar to previous tests, but we check if 
     pthread_mutex_{timed,clock}lock does not return EINTR.  */

> +  pthread_t thread;
> +  runs = 1;
> +  for (int i = 0; i < RETEST_TIMES * 1000; i++)
> +    {
> +      xpthread_mutex_lock (&mutex);
> +      thread = xpthread_create (NULL, worker, NULL);
> +      /* Sleep just a little bit to reach the lock on the worker thread.  */
> +      usleep (10);
> +      pthread_kill (thread, SIGUSR1);
> +      xpthread_mutex_unlock (&mutex);
> +      xpthread_join (thread);
> +    }
> +
> +  return 0;
> +}
> +
> +static int
> +do_test (void)
> +{
> +
> +  xsignal (SIGUSR1, signal_handler);
> +
> +  xpthread_mutex_init (&mutex, NULL);
> +
> +  run_test_set (worker_timedlock);
> +  clockid = CLOCK_REALTIME;
> +  run_test_set (worker_clocklock);
> +  clockid = CLOCK_MONOTONIC;
> +  run_test_set (worker_clocklock);
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
> diff --git a/sysdeps/nptl/futex-internal.h b/sysdeps/nptl/futex-internal.h
> index e67803952f..428621bf70 100644
> --- a/sysdeps/nptl/futex-internal.h
> +++ b/sysdeps/nptl/futex-internal.h
> @@ -418,19 +418,18 @@ static __always_inline int
>  __futex_clocklock64 (int *futex, clockid_t clockid,
>                       const struct __timespec64 *abstime, int private)
>  {
> -  int err = 0;
> -
>    if (__glibc_unlikely (atomic_compare_and_exchange_bool_acq (futex, 1, 0)))
>      {
>        while (atomic_exchange_acq (futex, 2) != 0)
>          {
> +	  int err = 0;
>            err = __futex_abstimed_wait64 ((unsigned int *) futex, 2, clockid,
>  					 abstime, private);
>            if (err == EINVAL || err == ETIMEDOUT || err == EOVERFLOW)
> -            break;
> +            return err;
>          }
>      }
> -  return err;
> +  return 0;
>  }
>  
>  #endif  /* futex-internal.h */
>