[Bug nptl/26964] New: pthread_mutex_timedlock returning EAGAIN after futex is locked

[Bug nptl/26964] New: pthread_mutex_timedlock returning EAGAIN after futex is locked

[lamm at linux dot ibm.com]

https://sourceware.org/bugzilla/show_bug.cgi?id=26964

            Bug ID: 26964
           Summary: pthread_mutex_timedlock returning EAGAIN after futex
                    is locked
           Product: glibc
           Version: 2.34
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: nptl
          Assignee: unassigned at sourceware dot org
          Reporter: lamm at linux dot ibm.com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

This bug was exposed by a PMDK (Persistent Memory Development Kit) testcase
where 25 threads uses the same mutex repetitively competing for it an checking
return error codes. On Fedora Rawhide ppc64le the testcase start receiving
EAGAIN from pthread_mutex_timedlock even without using recursive locks.

To reproduce just create a 50 more thread and send them to lock and unlock
using
pthread_mutex_timedlock. Returning EAGAIN even when the futex is successfully
acquired.

Test code:

#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define NANO_PER_ONE 1000000000LL
#define TIMEOUT (NANO_PER_ONE / 1000LL)
#define NUM_THREADS 50
#define WORKER_RUNS 10

static void *
worker(void *arg)
{
        for (unsigned run = 0; run < WORKER_RUNS; run++) {
                pthread_mutex_t *mtx = (pthread_mutex_t *)arg;
                struct timespec abs_time;
                clock_gettime(CLOCK_REALTIME, &abs_time);
                abs_time.tv_nsec += TIMEOUT;
                if (abs_time.tv_nsec >= NANO_PER_ONE) {
                        abs_time.tv_sec++;
                        abs_time.tv_nsec -= NANO_PER_ONE;
                }

                int ret = pthread_mutex_timedlock(mtx, &abs_time);

                if (ret == 0)
                        pthread_mutex_unlock(mtx);

                if (ret == EAGAIN) {
                        printf("EAGAIN returned \n");
                        return (void *)1;
                }
        }
        return NULL;
}

int
main(void)
{
        pthread_t *workers
                = (pthread_t *)malloc(NUM_THREADS * sizeof(pthread_t));

        pthread_mutex_t mutex;
        pthread_mutex_init(&mutex,NULL);

        int *thread_ret = NULL, ret = 0;

        for (unsigned long run = 0; run < 5; run++) {
                for (unsigned i = 0; i < NUM_THREADS; i++) {
                        pthread_create(&workers[i], NULL, worker,
                                (void *)&mutex);
                }
                for (unsigned i = 0; i < NUM_THREADS; i++) {
                        pthread_join(workers[i], (void **)&thread_ret);
                        if (thread_ret != NULL)
                                ++ret;
                }

        }

        free(workers);
        return ret;
}

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[Bug nptl/26964] pthread_mutex_timedlock returning EAGAIN after futex is locked

[tuliom at ascii dot art.br]

https://sourceware.org/bugzilla/show_bug.cgi?id=26964

Tulio Magno Quites Machado Filho <tuliom at ascii dot art.br> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|2.34                        |2.33
     Ever confirmed|0                           |1
                 CC|                            |tuliom at ascii dot art.br
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at sourceware dot org   |lamm at linux dot ibm.com
   Last reconfirmed|                            |2020-11-27

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[Bug nptl/26964] pthread_mutex_timedlock returning EAGAIN after futex is locked

[tuliom at ascii dot art.br]

https://sourceware.org/bugzilla/show_bug.cgi?id=26964

Tulio Magno Quites Machado Filho <tuliom at ascii dot art.br> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |2.33
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #1 from Tulio Magno Quites Machado Filho <tuliom at ascii dot art.br> ---
Fixed with:

commit 61855081017dff30c577855cda882740356b5d98
Author: Lucas A. M. Magalhaes <lamm@linux.ibm.com>
Date:   Tue Dec 1 18:05:07 2020 -0300

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

    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.

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