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From: "lamm at linux dot ibm.com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug nptl/26964] New: pthread_mutex_timedlock returning EAGAIN after
 futex is locked
Date: Fri, 27 Nov 2020 14:31:00 +0000
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X-Bugzilla-Version: 2.34
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D26964

            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 check=
ing
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 successful=
ly
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 =3D 0; run < WORKER_RUNS; run++) {
                pthread_mutex_t *mtx =3D (pthread_mutex_t *)arg;
                struct timespec abs_time;
                clock_gettime(CLOCK_REALTIME, &abs_time);
                abs_time.tv_nsec +=3D TIMEOUT;
                if (abs_time.tv_nsec >=3D NANO_PER_ONE) {
                        abs_time.tv_sec++;
                        abs_time.tv_nsec -=3D NANO_PER_ONE;
                }

                int ret =3D pthread_mutex_timedlock(mtx, &abs_time);

                if (ret =3D=3D 0)
                        pthread_mutex_unlock(mtx);

                if (ret =3D=3D EAGAIN) {
                        printf("EAGAIN returned \n");
                        return (void *)1;
                }
        }
        return NULL;
}

int
main(void)
{
        pthread_t *workers
                =3D (pthread_t *)malloc(NUM_THREADS * sizeof(pthread_t));

        pthread_mutex_t mutex;
        pthread_mutex_init(&mutex,NULL);

        int *thread_ret =3D NULL, ret =3D 0;

        for (unsigned long run =3D 0; run < 5; run++) {
                for (unsigned i =3D 0; i < NUM_THREADS; i++) {
                        pthread_create(&workers[i], NULL, worker,
                                (void *)&mutex);
                }
                for (unsigned i =3D 0; i < NUM_THREADS; i++) {
                        pthread_join(workers[i], (void **)&thread_ret);
                        if (thread_ret !=3D NULL)
                                ++ret;
                }

        }

        free(workers);
        return ret;
}

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