src/lib/lttng-ust/futex.c

   1 /*
   2  * SPDX-License-Identifier: LGPL-2.1-or-later
   3  *
   4  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   5  *
   6  * Userspace RCU library - sys_futex compatibility code
   7  */
   8
   9 #include <stdio.h>
  10 #include <pthread.h>
  11 #include <signal.h>
  12 #include <assert.h>
  13 #include <errno.h>
  14 #include <poll.h>
  15 #include <stdint.h>
  16
  17 #include <urcu/arch.h>
  18 #include <urcu/system.h>
  19
  20 #include "lib/lttng-ust/futex.h"
  21
  22 /*
  23  * This compat header originated in userspace RCU where it's used across
  24  * multiple shared objects hence the need to have the mutexes as public weak
  25  * symbols, in our case here, it's only used internally by liblttng-ust so we
  26  * can hide them. If we end up using this compat header in another library in
  27  * this project we will have to use the same scheme, but in the meantime, don't
  28  * expose those symbols in the ABI.
  29  */
  30
  31 /*
  32  * This comment will apply if we start using this compat header in multiple
  33  * libraires.
  34  *
  35  * Using attribute "weak" for __lttng_ust_compat_futex_lock and
  36  * __lttng_ust_compat_futex_cond. Those are globally visible by the entire
  37  * program, even though many shared objects may have their own version.
  38  * The first version that gets loaded will be used by the entire program
  39  * (executable and all shared objects).
  40  */
  41
  42 static pthread_mutex_t __lttng_ust_compat_futex_lock = PTHREAD_MUTEX_INITIALIZER;
  43 static pthread_cond_t __lttng_ust_compat_futex_cond = PTHREAD_COND_INITIALIZER;
  44
  45 /*
  46  * _NOT SIGNAL-SAFE_. pthread_cond is not signal-safe anyway. Though.
  47  * For now, timeout, uaddr2 and val3 are unused.
  48  * Waiter will relinquish the CPU until woken up.
  49  */
  50
  51 int lttng_ust_compat_futex_noasync(int32_t *uaddr, int op, int32_t val,
  52         const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
  53 {
  54         int ret = 0, lockret;
  55
  56         /*
  57          * Check if NULL. Don't let users expect that they are taken into
  58          * account.
  59          */
  60         assert(!timeout);
  61         assert(!uaddr2);
  62         assert(!val3);
  63
  64         /*
  65          * memory barriers to serialize with the previous uaddr modification.
  66          */
  67         cmm_smp_mb();
  68
  69         lockret = pthread_mutex_lock(&__lttng_ust_compat_futex_lock);
  70         if (lockret) {
  71                 errno = lockret;
  72                 ret = -1;
  73                 goto end;
  74         }
  75         switch (op) {
  76         case FUTEX_WAIT:
  77                 /*
  78                  * Wait until *uaddr is changed to something else than "val".
  79                  * Comparing *uaddr content against val figures out which
  80                  * thread has been awakened.
  81                  */
  82                 while (CMM_LOAD_SHARED(*uaddr) == val)
  83                         pthread_cond_wait(&__lttng_ust_compat_futex_cond,
  84                                 &__lttng_ust_compat_futex_lock);
  85                 break;
  86         case FUTEX_WAKE:
  87                 /*
  88                  * Each wake is sending a broadcast, thus attempting wakeup of
  89                  * all awaiting threads, independently of their respective
  90                  * uaddr.
  91                  */
  92                 pthread_cond_broadcast(&__lttng_ust_compat_futex_cond);
  93                 break;
  94         default:
  95                 errno = EINVAL;
  96                 ret = -1;
  97         }
  98         lockret = pthread_mutex_unlock(&__lttng_ust_compat_futex_lock);
  99         if (lockret) {
 100                 errno = lockret;
 101                 ret = -1;
 102         }
 103 end:
 104         return ret;
 105 }
 106
 107 /*
 108  * _ASYNC SIGNAL-SAFE_.
 109  * For now, timeout, uaddr2 and val3 are unused.
 110  * Waiter will busy-loop trying to read the condition.
 111  * It is OK to use compat_futex_async() on a futex address on which
 112  * futex() WAKE operations are also performed.
 113  */
 114
 115 int lttng_ust_compat_futex_async(int32_t *uaddr, int op, int32_t val,
 116         const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
 117 {
 118         int ret = 0;
 119
 120         /*
 121          * Check if NULL. Don't let users expect that they are taken into
 122          * account.
 123          */
 124         assert(!timeout);
 125         assert(!uaddr2);
 126         assert(!val3);
 127
 128         /*
 129          * Ensure previous memory operations on uaddr have completed.
 130          */
 131         cmm_smp_mb();
 132
 133         switch (op) {
 134         case FUTEX_WAIT:
 135                 while (CMM_LOAD_SHARED(*uaddr) == val) {
 136                         if (poll(NULL, 0, 10) < 0) {
 137                                 ret = -1;
 138                                 /* Keep poll errno. Caller handles EINTR. */
 139                                 goto end;
 140                         }
 141                 }
 142                 break;
 143         case FUTEX_WAKE:
 144                 break;
 145         default:
 146                 errno = EINVAL;
 147                 ret = -1;
 148         }
 149 end:
 150         return ret;
 151 }