nptl/pthread_cond_signal.c

   1 /* Copyright (C) 2003-2022 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <endian.h>
  19 #include <errno.h>
  20 #include <sysdep.h>
  21 #include <futex-internal.h>
  22 #include <pthread.h>
  23 #include <pthreadP.h>
  24 #include <atomic.h>
  25 #include <stdint.h>
  26
  27 #include <shlib-compat.h>
  28 #include <stap-probe.h>
  29
  30 #include "pthread_cond_common.c"
  31
  32 /* See __pthread_cond_wait for a high-level description of the algorithm.  */
  33 int
  34 ___pthread_cond_signal (pthread_cond_t *cond)
  35 {
  36   LIBC_PROBE (cond_signal, 1, cond);
  37
  38   /* First check whether there are waiters.  Relaxed MO is fine for that for
  39      the same reasons that relaxed MO is fine when observing __wseq (see
  40      below).  */
  41   unsigned int wrefs = atomic_load_relaxed (&cond->__data.__wrefs);
  42   if (wrefs >> 3 == 0)
  43     return 0;
  44   int private = __condvar_get_private (wrefs);
  45
  46   __condvar_acquire_lock (cond, private);
  47
  48   /* Load the waiter sequence number, which represents our relative ordering
  49      to any waiters.  Relaxed MO is sufficient for that because:
  50      1) We can pick any position that is allowed by external happens-before
  51         constraints.  In particular, if another __pthread_cond_wait call
  52         happened before us, this waiter must be eligible for being woken by
  53         us.  The only way do establish such a happens-before is by signaling
  54         while having acquired the mutex associated with the condvar and
  55         ensuring that the signal's critical section happens after the waiter.
  56         Thus, the mutex ensures that we see that waiter's __wseq increase.
  57      2) Once we pick a position, we do not need to communicate this to the
  58         program via a happens-before that we set up: First, any wake-up could
  59         be a spurious wake-up, so the program must not interpret a wake-up as
  60         an indication that the waiter happened before a particular signal;
  61         second, a program cannot detect whether a waiter has not yet been
  62         woken (i.e., it cannot distinguish between a non-woken waiter and one
  63         that has been woken but hasn't resumed execution yet), and thus it
  64         cannot try to deduce that a signal happened before a particular
  65         waiter.  */
  66   unsigned long long int wseq = __condvar_load_wseq_relaxed (cond);
  67   unsigned int g1 = (wseq & 1) ^ 1;
  68   wseq >>= 1;
  69   bool do_futex_wake = false;
  70
  71   /* If G1 is still receiving signals, we put the signal there.  If not, we
  72      check if G2 has waiters, and if so, quiesce and switch G1 to the former
  73      G2; if this results in a new G1 with waiters (G2 might have cancellations
  74      already, see __condvar_quiesce_and_switch_g1), we put the signal in the
  75      new G1.  */
  76   if ((cond->__data.__g_size[g1] != 0)
  77       || __condvar_quiesce_and_switch_g1 (cond, wseq, &g1, private))
  78     {
  79       /* Add a signal.  Relaxed MO is fine because signaling does not need to
  80          establish a happens-before relation (see above).  We do not mask the
  81          release-MO store when initializing a group in
  82          __condvar_quiesce_and_switch_g1 because we use an atomic
  83          read-modify-write and thus extend that store's release sequence.  */
  84       atomic_fetch_add_relaxed (cond->__data.__g_signals + g1, 2);
  85       cond->__data.__g_size[g1]--;
  86       /* TODO Only set it if there are indeed futex waiters.  */
  87       do_futex_wake = true;
  88     }
  89
  90   __condvar_release_lock (cond, private);
  91
  92   if (do_futex_wake)
  93     futex_wake (cond->__data.__g_signals + g1, 1, private);
  94
  95   return 0;
  96 }
  97 versioned_symbol (libpthread, ___pthread_cond_signal, pthread_cond_signal,
  98                   GLIBC_2_3_2);
  99 libc_hidden_ver (___pthread_cond_signal, __pthread_cond_signal)
 100 #ifndef SHARED
 101 strong_alias (___pthread_cond_signal, __pthread_cond_signal)
 102 #endif