nptl/pthread_cond_signal.c

   1 /* Copyright (C) 2003-2018 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <endian.h>
  20 #include <errno.h>
  21 #include <sysdep.h>
  22 #include <futex-internal.h>
  23 #include <pthread.h>
  24 #include <pthreadP.h>
  25 #include <atomic.h>
  26 #include <stdint.h>
  27
  28 #include <shlib-compat.h>
  29 #include <stap-probe.h>
  30
  31 #include "pthread_cond_common.c"
  32
  33 /* See __pthread_cond_wait for a high-level description of the algorithm.  */
  34 int
  35 __pthread_cond_signal (pthread_cond_t *cond)
  36 {
  37   LIBC_PROBE (cond_signal, 1, cond);
  38
  39   /* First check whether there are waiters.  Relaxed MO is fine for that for
  40      the same reasons that relaxed MO is fine when observing __wseq (see
  41      below).  */
  42   unsigned int wrefs = atomic_load_relaxed (&cond->__data.__wrefs);
  43   if (wrefs >> 3 == 0)
  44     return 0;
  45   int private = __condvar_get_private (wrefs);
  46
  47   __condvar_acquire_lock (cond, private);
  48
  49   /* Load the waiter sequence number, which represents our relative ordering
  50      to any waiters.  Relaxed MO is sufficient for that because:
  51      1) We can pick any position that is allowed by external happens-before
  52         constraints.  In particular, if another __pthread_cond_wait call
  53         happened before us, this waiter must be eligible for being woken by
  54         us.  The only way do establish such a happens-before is by signaling
  55         while having acquired the mutex associated with the condvar and
  56         ensuring that the signal's critical section happens after the waiter.
  57         Thus, the mutex ensures that we see that waiter's __wseq increase.
  58      2) Once we pick a position, we do not need to communicate this to the
  59         program via a happens-before that we set up: First, any wake-up could
  60         be a spurious wake-up, so the program must not interpret a wake-up as
  61         an indication that the waiter happened before a particular signal;
  62         second, a program cannot detect whether a waiter has not yet been
  63         woken (i.e., it cannot distinguish between a non-woken waiter and one
  64         that has been woken but hasn't resumed execution yet), and thus it
  65         cannot try to deduce that a signal happened before a particular
  66         waiter.  */
  67   unsigned long long int wseq = __condvar_load_wseq_relaxed (cond);
  68   unsigned int g1 = (wseq & 1) ^ 1;
  69   wseq >>= 1;
  70   bool do_futex_wake = false;
  71
  72   /* If G1 is still receiving signals, we put the signal there.  If not, we
  73      check if G2 has waiters, and if so, quiesce and switch G1 to the former
  74      G2; if this results in a new G1 with waiters (G2 might have cancellations
  75      already, see __condvar_quiesce_and_switch_g1), we put the signal in the
  76      new G1.  */
  77   if ((cond->__data.__g_size[g1] != 0)
  78       || __condvar_quiesce_and_switch_g1 (cond, wseq, &g1, private))
  79     {
  80       /* Add a signal.  Relaxed MO is fine because signaling does not need to
  81          establish a happens-before relation (see above).  We do not mask the
  82          release-MO store when initializing a group in
  83          __condvar_quiesce_and_switch_g1 because we use an atomic
  84          read-modify-write and thus extend that store's release sequence.  */
  85       atomic_fetch_add_relaxed (cond->__data.__g_signals + g1, 2);
  86       cond->__data.__g_size[g1]--;
  87       /* TODO Only set it if there are indeed futex waiters.  */
  88       do_futex_wake = true;
  89     }
  90
  91   __condvar_release_lock (cond, private);
  92
  93   if (do_futex_wake)
  94     futex_wake (cond->__data.__g_signals + g1, 1, private);
  95
  96   return 0;
  97 }
  98
  99 versioned_symbol (libpthread, __pthread_cond_signal, pthread_cond_signal,
 100                   GLIBC_2_3_2);