path: root/nptl/pthread_cond_wait.c

/* Copyright (C) 2003-2025 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#include <endian.h>
#include <errno.h>
#include <sysdep.h>
#include <futex-internal.h>
#include <pthread.h>
#include <pthreadP.h>
#include <sys/time.h>
#include <atomic.h>
#include <stdint.h>
#include <stdbool.h>

#include <shlib-compat.h>
#include <stap-probe.h>
#include <time.h>

#include "pthread_cond_common.c"


struct _condvar_cleanup_buffer
{
  uint64_t wseq;
  pthread_cond_t *cond;
  pthread_mutex_t *mutex;
  int private;
};


/* Decrease the waiter reference count.  */
static void
__condvar_confirm_wakeup (pthread_cond_t *cond, int private)
{
  /* If destruction is pending (i.e., the wake-request flag is nonzero) and we
     are the last waiter (prior value of __wrefs was 1 << 3), then wake any
     threads waiting in pthread_cond_destroy.  Release MO to synchronize with
     these threads.  Don't bother clearing the wake-up request flag.  */
  if ((atomic_fetch_add_release (&cond->__data.__wrefs, -8) >> 2) == 3)
    futex_wake (&cond->__data.__wrefs, INT_MAX, private);
}


/* Cancel waiting after having registered as a waiter previously.  SEQ is our
   position and G is our group index.
   The goal of cancellation is to make our group smaller if that is still
   possible.  If we are in a closed group, this is not possible anymore; in
   this case, we need to send a replacement signal for the one we effectively
   consumed because the signal should have gotten consumed by another waiter
   instead; we must not both cancel waiting and consume a signal.

   Must not be called while still holding a reference on the group.

   Returns true iff we consumed a signal.

   On some kind of timeouts, we may be able to pretend that a signal we
   effectively consumed happened before the timeout (i.e., similarly to first
   spinning on signals before actually checking whether the timeout has
   passed already).  Doing this would allow us to skip sending a replacement
   signal, but this case might happen rarely because the end of the timeout
   must race with someone else sending a signal.  Therefore, we don't bother
   trying to optimize this.  */
static void
__condvar_cancel_waiting (pthread_cond_t *cond, uint64_t seq, unsigned int g,
			  int private)
{
  bool consumed_signal = false;

  /* No deadlock with group switching is possible here because we do
     not hold a reference on the group.  */
  __condvar_acquire_lock (cond, private);

  uint64_t g1_start = __condvar_load_g1_start_relaxed (cond);
  if (g1_start > seq)
    {
      /* Our group is closed, so someone provided enough signals for it.
	 Thus, we effectively consumed a signal.  */
      consumed_signal = true;
    }
  else
    {
      if (g1_start + __condvar_get_orig_size (cond) <= seq)
	{
	  /* We are in the current G2 and thus cannot have consumed a signal.
	     Reduce its effective size or handle overflow.  Remember that in
	     G2, unsigned int size is zero or a negative value.  */
	  if (cond->__data.__g_size[g] + __PTHREAD_COND_MAX_GROUP_SIZE > 0)
	    {
	      cond->__data.__g_size[g]--;
	    }
	  else
	    {
	      /* Cancellations would overflow the maximum group size.  Just
		 wake up everyone spuriously to create a clean state.  This
		 also means we do not consume a signal someone else sent.  */
	      __condvar_release_lock (cond, private);
	      __pthread_cond_broadcast (cond);
	      return;
	    }
	}
      else
	{
	  /* We are in current G1.  If the group's size is zero, someone put
	     a signal in the group that nobody else but us can consume.  */
	  if (cond->__data.__g_size[g] == 0)
	    consumed_signal = true;
	  else
	    {
	      /* Otherwise, we decrease the size of the group.  This is
		 equivalent to atomically putting in a signal just for us and
		 consuming it right away.  We do not consume a signal sent
		 by someone else.  We also cannot have consumed a futex
		 wake-up because if we were cancelled or timed out in a futex
		 call, the futex will wake another waiter.  */
	      cond->__data.__g_size[g]--;
	    }
	}
    }

  __condvar_release_lock (cond, private);

  if (consumed_signal)
    {
      /* We effectively consumed a signal even though we didn't want to.
	 Therefore, we need to send a replacement signal.
	 If we would want to optimize this, we could do what
	 pthread_cond_signal does right in the critical section above.  */
      __pthread_cond_signal (cond);
    }
}

/* Clean-up for cancellation of waiters waiting for normal signals.  We cancel
   our registration as a waiter, confirm we have woken up, and re-acquire the
   mutex.  */
static void
__condvar_cleanup_waiting (void *arg)
{
  struct _condvar_cleanup_buffer *cbuffer =
    (struct _condvar_cleanup_buffer *) arg;
  pthread_cond_t *cond = cbuffer->cond;
  unsigned g = cbuffer->wseq & 1;

  __condvar_cancel_waiting (cond, cbuffer->wseq >> 1, g, cbuffer->private);
  /* FIXME With the current cancellation implementation, it is possible that
     a thread is cancelled after it has returned from a syscall.  This could
     result in a cancelled waiter consuming a futex wake-up that is then
     causing another waiter in the same group to not wake up.  To work around
     this issue until we have fixed cancellation, just add a futex wake-up
     conservatively.  */
  futex_wake (cond->__data.__g_signals + g, 1, cbuffer->private);

  __condvar_confirm_wakeup (cond, cbuffer->private