nptl/sem_waitcommon.c

   1 /* sem_waitcommon -- wait on a semaphore, shared code.
   2    Copyright (C) 2003-2024 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   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    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <kernel-features.h>
  20 #include <errno.h>
  21 #include <sysdep.h>
  22 #include <futex-internal.h>
  23 #include <internaltypes.h>
  24 #include <semaphore.h>
  25 #include <sys/time.h>
  26
  27 #include <pthreadP.h>
  28 #include <shlib-compat.h>
  29 #include <atomic.h>
  30
  31
  32 /* The semaphore provides two main operations: sem_post adds a token to the
  33    semaphore; sem_wait grabs a token from the semaphore, potentially waiting
  34    until there is a token available.  A sem_wait needs to synchronize with
  35    the sem_post that provided the token, so that whatever lead to the sem_post
  36    happens before the code after sem_wait.
  37
  38    Conceptually, available tokens can simply be counted; let's call that the
  39    value of the semaphore.  However, we also want to know whether there might
  40    be a sem_wait that is blocked on the value because it was zero (using a
  41    futex with the value being the futex variable); if there is no blocked
  42    sem_wait, sem_post does not need to execute a futex_wake call.  Therefore,
  43    we also need to count the number of potentially blocked sem_wait calls
  44    (which we call nwaiters).
  45
  46    What makes this tricky is that POSIX requires that a semaphore can be
  47    destroyed as soon as the last remaining sem_wait has returned, and no
  48    other sem_wait or sem_post calls are executing concurrently.  However, the
  49    sem_post call whose token was consumed by the last sem_wait is considered
  50    to have finished once it provided the token to the sem_wait.
  51    Thus, sem_post must not access the semaphore struct anymore after it has
  52    made a token available; IOW, it needs to be able to atomically provide
  53    a token and check whether any blocked sem_wait calls might exist.
  54
  55    This is straightforward to do if the architecture provides 64b atomics
  56    because we can just put both the value and nwaiters into one variable that
  57    we access atomically: This is the data field, the value is in the
  58    least-significant 32 bits, and nwaiters in the other bits.  When sem_post
  59    makes a value available, it can atomically check nwaiters.
  60
  61    If we have only 32b atomics available, we cannot put both nwaiters and
  62    value into one 32b value because then we might have too few bits for both
  63    of those counters.  Therefore, we need to use two distinct fields.
  64
  65    To allow sem_post to atomically make a token available and check for
  66    blocked sem_wait calls, we use one bit in value to indicate whether
  67    nwaiters is nonzero.  That allows sem_post to use basically the same
  68    algorithm as with 64b atomics, but requires sem_wait to update the bit; it
  69    can't do this atomically with another access to nwaiters, but it can compute
  70    a conservative value for the bit because it's benign if the bit is set
  71    even if nwaiters is zero (all we get is an unnecessary futex wake call by
  72    sem_post).
  73    Specifically, sem_wait will unset the bit speculatively if it believes that
  74    there is no other concurrently executing sem_wait.  If it misspeculated,
  75    it will have to clean up by waking any other sem_wait call (i.e., what
  76    sem_post would do otherwise).  This does not conflict with the destruction
  77    requirement because the semaphore must not be destructed while any sem_wait
  78    is still executing.  */
  79
  80 #if !__HAVE_64B_ATOMICS
  81 static void
  82 __sem_wait_32_finish (struct new_sem *sem);
  83 #endif
  84
  85 static void
  86 __sem_wait_cleanup (void *arg)
  87 {
  88   struct new_sem *sem = (struct new_sem *) arg;
  89
  90 #if __HAVE_64B_ATOMICS
  91   /* Stop being registered as a waiter.  See below for MO.  */
  92   atomic_fetch_add_relaxed (&sem->data, -((uint64_t) 1 << SEM_NWAITERS_SHIFT));
  93 #else
  94   __sem_wait_32_finish (sem);
  95 #endif
  96 }
  97
  98 /* Wait until at least one token is available, possibly with a timeout.
  99    This is in a separate function in order to make sure gcc
 100    puts the call site into an exception region, and thus the
 101    cleanups get properly run.  TODO still necessary?  Other futex_wait
 102    users don't seem to need it.  */
 103 static int
 104 __attribute__ ((noinline))
 105 do_futex_wait (struct new_sem *sem, clockid_t clockid,
 106                const struct __timespec64 *abstime)
 107 {
 108   int err;
 109
 110 #if __HAVE_64B_ATOMICS
 111   err = __futex_abstimed_wait_cancelable64 (
 112       (unsigned int *) &sem->data + SEM_VALUE_OFFSET, 0,
 113       clockid, abstime,
 114       sem->private);
 115 #else
 116   err = __futex_abstimed_wait_cancelable64 (&sem->value, SEM_NWAITERS_MASK,
 117                                             clockid, abstime, sem->private);
 118 #endif
 119
 120   return err;
 121 }
 122
 123 /* Fast path: Try to grab a token without blocking.  */
 124 static int
 125 __new_sem_wait_fast (struct new_sem *sem, int definitive_result)
 126 {
 127   /* We need acquire MO if we actually grab a token, so that this
 128      synchronizes with all token providers (i.e., the RMW operation we read
 129      from or all those before it in modification order; also see sem_post).
 130      We do not need to guarantee any ordering if we observed that there is
 131      no token (POSIX leaves it unspecified whether functions that fail
 132      synchronize memory); thus, relaxed MO is sufficient for the initial load
 133      and the failure path of the CAS.  If the weak CAS fails and we need a
 134      definitive result, retry.  */
 135 #if __HAVE_64B_ATOMICS
 136   uint64_t d = atomic_load_relaxed (&sem->data);
 137   do
 138     {
 139       if ((d & SEM_VALUE_MASK) == 0)
 140         break;
 141       if (atomic_compare_exchange_weak_acquire (&sem->data, &d, d - 1))
 142         return 0;
 143     }
 144   while (definitive_result);
 145   return -1;
 146 #else
 147   unsigned int v = atomic_load_relaxed (&sem->value);
 148   do
 149     {
 150       if ((v >> SEM_VALUE_SHIFT) == 0)
 151         break;
 152       if (atomic_compare_exchange_weak_acquire (&sem->value,
 153           &v, v - (1 << SEM_VALUE_SHIFT)))
 154         return 0;
 155     }
 156   while (definitive_result);
 157   return -1;
 158 #endif
 159 }
 160
 161 /* Slow path that blocks.  */
 162 static int
 163 __attribute__ ((noinline))
 164 __new_sem_wait_slow64 (struct new_sem *sem, clockid_t clockid,
 165                        const struct __timespec64 *abstime)
 166 {
 167   int err = 0;
 168
 169 #if __HAVE_64B_ATOMICS
 170   /* Add a waiter.  Relaxed MO is sufficient because we can rely on the
 171      ordering provided by the RMW operations we use.  */
 172   uint64_t d = atomic_fetch_add_relaxed (&sem->data,
 173       (uint64_t) 1 << SEM_NWAITERS_SHIFT);
 174
 175   pthread_cleanup_push (__sem_wait_cleanup, sem);
 176
 177   /* Wait for a token to be available.  Retry until we can grab one.  */
 178   for (;;)
 179     {
 180       /* If there is no token available, sleep until there is.  */
 181       if ((d & SEM_VALUE_MASK) == 0)
 182         {
 183           err = do_futex_wait (sem, clockid, abstime);
 184           /* A futex return value of 0 or EAGAIN is due to a real or spurious
 185              wake-up, or due to a change in the number of tokens.  We retry in
 186              these cases.
 187              If we timed out, forward this to the caller.
 188              EINTR is returned if we are interrupted by a signal; we
 189              forward this to the caller.  (See futex_wait and related
 190              documentation.  Before Linux 2.6.22, EINTR was also returned on
 191              spurious wake-ups; we only support more recent Linux versions,
 192              so do not need to consider this here.)  */
 193           if (err == ETIMEDOUT || err == EINTR || err == EOVERFLOW)
 194             {
 195               __set_errno (err);
 196               err = -1;
 197               /* Stop being registered as a waiter.  */
 198               atomic_fetch_add_relaxed (&sem->data,
 199                   -((uint64_t) 1 << SEM_NWAITERS_SHIFT));
 200               break;
 201             }
 202           /* Relaxed MO is sufficient; see below.  */
 203           d = atomic_load_relaxed (&sem->data);
 204         }
 205       else
 206         {
 207           /* Try to grab both a token and stop being a waiter.  We need
 208              acquire MO so this synchronizes with all token providers (i.e.,
 209              the RMW operation we read from or all those before it in
 210              modification order; also see sem_post).  On the failure path,
 211              relaxed MO is sufficient because we only eventually need the
 212              up-to-date value; the futex_wait or the CAS perform the real
 213              work.  */
 214           if (atomic_compare_exchange_weak_acquire (&sem->data,
 215               &d, d - 1 - ((uint64_t) 1 << SEM_NWAITERS_SHIFT)))
 216             {
 217               err = 0;
 218               break;
 219             }
 220         }
 221     }
 222
 223   pthread_cleanup_pop (0);
 224 #else
 225   /* The main difference to the 64b-atomics implementation is that we need to
 226      access value and nwaiters in separate steps, and that the nwaiters bit
 227      in the value can temporarily not be set even if nwaiters is nonzero.
 228      We work around incorrectly unsetting the nwaiters bit by letting sem_wait
 229      set the bit again and waking the number of waiters that could grab a
 230      token.  There are two additional properties we need to ensure:
 231      (1) We make sure that whenever unsetting the bit, we see the increment of
 232      nwaiters by the other thread that set the bit.  IOW, we will notice if
 233      we make a mistake.
 234      (2) When setting the nwaiters bit, we make sure that we see the unsetting
 235      of the bit by another waiter that happened before us.  This avoids having
 236      to blindly set the bit whenever we need to block on it.  We set/unset
 237      the bit while having incremented nwaiters (i.e., are a registered
 238      waiter), and the problematic case only happens when one waiter indeed
 239      followed another (i.e., nwaiters was never larger than 1); thus, this
 240      works similarly as with a critical section using nwaiters (see the MOs
 241      and related comments below).
 242
 243      An alternative approach would be to unset the bit after decrementing
 244      nwaiters; however, that would result in needing Dekker-like
 245      synchronization and thus full memory barriers.  We also would not be able
 246      to prevent misspeculation, so this alternative scheme does not seem
 247      beneficial.  */
 248   unsigned int v;
 249
 250   /* Add a waiter.  We need acquire MO so this synchronizes with the release
 251      MO we use when decrementing nwaiters below; it ensures that if another
 252      waiter unset the bit before us, we see that and set it again.  Also see
 253      property (2) above.  */
 254   atomic_fetch_add_acquire (&sem->nwaiters, 1);
 255
 256   pthread_cleanup_push (__sem_wait_cleanup, sem);
 257
 258   /* Wait for a token to be available.  Retry until we can grab one.  */
 259   /* We do not need any ordering wrt. to this load's reads-from, so relaxed
 260      MO is sufficient.  The acquire MO above ensures that in the problematic
 261      case, we do see the unsetting of the bit by another waiter.  */
 262   v = atomic_load_relaxed (&sem->value);
 263   do
 264     {
 265       do
 266         {
 267           /* We are about to block, so make sure that the nwaiters bit is
 268              set.  We need release MO on the CAS to ensure that when another
 269              waiter unsets the nwaiters bit, it will also observe that we
 270              incremented nwaiters in the meantime (also see the unsetting of
 271              the bit below).  Relaxed MO on CAS failure is sufficient (see
 272              above).  */
 273           do
 274             {
 275               if ((v & SEM_NWAITERS_MASK) != 0)
 276                 break;
 277             }
 278           while (!atomic_compare_exchange_weak_release (&sem->value,
 279               &v, v | SEM_NWAITERS_MASK));
 280           /* If there is no token, wait.  */
 281           if ((v >> SEM_VALUE_SHIFT) == 0)
 282             {
 283               /* See __HAVE_64B_ATOMICS variant.  */
 284               err = do_futex_wait (sem, clockid, abstime);
 285               if (err == ETIMEDOUT || err == EINTR)
 286                 {
 287                   __set_errno (err);
 288                   err = -1;
 289                   goto error;
 290                 }
 291               err = 0;
 292               /* We blocked, so there might be a token now.  Relaxed MO is
 293                  sufficient (see above).  */
 294               v = atomic_load_relaxed (&sem->value);
 295             }
 296         }
 297       /* If there is no token, we must not try to grab one.  */
 298       while ((v >> SEM_VALUE_SHIFT) == 0);
 299     }
 300   /* Try to grab a token.  We need acquire MO so this synchronizes with
 301      all token providers (i.e., the RMW operation we read from or all those
 302      before it in modification order; also see sem_post).  */
 303   while (!atomic_compare_exchange_weak_acquire (&sem->value,
 304       &v, v - (1 << SEM_VALUE_SHIFT)));
 305
 306 error:
 307   pthread_cleanup_pop (0);
 308
 309   __sem_wait_32_finish (sem);
 310 #endif
 311
 312   return err;
 313 }
 314
 315 /* Stop being a registered waiter (non-64b-atomics code only).  */
 316 #if !__HAVE_64B_ATOMICS
 317 static void
 318 __sem_wait_32_finish (struct new_sem *sem)
 319 {
 320   /* The nwaiters bit is still set, try to unset it now if this seems
 321      necessary.  We do this before decrementing nwaiters so that the unsetting
 322      is visible to other waiters entering after us.  Relaxed MO is sufficient
 323      because we are just speculating here; a stronger MO would not prevent
 324      misspeculation.  */
 325   unsigned int wguess = atomic_load_relaxed (&sem->nwaiters);
 326   if (wguess == 1)
 327     /* We might be the last waiter, so unset.  This needs acquire MO so that
 328        it synchronizes with the release MO when setting the bit above; if we
 329        overwrite someone else that set the bit, we'll read in the following
 330        decrement of nwaiters at least from that release sequence, so we'll
 331        see if the other waiter is still active or if another writer entered
 332        in the meantime (i.e., using the check below).  */
 333     atomic_fetch_and_acquire (&sem->value, ~SEM_NWAITERS_MASK);
 334
 335   /* Now stop being a waiter, and see whether our guess was correct.
 336      This needs release MO so that it synchronizes with the acquire MO when
 337      a waiter increments nwaiters; this makes sure that newer writers see that
 338      we reset the waiters_present bit.  */
 339   unsigned int wfinal = atomic_fetch_add_release (&sem->nwaiters, -1);
 340   if (wfinal > 1 && wguess == 1)
 341     {
 342       /* We guessed wrong, and so need to clean up after the mistake and
 343          unblock any waiters that could have not been woken.  There is no
 344          additional ordering that we need to set up, so relaxed MO is
 345          sufficient.  */
 346       unsigned int v = atomic_fetch_or_relaxed (&sem->value,
 347                                                 SEM_NWAITERS_MASK);
 348       /* If there are available tokens, then wake as many waiters.  If there
 349          aren't any, then there is no need to wake anyone because there is
 350          none to grab for another waiter.  If tokens become available
 351          subsequently, then the respective sem_post calls will do the wake-up
 352          due to us having set the nwaiters bit again.  */
 353       v >>= SEM_VALUE_SHIFT;
 354       if (v > 0)
 355         futex_wake (&sem->value, v, sem->private);
 356     }
 357 }
 358 #endif