rts/SpinLock.c

   1 /* ----------------------------------------------------------------------------
   2  *
   3  * (c) The GHC Team, 2006-2009
   4  *
   5  * Spin locks
   6  *
   7  * These are simple spin-only locks as opposed to Mutexes which
   8  * probably spin for a while before blocking in the kernel.  We use
   9  * these when we are sure that all our threads are actively running on
  10  * a CPU, eg. in the GC.
  11  *
  12  * TODO: measure whether we really need these, or whether Mutexes
  13  * would do (and be a bit safer if a CPU becomes loaded).
  14  *
  15  * Do not #include this file directly: #include "Rts.h" instead.
  16  *
  17  * To understand the structure of the RTS headers, see the wiki:
  18  *   https://gitlab.haskell.org/ghc/ghc/wikis/commentary/source-tree/includes
  19  *
  20  * -------------------------------------------------------------------------- */
  21
  22 #include "rts/PosixSource.h"
  23 #include "Rts.h"
  24
  25 #if defined(THREADED_RTS)
  26
  27 #if defined(PROF_SPIN)
  28
  29 ATTR_ALWAYS_INLINE static inline bool try_acquire_spin_slow_path(SpinLock * p)
  30 {
  31     StgWord r;
  32     r = cas((StgVolatilePtr)&(p->lock), 1, 0);
  33     if (r == 0) RELAXED_ADD(&p->spin, 1);
  34     return r != 0;
  35 }
  36
  37 #else /* !PROF_SPIN */
  38
  39 ATTR_ALWAYS_INLINE static inline bool try_acquire_spin_slow_path(SpinLock * p)
  40 {
  41     StgWord r;
  42     // Note
  43     //
  44     // Here we first check if we can obtain the lock without trying to cas.
  45     // The cas instruction will add extra inter-CPU traffic on most CPU
  46     // architectures as it has to invalidate cache lines. Rather than adding
  47     // this traffic in the spin loop, we rather restrict it to times when the
  48     // lock might be available.
  49     //
  50     // We do not need to do this when PROF_SPIN is enabled, since we write to
  51     // the lock in both cases (acquired/not acquired).
  52     r = RELAXED_LOAD(&p->lock);
  53     if (r != 0) {
  54         r = cas((StgVolatilePtr)&(p->lock), 1, 0);
  55     }
  56     return r != 0;
  57 }
  58
  59 #endif
  60
  61 void acquire_spin_lock_slow_path(SpinLock * p)
  62 {
  63     do {
  64         for (uint32_t i = 0; i < SPIN_COUNT; i++) {
  65             if (try_acquire_spin_slow_path(p)) return;
  66             busy_wait_nop();
  67         }
  68         IF_PROF_SPIN(RELAXED_ADD(&p->yield, 1));
  69         yieldThread();
  70     } while (1);
  71 }
  72
  73 #endif