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/* ----------------------------------------------------------------------------
*
* (c) The GHC Team, 2006-2009
*
* Spin locks
*
* These are simple spin-only locks as opposed to Mutexes which
* probably spin for a while before blocking in the kernel. We use
* these when we are sure that all our threads are actively running on
* a CPU, eg. in the GC.
*
* TODO: measure whether we really need these, or whether Mutexes
* would do (and be a bit safer if a CPU becomes loaded).
*
* Do not #include this file directly: #include "Rts.h" instead.
*
* To understand the structure of the RTS headers, see the wiki:
* https://gitlab.haskell.org/ghc/ghc/wikis/commentary/source-tree/includes
*
* -------------------------------------------------------------------------- */
#pragma once
#if defined(THREADED_RTS)
typedef struct SpinLock_
{
StgWord lock;
#if defined(PROF_SPIN)
StgWord64 spin; // incremented every time we spin in ACQUIRE_SPIN_LOCK
StgWord64 yield; // incremented every time we yield in ACQUIRE_SPIN_LOCK
#endif
} SpinLock;
// PROF_SPIN enables counting the number of times we spin on a lock
#if defined(PROF_SPIN)
#define IF_PROF_SPIN(x) x
#else
#define IF_PROF_SPIN(x)
#endif
// acquire spin lock
INLINE_HEADER void ACQUIRE_SPIN_LOCK(SpinLock * p)
{
do {
for (uint32_t i = 0; i < SPIN_COUNT; i++) {
StgWord32 r = cas((StgVolatilePtr)&(p->lock), 1, 0);
if (r != 0) return;
IF_PROF_SPIN(p->spin++);
busy_wait_nop();
}
IF_PROF_SPIN(p->yield++);
yieldThread();
} while (1);
}
// release spin lock
INLINE_HEADER void RELEASE_SPIN_LOCK(SpinLock * p)
{
write_barrier();
p->lock = 1;
}
// initialise spin lock
INLINE_HEADER void initSpinLock(SpinLock * p)
{
write_barrier();
p->lock = 1;
IF_PROF_SPIN(p->spin = 0);
IF_PROF_SPIN(p->yield = 0);
}
#else /* !THREADED_RTS */
// Using macros here means we don't have to ensure the argument is in scope
#define ACQUIRE_SPIN_LOCK(p) /* nothing */
#define RELEASE_SPIN_LOCK(p) /* nothing */
INLINE_HEADER void initSpinLock(void * p STG_UNUSED)
{ /* nothing */ }
#endif /* THREADED_RTS */
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