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/*-------------------------------------------------------------------------
*
* atomics.c
* Non-Inline parts of the atomics implementation
*
* Portions Copyright (c) 2013-2024, PostgreSQL Global Development Group
*
*
* IDENTIFICATION
* src/backend/port/atomics.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "miscadmin.h"
#include "port/atomics.h"
#include "storage/spin.h"
#ifdef PG_HAVE_MEMORY_BARRIER_EMULATION
#ifdef WIN32
#error "barriers are required (and provided) on WIN32 platforms"
#endif
#include <signal.h>
#endif
#ifdef PG_HAVE_MEMORY_BARRIER_EMULATION
void
pg_spinlock_barrier(void)
{
/*
* NB: we have to be reentrant here, some barriers are placed in signal
* handlers.
*
* We use kill(0) for the fallback barrier as we assume that kernels on
* systems old enough to require fallback barrier support will include an
* appropriate barrier while checking the existence of the postmaster pid.
*/
(void) kill(PostmasterPid, 0);
}
#endif
#ifdef PG_HAVE_COMPILER_BARRIER_EMULATION
void
pg_extern_compiler_barrier(void)
{
/* do nothing */
}
#endif
#ifdef PG_HAVE_ATOMIC_U64_SIMULATION
void
pg_atomic_init_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 val_)
{
StaticAssertDecl(sizeof(ptr->sema) >= sizeof(slock_t),
"size mismatch of atomic_uint64 vs slock_t");
SpinLockInit((slock_t *) &ptr->sema);
ptr->value = val_;
}
bool
pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
uint64 *expected, uint64 newval)
{
bool ret;
/*
* Do atomic op under a spinlock. It might look like we could just skip
* the cmpxchg if the lock isn't available, but that'd just emulate a
* 'weak' compare and swap. I.e. one that allows spurious failures. Since
* several algorithms rely on a strong variant and that is efficiently
* implementable on most major architectures let's emulate it here as
* well.
*/
SpinLockAcquire((slock_t *) &ptr->sema);
/* perform compare/exchange logic */
ret = ptr->value == *expected;
*expected = ptr->value;
if (ret)
ptr->value = newval;
/* and release lock */
SpinLockRelease((slock_t *) &ptr->sema);
return ret;
}
uint64
pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
{
uint64 oldval;
SpinLockAcquire((slock_t *) &ptr->sema);
oldval = ptr->value;
ptr->value += add_;
SpinLockRelease((slock_t *) &ptr->sema);
return oldval;
}
#endif /* PG_HAVE_ATOMIC_U64_SIMULATION */
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