atomics.c

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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_FLAG_SIMULATION
 
void
pg_atomic_init_flag_impl(volatile pg_atomic_flag *ptr)
{
    StaticAssertDecl(sizeof(ptr->sema) >= sizeof(slock_t),
                     "size mismatch of atomic_flag vs slock_t");
 
#ifndef HAVE_SPINLOCKS
 
    /*
     * NB: If we're using semaphore based TAS emulation, be careful to use a
     * separate set of semaphores. Otherwise we'd get in trouble if an atomic
     * var would be manipulated while spinlock is held.
     */
    s_init_lock_sema((slock_t *) &ptr->sema, true);
#else
    SpinLockInit((slock_t *) &ptr->sema);
#endif
 
    ptr->value = false;
}
 
bool
pg_atomic_test_set_flag_impl(volatile pg_atomic_flag *ptr)
{
    uint32      oldval;
 
    SpinLockAcquire((slock_t *) &ptr->sema);
    oldval = ptr->value;
    ptr->value = true;
    SpinLockRelease((slock_t *) &ptr->sema);
 
    return oldval == 0;
}
 
void
pg_atomic_clear_flag_impl(volatile pg_atomic_flag *ptr)
{
    SpinLockAcquire((slock_t *) &ptr->sema);
    ptr->value = false;
    SpinLockRelease((slock_t *) &ptr->sema);
}
 
bool
pg_atomic_unlocked_test_flag_impl(volatile pg_atomic_flag *ptr)
{
    return ptr->value == 0;
}
 
#endif                          /* PG_HAVE_ATOMIC_FLAG_SIMULATION */
 
#ifdef PG_HAVE_ATOMIC_U32_SIMULATION
void
pg_atomic_init_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val_)
{
    StaticAssertDecl(sizeof(ptr->sema) >= sizeof(slock_t),
                     "size mismatch of atomic_uint32 vs slock_t");
 
    /*
     * If we're using semaphore based atomic flags, be careful about nested
     * usage of atomics while a spinlock is held.
     */
#ifndef HAVE_SPINLOCKS
    s_init_lock_sema((slock_t *) &ptr->sema, true);
#else
    SpinLockInit((slock_t *) &ptr->sema);
#endif
    ptr->value = val_;
}
 
void
pg_atomic_write_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val)
{
    /*
     * One might think that an unlocked write doesn't need to acquire the
     * spinlock, but one would be wrong. Even an unlocked write has to cause a
     * concurrent pg_atomic_compare_exchange_u32() (et al) to fail.
     */
    SpinLockAcquire((slock_t *) &ptr->sema);
    ptr->value = val;
    SpinLockRelease((slock_t *) &ptr->sema);
}
 
bool
pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
                                    uint32 *expected, uint32 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;
}
 
uint32
pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
{
    uint32      oldval;
 
    SpinLockAcquire((slock_t *) &ptr->sema);
    oldval = ptr->value;
    ptr->value += add_;
    SpinLockRelease((slock_t *) &ptr->sema);
    return oldval;
}
 
#endif                          /* PG_HAVE_ATOMIC_U32_SIMULATION */
 
 
#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");
 
    /*
     * If we're using semaphore based atomic flags, be careful about nested
     * usage of atomics while a spinlock is held.
     */
#ifndef HAVE_SPINLOCKS
    s_init_lock_sema((slock_t *) &ptr->sema, true);
#else
    SpinLockInit((slock_t *) &ptr->sema);
#endif
    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 */