s_lock.h

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/*-------------------------------------------------------------------------
 *
 * s_lock.h
 *     Hardware-dependent implementation of spinlocks.
 *
 *  NOTE: none of the macros in this file are intended to be called directly.
 *  Call them through the hardware-independent macros in spin.h.
 *
 *  The following hardware-dependent macros must be provided for each
 *  supported platform:
 *
 *  void S_INIT_LOCK(slock_t *lock)
 *      Initialize a spinlock (to the unlocked state).
 *
 *  int S_LOCK(slock_t *lock)
 *      Acquire a spinlock, waiting if necessary.
 *      Time out and abort() if unable to acquire the lock in a
 *      "reasonable" amount of time --- typically ~ 1 minute.
 *      Should return number of "delays"; see s_lock.c
 *
 *  void S_UNLOCK(slock_t *lock)
 *      Unlock a previously acquired lock.
 *
 *  bool S_LOCK_FREE(slock_t *lock)
 *      Tests if the lock is free. Returns true if free, false if locked.
 *      This does *not* change the state of the lock.
 *
 *  void SPIN_DELAY(void)
 *      Delay operation to occur inside spinlock wait loop.
 *
 *  Note to implementors: there are default implementations for all these
 *  macros at the bottom of the file.  Check if your platform can use
 *  these or needs to override them.
 *
 *  Usually, S_LOCK() is implemented in terms of even lower-level macros
 *  TAS() and TAS_SPIN():
 *
 *  int TAS(slock_t *lock)
 *      Atomic test-and-set instruction.  Attempt to acquire the lock,
 *      but do *not* wait.  Returns 0 if successful, nonzero if unable
 *      to acquire the lock.
 *
 *  int TAS_SPIN(slock_t *lock)
 *      Like TAS(), but this version is used when waiting for a lock
 *      previously found to be contended.  By default, this is the
 *      same as TAS(), but on some architectures it's better to poll a
 *      contended lock using an unlocked instruction and retry the
 *      atomic test-and-set only when it appears free.
 *
 *  TAS() and TAS_SPIN() are NOT part of the API, and should never be called
 *  directly.
 *
 *  CAUTION: on some platforms TAS() and/or TAS_SPIN() may sometimes report
 *  failure to acquire a lock even when the lock is not locked.  For example,
 *  on Alpha TAS() will "fail" if interrupted.  Therefore a retry loop must
 *  always be used, even if you are certain the lock is free.
 *
 *  It is the responsibility of these macros to make sure that the compiler
 *  does not re-order accesses to shared memory to precede the actual lock
 *  acquisition, or follow the lock release.  Prior to PostgreSQL 9.5, this
 *  was the caller's responsibility, which meant that callers had to use
 *  volatile-qualified pointers to refer to both the spinlock itself and the
 *  shared data being accessed within the spinlocked critical section.  This
 *  was notationally awkward, easy to forget (and thus error-prone), and
 *  prevented some useful compiler optimizations.  For these reasons, we
 *  now require that the macros themselves prevent compiler re-ordering,
 *  so that the caller doesn't need to take special precautions.
 *
 *  On platforms with weak memory ordering, the TAS(), TAS_SPIN(), and
 *  S_UNLOCK() macros must further include hardware-level memory fence
 *  instructions to prevent similar re-ordering at the hardware level.
 *  TAS() and TAS_SPIN() must guarantee that loads and stores issued after
 *  the macro are not executed until the lock has been obtained.  Conversely,
 *  S_UNLOCK() must guarantee that loads and stores issued before the macro
 *  have been executed before the lock is released.
 *
 *  On most supported platforms, TAS() uses a tas() function written
 *  in assembly language to execute a hardware atomic-test-and-set
 *  instruction.  Equivalent OS-supplied mutex routines could be used too.
 *
 *  If no system-specific TAS() is available (ie, HAVE_SPINLOCKS is not
 *  defined), then we fall back on an emulation that uses SysV semaphores
 *  (see spin.c).  This emulation will be MUCH MUCH slower than a proper TAS()
 *  implementation, because of the cost of a kernel call per lock or unlock.
 *  An old report is that Postgres spends around 40% of its time in semop(2)
 *  when using the SysV semaphore code.
 *
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *    src/include/storage/s_lock.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef S_LOCK_H
#define S_LOCK_H
 
#ifdef FRONTEND
#error "s_lock.h may not be included from frontend code"
#endif
 
#ifdef HAVE_SPINLOCKS   /* skip spinlocks if requested */
 
#if defined(__GNUC__) || defined(__INTEL_COMPILER)
/*************************************************************************
 * All the gcc inlines
 * Gcc consistently defines the CPU as __cpu__.
 * Other compilers use __cpu or __cpu__ so we test for both in those cases.
 */
 
/*----------
 * Standard gcc asm format (assuming "volatile slock_t *lock"):
 
    __asm__ __volatile__(
        "   instruction \n"
        "   instruction \n"
        "   instruction \n"
:       "=r"(_res), "+m"(*lock)     // return register, in/out lock value
:       "r"(lock)                   // lock pointer, in input register
:       "memory", "cc");            // show clobbered registers here
 
 * The output-operands list (after first colon) should always include
 * "+m"(*lock), whether or not the asm code actually refers to this
 * operand directly.  This ensures that gcc believes the value in the
 * lock variable is used and set by the asm code.  Also, the clobbers
 * list (after third colon) should always include "memory"; this prevents
 * gcc from thinking it can cache the values of shared-memory fields
 * across the asm code.  Add "cc" if your asm code changes the condition
 * code register, and also list any temp registers the code uses.
 *----------
 */
 
 
#ifdef __i386__     /* 32-bit i386 */
#define HAS_TEST_AND_SET
 
typedef unsigned char slock_t;
 
#define TAS(lock) tas(lock)
 
static __inline__ int
tas(volatile slock_t *lock)
{
    slock_t     _res = 1;
 
    /*
     * Use a non-locking test before asserting the bus lock.  Note that the
     * extra test appears to be a small loss on some x86 platforms and a small
     * win on others; it's by no means clear that we should keep it.
     *
     * When this was last tested, we didn't have separate TAS() and TAS_SPIN()
     * macros.  Nowadays it probably would be better to do a non-locking test
     * in TAS_SPIN() but not in TAS(), like on x86_64, but no-one's done the
     * testing to verify that.  Without some empirical evidence, better to
     * leave it alone.
     */
    __asm__ __volatile__(
        "   cmpb    $0,%1   \n"
        "   jne     1f      \n"
        "   lock            \n"
        "   xchgb   %0,%1   \n"
        "1: \n"
:       "+q"(_res), "+m"(*lock)
:       /* no inputs */
:       "memory", "cc");
    return (int) _res;
}
 
#define SPIN_DELAY() spin_delay()
 
static __inline__ void
spin_delay(void)
{
    /*
     * This sequence is equivalent to the PAUSE instruction ("rep" is
     * ignored by old IA32 processors if the following instruction is
     * not a string operation); the IA-32 Architecture Software
     * Developer's Manual, Vol. 3, Section 7.7.2 describes why using
     * PAUSE in the inner loop of a spin lock is necessary for good
     * performance:
     *
     *     The PAUSE instruction improves the performance of IA-32
     *     processors supporting Hyper-Threading Technology when
     *     executing spin-wait loops and other routines where one
     *     thread is accessing a shared lock or semaphore in a tight
     *     polling loop. When executing a spin-wait loop, the
     *     processor can suffer a severe performance penalty when
     *     exiting the loop because it detects a possible memory order
     *     violation and flushes the core processor's pipeline. The
     *     PAUSE instruction provides a hint to the processor that the
     *     code sequence is a spin-wait loop. The processor uses this
     *     hint to avoid the memory order violation and prevent the
     *     pipeline flush. In addition, the PAUSE instruction
     *     de-pipelines the spin-wait loop to prevent it from
     *     consuming execution resources excessively.
     */
    __asm__ __volatile__(
        " rep; nop          \n");
}
 
#endif   /* __i386__ */
 
 
#ifdef __x86_64__       /* AMD Opteron, Intel EM64T */
#define HAS_TEST_AND_SET
 
typedef unsigned char slock_t;
 
#define TAS(lock) tas(lock)
 
/*
 * On Intel EM64T, it's a win to use a non-locking test before the xchg proper,
 * but only when spinning.
 *
 * See also Implementing Scalable Atomic Locks for Multi-Core Intel(tm) EM64T
 * and IA32, by Michael Chynoweth and Mary R. Lee. As of this writing, it is
 * available at:
 * http://software.intel.com/en-us/articles/implementing-scalable-atomic-locks-for-multi-core-intel-em64t-and-ia32-architectures
 */
#define TAS_SPIN(lock)    (*(lock) ? 1 : TAS(lock))
 
static __inline__ int
tas(volatile slock_t *lock)
{
    slock_t     _res = 1;
 
    __asm__ __volatile__(
        "   lock            \n"
        "   xchgb   %0,%1   \n"
:       "+q"(_res), "+m"(*lock)
:       /* no inputs */
:       "memory", "cc");
    return (int) _res;
}
 
#define SPIN_DELAY() spin_delay()
 
static __inline__ void
spin_delay(void)
{
    /*
     * Adding a PAUSE in the spin delay loop is demonstrably a no-op on
     * Opteron, but it may be of some use on EM64T, so we keep it.
     */
    __asm__ __volatile__(
        " rep; nop          \n");
}
 
#endif   /* __x86_64__ */
 
 
/*
 * On ARM and ARM64, we use __sync_lock_test_and_set(int *, int) if available.
 *
 * We use the int-width variant of the builtin because it works on more chips
 * than other widths.
 */
#if defined(__arm__) || defined(__arm) || defined(__aarch64__)
#ifdef HAVE_GCC__SYNC_INT32_TAS
#define HAS_TEST_AND_SET
 
#define TAS(lock) tas(lock)
 
typedef int slock_t;
 
static __inline__ int
tas(volatile slock_t *lock)
{
    return __sync_lock_test_and_set(lock, 1);
}
 
#define S_UNLOCK(lock) __sync_lock_release(lock)
 
/*
 * Using an ISB instruction to delay in spinlock loops appears beneficial on
 * high-core-count ARM64 processors.  It seems mostly a wash for smaller gear,
 * and ISB doesn't exist at all on pre-v7 ARM chips.
 */
#if defined(__aarch64__)
 
#define SPIN_DELAY() spin_delay()
 
static __inline__ void
spin_delay(void)
{
    __asm__ __volatile__(
        " isb;              \n");
}
 
#endif   /* __aarch64__ */
#endif   /* HAVE_GCC__SYNC_INT32_TAS */
#endif   /* __arm__ || __arm || __aarch64__ */
 
 
/* S/390 and S/390x Linux (32- and 64-bit zSeries) */
#if defined(__s390__) || defined(__s390x__)
#define HAS_TEST_AND_SET
 
typedef unsigned int slock_t;
 
#define TAS(lock)      tas(lock)
 
static __inline__ int
tas(volatile slock_t *lock)
{
    int         _res = 0;
 
    __asm__ __volatile__(
        "   cs  %0,%3,0(%2)     \n"
:       "+d"(_res), "+m"(*lock)
:       "a"(lock), "d"(1)
:       "memory", "cc");
    return _res;
}
 
#endif   /* __s390__ || __s390x__ */
 
 
#if defined(__sparc__)      /* Sparc */
/*
 * Solaris has always run sparc processors in TSO (total store) mode, but
 * linux didn't use to and the *BSDs still don't. So, be careful about
 * acquire/release semantics. The CPU will treat superfluous members as
 * NOPs, so it's just code space.
 */
#define HAS_TEST_AND_SET
 
typedef unsigned char slock_t;
 
#define TAS(lock) tas(lock)
 
static __inline__ int
tas(volatile slock_t *lock)
{
    slock_t     _res;
 
    /*
     *  See comment in src/backend/port/tas/sunstudio_sparc.s for why this
     *  uses "ldstub", and that file uses "cas".  gcc currently generates
     *  sparcv7-targeted binaries, so "cas" use isn't possible.
     */
    __asm__ __volatile__(
        "   ldstub  [%2], %0    \n"
:       "=r"(_res), "+m"(*lock)
:       "r"(lock)
:       "memory");
#if defined(__sparcv7) || defined(__sparc_v7__)
    /*
     * No stbar or membar available, luckily no actually produced hardware
     * requires a barrier.
     */
#elif defined(__sparcv8) || defined(__sparc_v8__)
    /* stbar is available (and required for both PSO, RMO), membar isn't */
    __asm__ __volatile__ ("stbar     \n":::"memory");
#else
    /*
     * #LoadStore (RMO) | #LoadLoad (RMO) together are the appropriate acquire
     * barrier for sparcv8+ upwards.
     */
    __asm__ __volatile__ ("membar #LoadStore | #LoadLoad \n":::"memory");
#endif
    return (int) _res;
}
 
#if defined(__sparcv7) || defined(__sparc_v7__)
/*
 * No stbar or membar available, luckily no actually produced hardware
 * requires a barrier.  We fall through to the default gcc definition of
 * S_UNLOCK in this case.
 */
#elif defined(__sparcv8) || defined(__sparc_v8__)
/* stbar is available (and required for both PSO, RMO), membar isn't */
#define S_UNLOCK(lock)  \
do \
{ \
    __asm__ __volatile__ ("stbar     \n":::"memory"); \
    *((volatile slock_t *) (lock)) = 0; \
} while (0)
#else
/*
 * #LoadStore (RMO) | #StoreStore (RMO, PSO) together are the appropriate
 * release barrier for sparcv8+ upwards.
 */
#define S_UNLOCK(lock)  \
do \
{ \
    __asm__ __volatile__ ("membar #LoadStore | #StoreStore \n":::"memory"); \
    *((volatile slock_t *) (lock)) = 0; \
} while (0)
#endif
 
#endif   /* __sparc__ */
 
 
/* PowerPC */
#if defined(__ppc__) || defined(__powerpc__) || defined(__ppc64__) || defined(__powerpc64__)
#define HAS_TEST_AND_SET
 
typedef unsigned int slock_t;
 
#define TAS(lock) tas(lock)
 
/* On PPC, it's a win to use a non-locking test before the lwarx */
#define TAS_SPIN(lock)  (*(lock) ? 1 : TAS(lock))
 
/*
 * The second operand of addi can hold a constant zero or a register number,
 * hence constraint "=&b" to avoid allocating r0.  "b" stands for "address
 * base register"; most operands having this register-or-zero property are
 * address bases, e.g. the second operand of lwax.
 *
 * NOTE: per the Enhanced PowerPC Architecture manual, v1.0 dated 7-May-2002,
 * an isync is a sufficient synchronization barrier after a lwarx/stwcx loop.
 * But if the spinlock is in ordinary memory, we can use lwsync instead for
 * better performance.
 */
static __inline__ int
tas(volatile slock_t *lock)
{
    slock_t _t;
    int _res;
 
    __asm__ __volatile__(
"   lwarx   %0,0,%3,1   \n"
"   cmpwi   %0,0        \n"
"   bne     1f          \n"
"   addi    %0,%0,1     \n"
"   stwcx.  %0,0,%3     \n"
"   beq     2f          \n"
"1: \n"
"   li      %1,1        \n"
"   b       3f          \n"
"2: \n"
"   lwsync              \n"
"   li      %1,0        \n"
"3: \n"
:   "=&b"(_t), "=r"(_res), "+m"(*lock)
:   "r"(lock)
:   "memory", "cc");
    return _res;
}
 
/*
 * PowerPC S_UNLOCK is almost standard but requires a "sync" instruction.
 * But we can use lwsync instead for better performance.
 */
#define S_UNLOCK(lock)  \
do \
{ \
    __asm__ __volatile__ (" lwsync \n" ::: "memory"); \
    *((volatile slock_t *) (lock)) = 0; \
} while (0)
 
#endif /* powerpc */
 
 
#if defined(__mips__) && !defined(__sgi)    /* non-SGI MIPS */
#define HAS_TEST_AND_SET
 
typedef unsigned int slock_t;
 
#define TAS(lock) tas(lock)
 
/*
 * Original MIPS-I processors lacked the LL/SC instructions, but if we are
 * so unfortunate as to be running on one of those, we expect that the kernel
 * will handle the illegal-instruction traps and emulate them for us.  On
 * anything newer (and really, MIPS-I is extinct) LL/SC is the only sane
 * choice because any other synchronization method must involve a kernel
 * call.  Unfortunately, many toolchains still default to MIPS-I as the
 * codegen target; if the symbol __mips shows that that's the case, we
 * have to force the assembler to accept LL/SC.
 *
 * R10000 and up processors require a separate SYNC, which has the same
 * issues as LL/SC.
 */
#if __mips < 2
#define MIPS_SET_MIPS2  "       .set mips2          \n"
#else
#define MIPS_SET_MIPS2
#endif
 
static __inline__ int
tas(volatile slock_t *lock)
{
    volatile slock_t *_l = lock;
    int         _res;
    int         _tmp;
 
    __asm__ __volatile__(
        "       .set push           \n"
        MIPS_SET_MIPS2
        "       .set noreorder      \n"
        "       .set nomacro        \n"
        "       ll      %0, %2      \n"
        "       or      %1, %0, 1   \n"
        "       sc      %1, %2      \n"
        "       xori    %1, 1       \n"
        "       or      %0, %0, %1  \n"
        "       sync                \n"
        "       .set pop              "
:       "=&r" (_res), "=&r" (_tmp), "+R" (*_l)
:       /* no inputs */
:       "memory");
    return _res;
}
 
/* MIPS S_UNLOCK is almost standard but requires a "sync" instruction */
#define S_UNLOCK(lock)  \
do \
{ \
    __asm__ __volatile__( \
        "       .set push           \n" \
        MIPS_SET_MIPS2 \
        "       .set noreorder      \n" \
        "       .set nomacro        \n" \
        "       sync                \n" \
        "       .set pop              " \
:       /* no outputs */ \
:       /* no inputs */ \
:       "memory"); \
    *((volatile slock_t *) (lock)) = 0; \
} while (0)
 
#endif /* __mips__ && !__sgi */
 
 
#if defined(__hppa) || defined(__hppa__)    /* HP PA-RISC */
/*
 * HP's PA-RISC
 *
 * Because LDCWX requires a 16-byte-aligned address, we declare slock_t as a
 * 16-byte struct.  The active word in the struct is whichever has the aligned
 * address; the other three words just sit at -1.
 */
#define HAS_TEST_AND_SET
 
typedef struct
{
    int         sema[4];
} slock_t;
 
#define TAS_ACTIVE_WORD(lock)   ((volatile int *) (((uintptr_t) (lock) + 15) & ~15))
 
static __inline__ int
tas(volatile slock_t *lock)
{
    volatile int *lockword = TAS_ACTIVE_WORD(lock);
    int         lockval;
 
    /*
     * The LDCWX instruction atomically clears the target word and
     * returns the previous value.  Hence, if the instruction returns
     * 0, someone else has already acquired the lock before we tested
     * it (i.e., we have failed).
     *
     * Notice that this means that we actually clear the word to set
     * the lock and set the word to clear the lock.  This is the
     * opposite behavior from the SPARC LDSTUB instruction.  For some
     * reason everything that H-P does is rather baroque...
     *
     * For details about the LDCWX instruction, see the "Precision
     * Architecture and Instruction Reference Manual" (09740-90014 of June
     * 1987), p. 5-38.
     */
    __asm__ __volatile__(
        "   ldcwx   0(0,%2),%0  \n"
:       "=r"(lockval), "+m"(*lockword)
:       "r"(lockword)
:       "memory");
    return (lockval == 0);
}
 
#define S_UNLOCK(lock)  \
    do { \
        __asm__ __volatile__("" : : : "memory"); \
        *TAS_ACTIVE_WORD(lock) = -1; \
    } while (0)
 
#define S_INIT_LOCK(lock) \
    do { \
        volatile slock_t *lock_ = (lock); \
        lock_->sema[0] = -1; \
        lock_->sema[1] = -1; \
        lock_->sema[2] = -1; \
        lock_->sema[3] = -1; \
    } while (0)
 
#define S_LOCK_FREE(lock)   (*TAS_ACTIVE_WORD(lock) != 0)
 
#endif   /* __hppa || __hppa__ */
 
 
/*
 * If we have no platform-specific knowledge, but we found that the compiler
 * provides __sync_lock_test_and_set(), use that.  Prefer the int-width
 * version over the char-width version if we have both, on the rather dubious
 * grounds that that's known to be more likely to work in the ARM ecosystem.
 * (But we dealt with ARM above.)
 */
#if !defined(HAS_TEST_AND_SET)
 
#if defined(HAVE_GCC__SYNC_INT32_TAS)
#define HAS_TEST_AND_SET
 
#define TAS(lock) tas(lock)
 
typedef int slock_t;
 
static __inline__ int
tas(volatile slock_t *lock)
{
    return __sync_lock_test_and_set(lock, 1);
}
 
#define S_UNLOCK(lock) __sync_lock_release(lock)
 
#elif defined(HAVE_GCC__SYNC_CHAR_TAS)
#define HAS_TEST_AND_SET
 
#define TAS(lock) tas(lock)
 
typedef char slock_t;
 
static __inline__ int
tas(volatile slock_t *lock)
{
    return __sync_lock_test_and_set(lock, 1);
}
 
#define S_UNLOCK(lock) __sync_lock_release(lock)
 
#endif   /* HAVE_GCC__SYNC_INT32_TAS */
 
#endif  /* !defined(HAS_TEST_AND_SET) */
 
 
/*
 * Default implementation of S_UNLOCK() for gcc/icc.
 *
 * Note that this implementation is unsafe for any platform that can reorder
 * a memory access (either load or store) after a following store.  That
 * happens not to be possible on x86 and most legacy architectures (some are
 * single-processor!), but many modern systems have weaker memory ordering.
 * Those that do must define their own version of S_UNLOCK() rather than
 * relying on this one.
 */
#if !defined(S_UNLOCK)
#define S_UNLOCK(lock)  \
    do { __asm__ __volatile__("" : : : "memory");  *(lock) = 0; } while (0)
#endif
 
#endif  /* defined(__GNUC__) || defined(__INTEL_COMPILER) */
 
 
/*
 * ---------------------------------------------------------------------
 * Platforms that use non-gcc inline assembly:
 * ---------------------------------------------------------------------
 */
 
#if !defined(HAS_TEST_AND_SET)  /* We didn't trigger above, let's try here */
 
/* These are in sunstudio_(sparc|x86).s */
 
#if defined(__SUNPRO_C) && (defined(__i386) || defined(__x86_64__) || defined(__sparc__) || defined(__sparc))
#define HAS_TEST_AND_SET
 
#if defined(__i386) || defined(__x86_64__) || defined(__sparcv9) || defined(__sparcv8plus)
typedef unsigned int slock_t;
#else
typedef unsigned char slock_t;
#endif
 
extern slock_t pg_atomic_cas(volatile slock_t *lock, slock_t with,
                                      slock_t cmp);
 
#define TAS(a) (pg_atomic_cas((a), 1, 0) != 0)
#endif
 
 
#ifdef _MSC_VER
typedef LONG slock_t;
 
#define HAS_TEST_AND_SET
#define TAS(lock) (InterlockedCompareExchange(lock, 1, 0))
 
#define SPIN_DELAY() spin_delay()
 
/* If using Visual C++ on Win64, inline assembly is unavailable.
 * Use a _mm_pause intrinsic instead of rep nop.
 */
#if defined(_WIN64)
static __forceinline void
spin_delay(void)
{
    _mm_pause();
}
#else
static __forceinline void
spin_delay(void)
{
    /* See comment for gcc code. Same code, MASM syntax */
    __asm rep nop;
}
#endif
 
#include <intrin.h>
#pragma intrinsic(_ReadWriteBarrier)
 
#define S_UNLOCK(lock)  \
    do { _ReadWriteBarrier(); (*(lock)) = 0; } while (0)
 
#endif
 
 
#endif  /* !defined(HAS_TEST_AND_SET) */
 
 
/* Blow up if we didn't have any way to do spinlocks */
#ifndef HAS_TEST_AND_SET
#error PostgreSQL does not have native spinlock support on this platform.  To continue the compilation, rerun configure using --disable-spinlocks.  However, performance will be poor.  Please report this to pgsql-bugs@lists.postgresql.org.
#endif
 
 
#else   /* !HAVE_SPINLOCKS */
 
 
/*
 * Fake spinlock implementation using semaphores --- slow and prone
 * to fall foul of kernel limits on number of semaphores, so don't use this
 * unless you must!  The subroutines appear in spin.c.
 */
typedef int slock_t;
 
extern bool s_lock_free_sema(volatile slock_t *lock);
extern void s_unlock_sema(volatile slock_t *lock);
extern void s_init_lock_sema(volatile slock_t *lock, bool nested);
extern int  tas_sema(volatile slock_t *lock);
 
#define S_LOCK_FREE(lock)   s_lock_free_sema(lock)
#define S_UNLOCK(lock)   s_unlock_sema(lock)
#define S_INIT_LOCK(lock)   s_init_lock_sema(lock, false)
#define TAS(lock)   tas_sema(lock)
 
 
#endif  /* HAVE_SPINLOCKS */
 
 
/*
 * Default Definitions - override these above as needed.
 */
 
#if !defined(S_LOCK)
#define S_LOCK(lock) \
    (TAS(lock) ? s_lock((lock), __FILE__, __LINE__, __func__) : 0)
#endif   /* S_LOCK */
 
#if !defined(S_LOCK_FREE)
#define S_LOCK_FREE(lock)   (*(lock) == 0)
#endif   /* S_LOCK_FREE */
 
#if !defined(S_UNLOCK)
/*
 * Our default implementation of S_UNLOCK is essentially *(lock) = 0.  This
 * is unsafe if the platform can reorder a memory access (either load or
 * store) after a following store; platforms where this is possible must
 * define their own S_UNLOCK.  But CPU reordering is not the only concern:
 * if we simply defined S_UNLOCK() as an inline macro, the compiler might
 * reorder instructions from inside the critical section to occur after the
 * lock release.  Since the compiler probably can't know what the external
 * function s_unlock is doing, putting the same logic there should be adequate.
 * A sufficiently-smart globally optimizing compiler could break that
 * assumption, though, and the cost of a function call for every spinlock
 * release may hurt performance significantly, so we use this implementation
 * only for platforms where we don't know of a suitable intrinsic.  For the
 * most part, those are relatively obscure platform/compiler combinations to
 * which the PostgreSQL project does not have access.
 */
#define USE_DEFAULT_S_UNLOCK
extern void s_unlock(volatile slock_t *lock);
#define S_UNLOCK(lock)      s_unlock(lock)
#endif   /* S_UNLOCK */
 
#if !defined(S_INIT_LOCK)
#define S_INIT_LOCK(lock)   S_UNLOCK(lock)
#endif   /* S_INIT_LOCK */
 
#if !defined(SPIN_DELAY)
#define SPIN_DELAY()    ((void) 0)
#endif   /* SPIN_DELAY */
 
#if !defined(TAS)
extern int  tas(volatile slock_t *lock);        /* in port/.../tas.s, or
                                                 * s_lock.c */
 
#define TAS(lock)       tas(lock)
#endif   /* TAS */
 
#if !defined(TAS_SPIN)
#define TAS_SPIN(lock)  TAS(lock)
#endif   /* TAS_SPIN */
 
 
/*
 * Platform-independent out-of-line support routines
 */
extern int s_lock(volatile slock_t *lock, const char *file, int line, const char *func);
 
/* Support for dynamic adjustment of spins_per_delay */
#define DEFAULT_SPINS_PER_DELAY  100
 
extern void set_spins_per_delay(int shared_spins_per_delay);
extern int  update_spins_per_delay(int shared_spins_per_delay);
 
/*
 * Support for spin delay which is useful in various places where
 * spinlock-like procedures take place.
 */
typedef struct
{
    int         spins;
    int         delays;
    int         cur_delay;
    const char *file;
    int         line;
    const char *func;
} SpinDelayStatus;
 
static inline void
init_spin_delay(SpinDelayStatus *status,
                const char *file, int line, const char *func)
{
    status->spins = 0;
    status->delays = 0;
    status->cur_delay = 0;
    status->file = file;
    status->line = line;
    status->func = func;
}
 
#define init_local_spin_delay(status) init_spin_delay(status, __FILE__, __LINE__, __func__)
extern void perform_spin_delay(SpinDelayStatus *status);
extern void finish_spin_delay(SpinDelayStatus *status);
 
#endif   /* S_LOCK_H */