Age         Owner                  TLA  Line data    Source code

                                  1                 : /*-------------------------------------------------------------------------
                                  2                 :  *
                                  3                 :  * spin.c
                                  4                 :  *     Hardware-independent implementation of spinlocks.
                                  5                 :  *
                                  6                 :  *
                                  7                 :  * For machines that have test-and-set (TAS) instructions, s_lock.h/.c
                                  8                 :  * define the spinlock implementation.  This file contains only a stub
                                  9                 :  * implementation for spinlocks using PGSemaphores.  Unless semaphores
                                 10                 :  * are implemented in a way that doesn't involve a kernel call, this
                                 11                 :  * is too slow to be very useful :-(
                                 12                 :  *
                                 13                 :  *
                                 14                 :  * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
                                 15                 :  * Portions Copyright (c) 1994, Regents of the University of California
                                 16                 :  *
                                 17                 :  *
                                 18                 :  * IDENTIFICATION
                                 19                 :  *    src/backend/storage/lmgr/spin.c
                                 20                 :  *
                                 21                 :  *-------------------------------------------------------------------------
                                 22                 :  */
                                 23                 : #include "postgres.h"
                                 24                 : 
                                 25                 : #include "storage/pg_sema.h"
                                 26                 : #include "storage/shmem.h"
                                 27                 : #include "storage/spin.h"
                                 28                 : 
                                 29                 : 
                                 30                 : #ifndef HAVE_SPINLOCKS
                                 31                 : 
                                 32                 : /*
                                 33                 :  * No TAS, so spinlocks are implemented as PGSemaphores.
                                 34                 :  */
                                 35                 : 
                                 36                 : #ifndef HAVE_ATOMICS
                                 37                 : #define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES + NUM_ATOMICS_SEMAPHORES)
                                 38                 : #else
                                 39                 : #define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES)
                                 40                 : #endif                          /* DISABLE_ATOMICS */
                                 41                 : 
                                 42                 : PGSemaphore *SpinlockSemaArray;
                                 43                 : 
                                 44                 : #else                           /* !HAVE_SPINLOCKS */
                                 45                 : 
                                 46                 : #define NUM_EMULATION_SEMAPHORES 0
                                 47                 : 
                                 48                 : #endif                          /* HAVE_SPINLOCKS */
                                 49                 : 
                                 50                 : /*
                                 51                 :  * Report the amount of shared memory needed to store semaphores for spinlock
                                 52                 :  * support.
                                 53                 :  */
                                 54                 : Size
 3378 rhaas                      55 CBC        2738 : SpinlockSemaSize(void)
                                 56                 : {
andres                     57            2738 :     return NUM_EMULATION_SEMAPHORES * sizeof(PGSemaphore);
                                 58                 : }
                                 59                 : 
                                 60                 : /*
                                 61                 :  * Report number of semaphores needed to support spinlocks.
                                 62                 :  */
                                 63                 : int
tgl                        64            2738 : SpinlockSemas(void)
                                 65                 : {
andres                     66            2738 :     return NUM_EMULATION_SEMAPHORES;
                                 67                 : }
                                 68                 : 
                                 69                 : #ifndef HAVE_SPINLOCKS
                                 70                 : 
                                 71                 : /*
                                 72                 :  * Initialize spinlock emulation.
                                 73                 :  *
                                 74                 :  * This must be called after PGReserveSemaphores().
                                 75                 :  */
                                 76                 : void
                                 77                 : SpinlockSemaInit(void)
                                 78                 : {
                                 79                 :     PGSemaphore *spinsemas;
                                 80                 :     int         nsemas = SpinlockSemas();
                                 81                 :     int         i;
                                 82                 : 
                                 83                 :     /*
                                 84                 :      * We must use ShmemAllocUnlocked(), since the spinlock protecting
                                 85                 :      * ShmemAlloc() obviously can't be ready yet.
                                 86                 :      */
                                 87                 :     spinsemas = (PGSemaphore *) ShmemAllocUnlocked(SpinlockSemaSize());
                                 88                 :     for (i = 0; i < nsemas; ++i)
                                 89                 :         spinsemas[i] = PGSemaphoreCreate();
                                 90                 :     SpinlockSemaArray = spinsemas;
                                 91                 : }
                                 92                 : 
                                 93                 : /*
                                 94                 :  * s_lock.h hardware-spinlock emulation using semaphores
                                 95                 :  *
                                 96                 :  * We map all spinlocks onto NUM_EMULATION_SEMAPHORES semaphores.  It's okay to
                                 97                 :  * map multiple spinlocks onto one semaphore because no process should ever
                                 98                 :  * hold more than one at a time.  We just need enough semaphores so that we
                                 99                 :  * aren't adding too much extra contention from that.
                                100                 :  *
                                101                 :  * There is one exception to the restriction of only holding one spinlock at a
                                102                 :  * time, which is that it's ok if emulated atomic operations are nested inside
                                103                 :  * spinlocks. To avoid the danger of spinlocks and atomic using the same sema,
                                104                 :  * we make sure "normal" spinlocks and atomics backed by spinlocks use
                                105                 :  * distinct semaphores (see the nested argument to s_init_lock_sema).
                                106                 :  *
                                107                 :  * slock_t is just an int for this implementation; it holds the spinlock
                                108                 :  * number from 1..NUM_EMULATION_SEMAPHORES.  We intentionally ensure that 0
                                109                 :  * is not a valid value, so that testing with this code can help find
                                110                 :  * failures to initialize spinlocks.
                                111                 :  */
                                112                 : 
                                113                 : static inline void
                                114                 : s_check_valid(int lockndx)
                                115                 : {
                                116                 :     if (unlikely(lockndx <= 0 || lockndx > NUM_EMULATION_SEMAPHORES))
                                117                 :         elog(ERROR, "invalid spinlock number: %d", lockndx);
                                118                 : }
                                119                 : 
                                120                 : void
                                121                 : s_init_lock_sema(volatile slock_t *lock, bool nested)
                                122                 : {
                                123                 :     static uint32 counter = 0;
                                124                 :     uint32      offset;
                                125                 :     uint32      sema_total;
                                126                 :     uint32      idx;
                                127                 : 
                                128                 :     if (nested)
                                129                 :     {
                                130                 :         /*
                                131                 :          * To allow nesting atomics inside spinlocked sections, use a
                                132                 :          * different spinlock. See comment above.
                                133                 :          */
                                134                 :         offset = 1 + NUM_SPINLOCK_SEMAPHORES;
                                135                 :         sema_total = NUM_ATOMICS_SEMAPHORES;
                                136                 :     }
                                137                 :     else
                                138                 :     {
                                139                 :         offset = 1;
                                140                 :         sema_total = NUM_SPINLOCK_SEMAPHORES;
                                141                 :     }
                                142                 : 
                                143                 :     idx = (counter++ % sema_total) + offset;
                                144                 : 
                                145                 :     /* double check we did things correctly */
                                146                 :     s_check_valid(idx);
                                147                 : 
                                148                 :     *lock = idx;
                                149                 : }
                                150                 : 
                                151                 : void
                                152                 : s_unlock_sema(volatile slock_t *lock)
                                153                 : {
                                154                 :     int         lockndx = *lock;
                                155                 : 
                                156                 :     s_check_valid(lockndx);
                                157                 : 
                                158                 :     PGSemaphoreUnlock(SpinlockSemaArray[lockndx - 1]);
                                159                 : }
                                160                 : 
                                161                 : bool
                                162                 : s_lock_free_sema(volatile slock_t *lock)
                                163                 : {
                                164                 :     /* We don't currently use S_LOCK_FREE anyway */
                                165                 :     elog(ERROR, "spin.c does not support S_LOCK_FREE()");
                                166                 :     return false;
                                167                 : }
                                168                 : 
                                169                 : int
                                170                 : tas_sema(volatile slock_t *lock)
                                171                 : {
                                172                 :     int         lockndx = *lock;
                                173                 : 
                                174                 :     s_check_valid(lockndx);
                                175                 : 
                                176                 :     /* Note that TAS macros return 0 if *success* */
                                177                 :     return !PGSemaphoreTryLock(SpinlockSemaArray[lockndx - 1]);
                                178                 : }
                                179                 : 
                                180                 : #endif                          /* !HAVE_SPINLOCKS */