mirror of https://github.com/postgres/postgres
A later change will require atomic support, so it wouldn't make sense for a hypothetical new system not to be able to implement spinlocks. Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (concept, not the patch) Reviewed-by: Andres Freund <andres@anarazel.de> (concept, not the patch) Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.uspull/173/head
Thomas Munro
11 months ago
parent
1330843bb7
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/*-------------------------------------------------------------------------
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* |
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* spin.c |
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* Hardware-independent implementation of spinlocks. |
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* |
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* |
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* For machines that have test-and-set (TAS) instructions, s_lock.h/.c |
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* define the spinlock implementation. This file contains only a stub |
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* implementation for spinlocks using PGSemaphores. Unless semaphores |
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* are implemented in a way that doesn't involve a kernel call, this |
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* is too slow to be very useful :-( |
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* |
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* |
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* Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group |
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* Portions Copyright (c) 1994, Regents of the University of California |
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* |
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* |
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* IDENTIFICATION |
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* src/backend/storage/lmgr/spin.c |
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* |
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*------------------------------------------------------------------------- |
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*/ |
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#include "postgres.h" |
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#include "storage/pg_sema.h" |
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#include "storage/shmem.h" |
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#include "storage/spin.h" |
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#ifndef HAVE_SPINLOCKS |
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/*
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* No TAS, so spinlocks are implemented as PGSemaphores. |
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*/ |
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#ifndef HAVE_ATOMICS |
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#define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES + NUM_ATOMICS_SEMAPHORES) |
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#else |
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#define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES) |
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#endif /* HAVE_ATOMICS */ |
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PGSemaphore *SpinlockSemaArray; |
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#else /* !HAVE_SPINLOCKS */ |
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#define NUM_EMULATION_SEMAPHORES 0 |
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#endif /* HAVE_SPINLOCKS */ |
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/*
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* Report the amount of shared memory needed to store semaphores for spinlock |
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* support. |
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*/ |
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Size |
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SpinlockSemaSize(void) |
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{ |
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return NUM_EMULATION_SEMAPHORES * sizeof(PGSemaphore); |
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} |
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/*
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* Report number of semaphores needed to support spinlocks. |
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*/ |
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int |
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SpinlockSemas(void) |
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{ |
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return NUM_EMULATION_SEMAPHORES; |
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} |
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#ifndef HAVE_SPINLOCKS |
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/*
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* Initialize spinlock emulation. |
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* |
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* This must be called after PGReserveSemaphores(). |
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*/ |
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void |
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SpinlockSemaInit(void) |
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{ |
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PGSemaphore *spinsemas; |
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int nsemas = SpinlockSemas(); |
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int i; |
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/*
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* We must use ShmemAllocUnlocked(), since the spinlock protecting |
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* ShmemAlloc() obviously can't be ready yet. |
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*/ |
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spinsemas = (PGSemaphore *) ShmemAllocUnlocked(SpinlockSemaSize()); |
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for (i = 0; i < nsemas; ++i) |
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spinsemas[i] = PGSemaphoreCreate(); |
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SpinlockSemaArray = spinsemas; |
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} |
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/*
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* s_lock.h hardware-spinlock emulation using semaphores |
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* |
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* We map all spinlocks onto NUM_EMULATION_SEMAPHORES semaphores. It's okay to |
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* map multiple spinlocks onto one semaphore because no process should ever |
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* hold more than one at a time. We just need enough semaphores so that we |
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* aren't adding too much extra contention from that. |
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* |
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* There is one exception to the restriction of only holding one spinlock at a |
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* time, which is that it's ok if emulated atomic operations are nested inside |
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* spinlocks. To avoid the danger of spinlocks and atomic using the same sema, |
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* we make sure "normal" spinlocks and atomics backed by spinlocks use |
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* distinct semaphores (see the nested argument to s_init_lock_sema). |
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* |
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* slock_t is just an int for this implementation; it holds the spinlock |
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* number from 1..NUM_EMULATION_SEMAPHORES. We intentionally ensure that 0 |
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* is not a valid value, so that testing with this code can help find |
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* failures to initialize spinlocks. |
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*/ |
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static inline void |
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s_check_valid(int lockndx) |
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{ |
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if (unlikely(lockndx <= 0 || lockndx > NUM_EMULATION_SEMAPHORES)) |
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elog(ERROR, "invalid spinlock number: %d", lockndx); |
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} |
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void |
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s_init_lock_sema(volatile slock_t *lock, bool nested) |
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{ |
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static uint32 counter = 0; |
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uint32 offset; |
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uint32 sema_total; |
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uint32 idx; |
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if (nested) |
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{ |
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/*
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* To allow nesting atomics inside spinlocked sections, use a |
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* different spinlock. See comment above. |
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*/ |
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offset = 1 + NUM_SPINLOCK_SEMAPHORES; |
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sema_total = NUM_ATOMICS_SEMAPHORES; |
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} |
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else |
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{ |
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offset = 1; |
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sema_total = NUM_SPINLOCK_SEMAPHORES; |
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} |
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idx = (counter++ % sema_total) + offset; |
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/* double check we did things correctly */ |
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s_check_valid(idx); |
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*lock = idx; |
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} |
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void |
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s_unlock_sema(volatile slock_t *lock) |
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{ |
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int lockndx = *lock; |
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s_check_valid(lockndx); |
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PGSemaphoreUnlock(SpinlockSemaArray[lockndx - 1]); |
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} |
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bool |
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s_lock_free_sema(volatile slock_t *lock) |
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{ |
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/* We don't currently use S_LOCK_FREE anyway */ |
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elog(ERROR, "spin.c does not support S_LOCK_FREE()"); |
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return false; |
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} |
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int |
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tas_sema(volatile slock_t *lock) |
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{ |
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int lockndx = *lock; |
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s_check_valid(lockndx); |
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/* Note that TAS macros return 0 if *success* */ |
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return !PGSemaphoreTryLock(SpinlockSemaArray[lockndx - 1]); |
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} |
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#endif /* !HAVE_SPINLOCKS */ |
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