/*------------------------------------------------------------------------- * * s_lock.c * Hardware-dependent implementation of spinlocks. * * When waiting for a contended spinlock we loop tightly for awhile, then * delay using pg_usleep() and try again. Preferably, "awhile" should be a * small multiple of the maximum time we expect a spinlock to be held. 100 * iterations seems about right as an initial guess. However, on a * uniprocessor the loop is a waste of cycles, while in a multi-CPU scenario * it's usually better to spin a bit longer than to call the kernel, so we try * to adapt the spin loop count depending on whether we seem to be in a * uniprocessor or multiprocessor. * * Note: you might think MIN_SPINS_PER_DELAY should be just 1, but you'd * be wrong; there are platforms where that can result in a "stuck * spinlock" failure. This has been seen particularly on Alphas; it seems * that the first TAS after returning from kernel space will always fail * on that hardware. * * Once we do decide to block, we use randomly increasing pg_usleep() * delays. The first delay is 1 msec, then the delay randomly increases to * about one second, after which we reset to 1 msec and start again. The * idea here is that in the presence of heavy contention we need to * increase the delay, else the spinlock holder may never get to run and * release the lock. (Consider situation where spinlock holder has been * nice'd down in priority by the scheduler --- it will not get scheduled * until all would-be acquirers are sleeping, so if we always use a 1-msec * sleep, there is a real possibility of starvation.) But we can't just * clamp the delay to an upper bound, else it would take a long time to * make a reasonable number of tries. * * We time out and declare error after NUM_DELAYS delays (thus, exactly * that many tries). With the given settings, this will usually take 2 or * so minutes. It seems better to fix the total number of tries (and thus * the probability of unintended failure) than to fix the total time * spent. * * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * * * IDENTIFICATION * src/backend/storage/lmgr/s_lock.c * *------------------------------------------------------------------------- */ #include "postgres.h" #include #include #include "storage/s_lock.h" #include "port/atomics.h" #define MIN_SPINS_PER_DELAY 10 #define MAX_SPINS_PER_DELAY 1000 #define NUM_DELAYS 1000 #define MIN_DELAY_USEC 1000L #define MAX_DELAY_USEC 1000000L slock_t dummy_spinlock; static int spins_per_delay = DEFAULT_SPINS_PER_DELAY; /* * s_lock_stuck() - complain about a stuck spinlock */ static void s_lock_stuck(const char *file, int line, const char *func) { if (!func) func = "(unknown)"; #if defined(S_LOCK_TEST) fprintf(stderr, "\nStuck spinlock detected at %s, %s:%d.\n", func, file, line); exit(1); #else elog(PANIC, "stuck spinlock detected at %s, %s:%d", func, file, line); #endif } /* * s_lock(lock) - platform-independent portion of waiting for a spinlock. */ int s_lock(volatile slock_t *lock, const char *file, int line, const char *func) { SpinDelayStatus delayStatus; init_spin_delay(&delayStatus, file, line, func); while (TAS_SPIN(lock)) { perform_spin_delay(&delayStatus); } finish_spin_delay(&delayStatus); return delayStatus.delays; } #ifdef USE_DEFAULT_S_UNLOCK void s_unlock(volatile slock_t *lock) { #ifdef TAS_ACTIVE_WORD /* HP's PA-RISC */ *TAS_ACTIVE_WORD(lock) = -1; #else *lock = 0; #endif } #endif /* * Wait while spinning on a contended spinlock. */ void perform_spin_delay(SpinDelayStatus *status) { /* CPU-specific delay each time through the loop */ SPIN_DELAY(); /* Block the process every spins_per_delay tries */ if (++(status->spins) >= spins_per_delay) { if (++(status->delays) > NUM_DELAYS) s_lock_stuck(status->file, status->line, status->func); if (status->cur_delay == 0) /* first time to delay? */ status->cur_delay = MIN_DELAY_USEC; pg_usleep(status->cur_delay); #if defined(S_LOCK_TEST) fprintf(stdout, "*"); fflush(stdout); #endif /* increase delay by a random fraction between 1X and 2X */ status->cur_delay += (int) (status->cur_delay * ((double) random() / (double) MAX_RANDOM_VALUE) + 0.5); /* wrap back to minimum delay when max is exceeded */ if (status->cur_delay > MAX_DELAY_USEC) status->cur_delay = MIN_DELAY_USEC; status->spins = 0; } } /* * After acquiring a spinlock, update estimates about how long to loop. * * If we were able to acquire the lock without delaying, it's a good * indication we are in a multiprocessor. If we had to delay, it's a sign * (but not a sure thing) that we are in a uniprocessor. Hence, we * decrement spins_per_delay slowly when we had to delay, and increase it * rapidly when we didn't. It's expected that spins_per_delay will * converge to the minimum value on a uniprocessor and to the maximum * value on a multiprocessor. * * Note: spins_per_delay is local within our current process. We want to * average these observations across multiple backends, since it's * relatively rare for this function to even get entered, and so a single * backend might not live long enough to converge on a good value. That * is handled by the two routines below. */ void finish_spin_delay(SpinDelayStatus *status) { if (status->cur_delay == 0) { /* we never had to delay */ if (spins_per_delay < MAX_SPINS_PER_DELAY) spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY); } else { if (spins_per_delay > MIN_SPINS_PER_DELAY) spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY); } } /* * Set local copy of spins_per_delay during backend startup. * * NB: this has to be pretty fast as it is called while holding a spinlock */ void set_spins_per_delay(int shared_spins_per_delay) { spins_per_delay = shared_spins_per_delay; } /* * Update shared estimate of spins_per_delay during backend exit. * * NB: this has to be pretty fast as it is called while holding a spinlock */ int update_spins_per_delay(int shared_spins_per_delay) { /* * We use an exponential moving average with a relatively slow adaption * rate, so that noise in any one backend's result won't affect the shared * value too much. As long as both inputs are within the allowed range, * the result must be too, so we need not worry about clamping the result. * * We deliberately truncate rather than rounding; this is so that single * adjustments inside a backend can affect the shared estimate (see the * asymmetric adjustment rules above). */ return (shared_spins_per_delay * 15 + spins_per_delay) / 16; } /* * Various TAS implementations that cannot live in s_lock.h as no inline * definition exists (yet). * In the future, get rid of tas.[cso] and fold it into this file. * * If you change something here, you will likely need to modify s_lock.h too, * because the definitions for these are split between this file and s_lock.h. */ #ifdef HAVE_SPINLOCKS /* skip spinlocks if requested */ #if defined(__GNUC__) /* * All the gcc flavors that are not inlined */ /* * Note: all the if-tests here probably ought to be testing gcc version * rather than platform, but I don't have adequate info to know what to * write. Ideally we'd flush all this in favor of the inline version. */ #if defined(__m68k__) && !defined(__linux__) /* really means: extern int tas(slock_t* **lock); */ static void tas_dummy() { __asm__ __volatile__( #if defined(__NetBSD__) && defined(__ELF__) /* no underscore for label and % for registers */ "\ .global tas \n\ tas: \n\ movel %sp@(0x4),%a0 \n\ tas %a0@ \n\ beq _success \n\ moveq #-128,%d0 \n\ rts \n\ _success: \n\ moveq #0,%d0 \n\ rts \n" #else "\ .global _tas \n\ _tas: \n\ movel sp@(0x4),a0 \n\ tas a0@ \n\ beq _success \n\ moveq #-128,d0 \n\ rts \n\ _success: \n\ moveq #0,d0 \n\ rts \n" #endif /* __NetBSD__ && __ELF__ */ ); } #endif /* __m68k__ && !__linux__ */ #endif /* not __GNUC__ */ #endif /* HAVE_SPINLOCKS */ /*****************************************************************************/ #if defined(S_LOCK_TEST) /* * test program for verifying a port's spinlock support. */ struct test_lock_struct { char pad1; slock_t lock; char pad2; }; volatile struct test_lock_struct test_lock; int main() { srandom((unsigned int) time(NULL)); test_lock.pad1 = test_lock.pad2 = 0x44; S_INIT_LOCK(&test_lock.lock); if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44) { printf("S_LOCK_TEST: failed, declared datatype is wrong size\n"); return 1; } if (!S_LOCK_FREE(&test_lock.lock)) { printf("S_LOCK_TEST: failed, lock not initialized\n"); return 1; } S_LOCK(&test_lock.lock); if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44) { printf("S_LOCK_TEST: failed, declared datatype is wrong size\n"); return 1; } if (S_LOCK_FREE(&test_lock.lock)) { printf("S_LOCK_TEST: failed, lock not locked\n"); return 1; } S_UNLOCK(&test_lock.lock); if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44) { printf("S_LOCK_TEST: failed, declared datatype is wrong size\n"); return 1; } if (!S_LOCK_FREE(&test_lock.lock)) { printf("S_LOCK_TEST: failed, lock not unlocked\n"); return 1; } S_LOCK(&test_lock.lock); if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44) { printf("S_LOCK_TEST: failed, declared datatype is wrong size\n"); return 1; } if (S_LOCK_FREE(&test_lock.lock)) { printf("S_LOCK_TEST: failed, lock not re-locked\n"); return 1; } printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS); printf(" exit with a 'stuck spinlock' message\n"); printf(" if S_LOCK() and TAS() are working.\n"); fflush(stdout); s_lock(&test_lock.lock, __FILE__, __LINE__); printf("S_LOCK_TEST: failed, lock not locked\n"); return 1; } #endif /* S_LOCK_TEST */