*
* 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) 2020 Huawei Technologies Co.,Ltd.
* Portions Copyright (c) 1996-2019, 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 <time.h>
#include <unistd.h>
#include "storage/lock/s_lock.h"
#ifdef FRONTEND
#undef FRONTEND
#include "atomics.h"
#define FRONTEND
#else
#include "atomics.h"
#endif
#include "logger.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)
{
#if defined(S_LOCK_TEST)
fprintf(stderr,
"\nStuck spinlock detected at %s:%d.\n",
file, line);
exit(1);
#else
elog(FATAL, "stuck spinlock detected at %s:%d",
file, line);
#endif
}
#ifdef init_spin_delay
#undef init_spin_delay
#endif
static void
init_spin_delay(SpinDelayStatus *status,const char *file, int line)
{
status->spins = 0;
status->delays = 0;
status->cur_delay = 0;
status->file = file;
status->line = line;
}
* s_lock(lock) - platform-independent portion of waiting for a spinlock.
*/
int
s_lock(volatile slock_t *lock, const char *file, int line)
{
SpinDelayStatus delayStatus;
init_spin_delay(&delayStatus, file, line);
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
*TAS_ACTIVE_WORD(lock) = -1;
#else
*lock = 0;
#endif
}
#endif
* Wait while spinning on a contended spinlock.
*/
void
perform_spin_delay(SpinDelayStatus *status)
{
SPIN_DELAY();
if (++(status->spins) >= spins_per_delay)
{
if (++(status->delays) > NUM_DELAYS)
s_lock_stuck(status->file, status->line);
if (status->cur_delay == 0)
status->cur_delay = MIN_DELAY_USEC;
pg_usleep(status->cur_delay);
#if defined(S_LOCK_TEST)
fprintf(stdout, "*");
fflush(stdout);
#endif
status->cur_delay += (int) (status->cur_delay *
((double) random() / (double) MAX_RANDOM_VALUE) + 0.5);
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)
{
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
#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__)
static void
tas_dummy()
{
__asm__ __volatile__(
#if (defined(__NetBSD__) || defined(__OpenBSD__)) && defined(__ELF__)
"\
.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
);
}
#endif
#endif
#endif
#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