1 files changed, 324 insertions, 0 deletions
diff --git a/src/backend/storage/lmgr/s_lock.c b/src/backend/storage/lmgr/s_lock.c
new file mode 100644
index 0000000..327ac64
--- /dev/null
+++ b/src/backend/storage/lmgr/s_lock.c
@@ -0,0 +1,324 @@
+/*-------------------------------------------------------------------------
+ *
+ * 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) 1996-2023, 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 "common/pg_prng.h"
+#include "port/atomics.h"
+#include "storage/s_lock.h"
+#include "utils/wait_event.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;
+
+		/*
+		 * Once we start sleeping, the overhead of reporting a wait event is
+		 * justified. Actively spinning easily stands out in profilers, but
+		 * sleeping with an exponential backoff is harder to spot...
+		 *
+		 * We might want to report something more granular at some point, but
+		 * this is better than nothing.
+		 */
+		pgstat_report_wait_start(WAIT_EVENT_SPIN_DELAY);
+		pg_usleep(status->cur_delay);
+		pgstat_report_wait_end();
+
+#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 *
+									pg_prng_double(&pg_global_prng_state) + 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;
+}
+
+
+/*****************************************************************************/
+#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()
+{
+	pg_prng_seed(&pg_global_prng_state, (uint64) 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__, __func__);
+
+	printf("S_LOCK_TEST: failed, lock not locked\n");
+	return 1;
+}
+
+#endif							/* S_LOCK_TEST */