arch/tile/lib/spinlock_32.c

/*
 * Copyright 2010 Tilera Corporation. All Rights Reserved.
 *
 *   This program is free software; you can redistribute it and/or
 *   modify it under the terms of the GNU General Public License
 *   as published by the Free Software Foundation, version 2.
 *
 *   This program is distributed in the hope that it will be useful, but
 *   WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
 *   NON INFRINGEMENT.  See the GNU General Public License for
 *   more details.
 */

#include <linux/spinlock.h>
#include <linux/module.h>
#include <asm/processor.h>

#include "spinlock_common.h"

void arch_spin_lock(arch_spinlock_t *lock)
{
	int my_ticket;
	int iterations = 0;
	int delta;

	while ((my_ticket = __insn_tns((void *)&lock->next_ticket)) & 1)
		delay_backoff(iterations++);

	/* Increment the next ticket number, implicitly releasing tns lock. */
	lock->next_ticket = my_ticket + TICKET_QUANTUM;

	/* Wait until it's our turn. */
	while ((delta = my_ticket - lock->current_ticket) != 0)
		relax((128 / CYCLES_PER_RELAX_LOOP) * delta);
}
EXPORT_SYMBOL(arch_spin_lock);

int arch_spin_trylock(arch_spinlock_t *lock)
{
	/*
	 * Grab a ticket; no need to retry if it's busy, we'll just
	 * treat that the same as "locked", since someone else
	 * will lock it momentarily anyway.
	 */
	int my_ticket = __insn_tns((void *)&lock->next_ticket);

	if (my_ticket == lock->current_ticket) {
		/* Not currently locked, so lock it by keeping this ticket. */
		lock->next_ticket = my_ticket + TICKET_QUANTUM;
		/* Success! */
		return 1;
	}

	if (!(my_ticket & 1)) {
		/* Release next_ticket. */
		lock->next_ticket = my_ticket;
	}

	return 0;
}
EXPORT_SYMBOL(arch_spin_trylock);

void arch_spin_unlock_wait(arch_spinlock_t *lock)
{
	u32 iterations = 0;
	while (arch_spin_is_locked(lock))
		delay_backoff(iterations++);
}
EXPORT_SYMBOL(arch_spin_unlock_wait);

/*
 * The low byte is always reserved to be the marker for a "tns" operation
 * since the low bit is set to "1" by a tns.  The next seven bits are
 * zeroes.  The next byte holds the "next" writer value, i.e. the ticket
 * available for the next task that wants to write.  The third byte holds
 * the current writer value, i.e. the writer who holds the current ticket.
 * If current == next == 0, there are no interested writers.
 */
#define WR_NEXT_SHIFT   _WR_NEXT_SHIFT
#define WR_CURR_SHIFT   _WR_CURR_SHIFT
#define WR_WIDTH        _WR_WIDTH
#define WR_MASK         ((1 << WR_WIDTH) - 1)

/*
 * The last eight bits hold the active reader count.  This has to be
 * zero before a writer can start to write.
 */
#define RD_COUNT_SHIFT  _RD_COUNT_SHIFT
#define RD_COUNT_WIDTH  _RD_COUNT_WIDTH
#define RD_COUNT_MASK   ((1 << RD_COUNT_WIDTH) - 1)


/* Lock the word, spinning until there are no tns-ers. */
static inline u32 get_rwlock(arch_rwlock_t *rwlock)
{
	u32 iterations = 0;
	for (;;) {
		u32 val = __insn_tns((int *)&rwlock->lock);
		if (unlikely(val & 1)) {
			delay_backoff(iterations++);
			continue;
		}
		return val;
	}
}

int arch_read_trylock_slow(arch_rwlock_t *rwlock)
{
	u32 val = get_rwlock(rwlock);
	int locked = (val << RD_COUNT_WIDTH) == 0;
	rwlock->lock = val + (locked << RD_COUNT_SHIFT);
	return locked;
}
EXPORT_SYMBOL(arch_read_trylock_slow);

void arch_read_unlock_slow(arch_rwlock_t *rwlock)
{
	u32 val = get_rwlock(rwlock);
	rwlock->lock = val - (1 << RD_COUNT_SHIFT);
}
EXPORT_SYMBOL(arch_read_unlock_slow);

void arch_write_unlock_slow(arch_rwlock_t *rwlock, u32 val)
{
	u32 eq, mask = 1 << WR_CURR_SHIFT;
	while (unlikely(val & 1)) {
		/* Limited backoff since we are the highest-priority task. */
		relax(4);
		val = __insn_tns((int *)&rwlock->lock);
	}
	val = __insn_addb(val, mask);
	eq = __insn_seqb(val, val << (WR_CURR_SHIFT - WR_NEXT_SHIFT));
	val = __insn_mz(eq & mask, val);
	rwlock->lock = val;
}
EXPORT_SYMBOL(arch_write_unlock_slow);

/*
 * We spin until everything but the reader bits (which are in the high
 * part of the word) are zero, i.e. no active or waiting writers, no tns.
 *
 * ISSUE: This approach can permanently starve readers.  A reader who sees
 * a writer could instead take a ticket lock (just like a writer would),
 * and atomically enter read mode (with 1 reader) when it gets the ticket.
 * This way both readers and writers will always make forward progress
 * in a finite time.
 */
void arch_read_lock_slow(arch_rwlock_t *rwlock, u32 val)
{
	u32 iterations = 0;
	do {
		if (!(val & 1))
			rwlock->lock = val;
		delay_backoff(iterations++);
		val = __insn_tns((int *)&rwlock->lock);
	} while ((val << RD_COUNT_WIDTH) != 0);
	rwlock->lock = val + (1 << RD_COUNT_SHIFT);
}
EXPORT_SYMBOL(arch_read_lock_slow);

void arch_write_lock_slow(arch_rwlock_t *rwlock, u32 val)
{
	/*
	 * The trailing underscore on this variable (and curr_ below)
	 * reminds us that the high bits are garbage; we mask them out
	 * when we compare them.
	 */
	u32 my_ticket_;

	/* Take out the next ticket; this will also stop would-be readers. */
	if (val & 1)
		val = get_rwlock(rwlock);
	rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT);

	/* Extract my ticket value from the original word. */
	my_ticket_ = val >> WR_NEXT_SHIFT;

	/*
	 * Wait until the "current" field matches our ticket, and
	 * there are no remaining readers.
	 */
	for (;;) {
		u32 curr_ = val >> WR_CURR_SHIFT;
		u32 readers = val >> RD_COUNT_SHIFT;
		u32 delta = ((my_ticket_ - curr_) & WR_MASK) + !!readers;
		if (likely(delta == 0))
			break;

		/* Delay based on how many lock-holders are still out there. */
		relax((256 / CYCLES_PER_RELAX_LOOP) * delta);

		/*
		 * Get a non-tns value to check; we don't need to tns
		 * it ourselves.  Since we're not tns'ing, we retry
		 * more rapidly to get a valid value.
		 */
		while ((val = rwlock->lock) & 1)
			relax(4);
	}
}
EXPORT_SYMBOL(arch_write_lock_slow);

int __tns_atomic_acquire(atomic_t *lock)
{
	int ret;
	u32 iterations = 0;

	BUG_ON(__insn_mfspr(SPR_INTERRUPT_CRITICAL_SECTION));
	__insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 1);

	while ((ret = __insn_tns((void *)&lock->counter)) == 1)
		delay_backoff(iterations++);
	return ret;
}

void __tns_atomic_release(atomic_t *p, int v)
{
	p->counter = v;
	__insn_mtspr(SPR_INTERRUPT_CRITICAL_SECTION, 0);
}