/*
* QemuLockCnt implementation
*
* Copyright Red Hat, Inc. 2017
*
* Author:
* Paolo Bonzini <pbonzini@redhat.com>
*/
#include "qemu/osdep.h"
#include "qemu/thread.h"
#include "qemu/atomic.h"
#include "trace.h"
#ifdef CONFIG_LINUX
#include "qemu/futex.h"
/* On Linux, bits 0-1 are a futex-based lock, bits 2-31 are the counter.
* For the mutex algorithm see Ulrich Drepper's "Futexes Are Tricky" (ok,
* this is not the most relaxing citation I could make...). It is similar
* to mutex2 in the paper.
*/
#define QEMU_LOCKCNT_STATE_MASK 3
#define QEMU_LOCKCNT_STATE_FREE 0 /* free, uncontended */
#define QEMU_LOCKCNT_STATE_LOCKED 1 /* locked, uncontended */
#define QEMU_LOCKCNT_STATE_WAITING 2 /* locked, contended */
#define QEMU_LOCKCNT_COUNT_STEP 4
#define QEMU_LOCKCNT_COUNT_SHIFT 2
void qemu_lockcnt_init(QemuLockCnt *lockcnt)
{
lockcnt->count = 0;
}
void qemu_lockcnt_destroy(QemuLockCnt *lockcnt)
{
}
/* *val is the current value of lockcnt->count.
*
* If the lock is free, try a cmpxchg from *val to new_if_free; return
* true and set *val to the old value found by the cmpxchg in
* lockcnt->count.
*
* If the lock is taken, wait for it to be released and return false
* *without trying again to take the lock*. Again, set *val to the
* new value of lockcnt->count.
*
* If *waited is true on return, new_if_free's bottom two bits must not
* be QEMU_LOCKCNT_STATE_LOCKED on subsequent calls, because the caller
* does not know if there are other waiters. Furthermore, after *waited
* is set the caller has effectively acquired the lock. If it returns
* with the lock not taken, it must wake another futex waiter.
*/
static bool qemu_lockcnt_cmpxchg_or_wait(QemuLockCnt *lockcnt, int *val,
int new_if_free, bool *waited)
{
/* Fast path for when the lock is free. */
if ((*val & QEMU_LOCKCNT_STATE_MASK) == QEMU_LOCKCNT_STATE_FREE) {
int expected = *val;
trace_lockcnt_fast_path_attempt(lockcnt, expected, new_if_free);
*val = qatomic_cmpxchg(&lockcnt->count, expected, new_if_free);
if (*val == expected) {
trace_lockcnt_fast_path_success(lockcnt, expected, new_if_free);
*val = new_if_free;
return true;
}
}
/* The slow path moves from locked to waiting if necessary, then
* does a futex wait. Both steps can be repeated ad nauseam,
* only getting out of the loop if we can have another shot at the
* fast path. Once we can, get out to compute the new destination
* value for the fast path.
*/
while ((*val & QEMU_LOCKCNT_STATE_MASK) != QEMU_LOCKCNT_STATE_FREE) {
if ((*val & QEMU_LOCKCNT_STATE_MASK) == QEMU_LOCKCNT_STATE_LOCKED) {
int expected = *val;
int new = expected - QEMU_LOCKCNT_STATE_LOCKED + QEMU_LOCKCNT_STATE_WAITING;
trace_lockcnt_futex_wait_prepare(lockcnt, expected, new);
*val = qatomic_cmpxchg(&lockcnt->count, expected, new);
if (*val == expected) {
*val = new;
}
continue;
}
if ((*val & QEMU_LOCKCNT_STATE_MASK) == QEMU_LOCKCNT_STATE_WAITING) {
*waited = true;
trace_lockcnt_futex_wait(lockcnt, *val);
qemu_futex_wait(&lockcnt->count, *val);
*val = qatomic_read(&lockcnt->count);
trace_lockcnt_futex_wait_resume(lockcnt, *val);
continue;
}
abort();
}
return false;
}
static void lockcnt_wake(QemuLockCnt *lockcnt)
{
trace_lockcnt_futex_wake(lockcnt);
qemu_futex_wake(&lockcnt->count, 1);
}
void qemu_lockcnt_inc(QemuLockCnt *lockcnt)
{
int val = qatomic_read(&lockcnt->count);
bool waited = false;
for (;;) {
if (val >= QEMU_LOCKCNT_COUNT_STEP) {
int expected = val;
val = qatomic_cmpxchg(&lockcnt->count, val,
val + QEMU_LOCKCNT_COUNT_STEP);
if (val == expected) {
break;
}
} else {
/* The fast path is (0, unlocked)->(1, unlocked). */
if (qemu_lockcnt_cmpxchg_or_wait(lockcnt, &val, QEMU_LOCKCNT_COUNT_STEP,
&waited)) {
break;
}
}
}
/* If we were woken by another thread, we should also wake one because
* we are effectively releasing the lock that was given to us. This is
* the case where qemu_lockcnt_lock would leave QEMU_LOCKCNT_STATE_WAITING
* in the low bits, and qemu_lockcnt_inc_and_unlock would find it and
* wake someone.
*/
if (waited) {
lockcnt_wake(lockcnt);
}
}
void qemu_lockcnt_dec(QemuLockCnt *lockcnt)
{
qatomic_sub(&lockcnt->count, QEMU_LOCKCNT_COUNT_STEP);
}
/* Decrement a counter, and return locked if it is decremented to zero.
* If the function returns true, it is impossible for the counter to
* become nonzero until the next qemu_lockcnt_unlock.
*/
bool qemu_lockcnt_dec_and_lock(QemuLockCnt *lockcnt)
{
int val = qatomic_read(&lockcnt->count);
int locked_state = QEMU_LOCKCNT_STATE_LOCKED;
bool waited = false;
for (;;) {
if (val >= 2 * QEMU_LOCKCNT_COUNT_STEP) {
int expected = val;
val = qatomic_cmpxchg(&lockcnt->count, val,
val - QEMU_LOCKCNT_COUNT_STEP);
if (val == expected) {
break;
}
} else {
/* If count is going 1->0, take the lock. The fast path is
* (1, unlocked)->(0, locked) or (1, unlocked)->(0, waiting).
*/
if (qemu_lockcnt_cmpxchg_or_wait(lockcnt, &val, locked_state, &waited)) {
return true;
}
if (waited) {
/* At this point we do not know if there are more waiters. Assume
* there are.
*/
locked_state = QEMU_LOCKCNT_STATE_WAITING;
}
}
}
/* If we were woken by another thread, but we're returning in unlocked
* state, we should also wake a thread because we are effectively
* releasing the lock that was given to us. This is the case where
* qemu_lockcnt_lock would leave QEMU_LOCKCNT_STATE_WAITING in the low
* bits, and qemu_lockcnt_unlock would find it and wake someone.
*/
if (waited) {
lockcnt_wake(lockcnt);
}
return false;
}
/* If the counter is one, decrement it and return locked. Otherwise do
* nothing.
*
* If the function returns true, it is impossible for the counter to
* become nonzero until the next qemu_lockcnt_unlock.
*/
bool qemu_lockcnt_dec_if_lock(QemuLockCnt *lockcnt)
{
int val = qatomic_read(&lockcnt->count);
int locked_state = QEMU_LOCKCNT_STATE_LOCKED;
bool waited = false;
while (val < 2 * QEMU_LOCKCNT_COUNT_STEP) {
/* If count is going 1->0, take the lock. The fast path is
* (1, unlocked)->(0, locked) or (1, unlocked)->(0, waiting).
*/
if (qemu_lockcnt_cmpxchg_or_wait(lockcnt, &val, locked_state, &waited)) {
return true;
}
if (waited) {
/* At this point we do not know if there are more waiters. Assume
* there are.
*/
locked_state = QEMU_LOCKCNT_STATE_WAITING;
}
}
/* If we were woken by another thread, but we're returning in unlocked
* state, we should also wake a thread because we are effectively
* releasing the lock that was given to us. This is the case where
* qemu_lockcnt_lock would leave QEMU_LOCKCNT_STATE_WAITING in the low
* bits, and qemu_lockcnt_inc_and_unlock would find it and wake someone.
*/
if (waited) {
lockcnt_wake(lockcnt);
}
return false;
}
void qemu_lockcnt_lock(QemuLockCnt *lockcnt)
{
int val = qatomic_read(&lockcnt->count);
int step = QEMU_LOCKCNT_STATE_LOCKED;
bool waited = false;
/* The third argument is only used if the low bits of val are 0
* (QEMU_LOCKCNT_STATE_FREE), so just blindly mix in the desired
* state.
*/
while (!qemu_lockcnt_cmpxchg_or_wait(lockcnt, &val, val + step, &waited)) {
if (waited) {
/* At this point we do not know if there are more waiters. Assume
* there are.
*/
step = QEMU_LOCKCNT_STATE_WAITING;
}
}
}
void qemu_lockcnt_inc_and_unlock(QemuLockCnt *lockcnt)
{
int expected, new, val;
val = qatomic_read(&lockcnt->count);
do {
expected = val;
new = (val + QEMU_LOCKCNT_COUNT_STEP) & ~QEMU_LOCKCNT_STATE_MASK;
trace_lockcnt_unlock_attempt(lockcnt, val, new);
val = qatomic_cmpxchg(&lockcnt->count, val, new);
} while (val != expected);
trace_lockcnt_unlock_success(lockcnt, val, new);
if (val & QEMU_LOCKCNT_STATE_WAITING) {
lockcnt_wake(lockcnt);
}
}
void qemu_lockcnt_unlock(QemuLockCnt *lockcnt)
{
int expected, new, val;
val = qatomic_read(&lockcnt->count);
do {
expected = val;
new = val & ~QEMU_LOCKCNT_STATE_MASK;
trace_lockcnt_unlock_attempt(lockcnt, val, new);
val = qatomic_cmpxchg(&lockcnt->count, val, new);
} while (val != expected);
trace_lockcnt_unlock_success(lockcnt, val, new);
if (val & QEMU_LOCKCNT_STATE_WAITING) {
lockcnt_wake(lockcnt);
}
}
unsigned qemu_lockcnt_count(QemuLockCnt *lockcnt)
{
return qatomic_read(&lockcnt->count) >> QEMU_LOCKCNT_COUNT_SHIFT;
}
#else
void qemu_lockcnt_init(QemuLockCnt *lockcnt)
{
qemu_mutex_init(&lockcnt->mutex);
lockcnt->count = 0;
}
void qemu_lockcnt_destroy(QemuLockCnt *lockcnt)
{
qemu_mutex_destroy(&lockcnt->mutex);
}
void qemu_lockcnt_inc(QemuLockCnt *lockcnt)
{
int old;
for (;;) {
old = qatomic_read(&lockcnt->count);
if (old == 0) {
qemu_lockcnt_lock(lockcnt);
qemu_lockcnt_inc_and_unlock(lockcnt);
return;
} else {
if (qatomic_cmpxchg(&lockcnt->count, old, old + 1) == old) {
return;
}
}
}
}
void qemu_lockcnt_dec(QemuLockCnt *lockcnt)
{
qatomic_dec(&lockcnt->count);
}
/* Decrement a counter, and return locked if it is decremented to zero.
* It is impossible for the counter to become nonzero while the mutex
* is taken.
*/
bool qemu_lockcnt_dec_and_lock(QemuLockCnt *lockcnt)
{
int val = qatomic_read(&lockcnt->count);
while (val > 1) {
int old = qatomic_cmpxchg(&lockcnt->count, val, val - 1);
if (old != val) {
val = old;
continue;
}
return false;
}
qemu_lockcnt_lock(lockcnt);
if (qatomic_fetch_dec(&lockcnt->count) == 1) {
return true;
}
qemu_lockcnt_unlock(lockcnt);
return false;
}
/* Decrement a counter and return locked if it is decremented to zero.
* Otherwise do nothing.
*
* It is impossible for the counter to become nonzero while the mutex
* is taken.
*/
bool qemu_lockcnt_dec_if_lock(QemuLockCnt *lockcnt)
{
/* No need for acquire semantics if we return false. */
int val = qatomic_read(&lockcnt->count);
if (val > 1) {
return false;
}
qemu_lockcnt_lock(lockcnt);
if (qatomic_fetch_dec(&lockcnt->count) == 1) {
return true;
}
qemu_lockcnt_inc_and_unlock(lockcnt);
return false;
}
void qemu_lockcnt_lock(QemuLockCnt *lockcnt)
{
qemu_mutex_lock(&lockcnt->mutex);
}
void qemu_lockcnt_inc_and_unlock(QemuLockCnt *lockcnt)
{
qatomic_inc(&lockcnt->count);
qemu_mutex_unlock(&lockcnt->mutex);
}
void qemu_lockcnt_unlock(QemuLockCnt *lockcnt)
{
qemu_mutex_unlock(&lockcnt->mutex);
}
unsigned qemu_lockcnt_count(QemuLockCnt *lockcnt)
{
return qatomic_read(&lockcnt->count);
}
#endif
