399 lines
12 KiB
C
399 lines
12 KiB
C
/*
|
|
* 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
|