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qemu-thread: optimize QemuLockCnt with futexes on Linux
This is complex, but I think it is reasonably documented in the source. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Message-id: 20170112180800.21085-5-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Paolo Bonzini
Stefan Hajnoczi
parent
d7c99a1282
commit
fbcc3e5004
7 changed files with 342 additions and 35 deletions
283
util/lockcnt.c
283
util/lockcnt.c
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@ -9,7 +9,289 @@
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#include "qemu/osdep.h"
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#include "qemu/thread.h"
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#include "qemu/atomic.h"
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#include "trace.h"
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#ifdef CONFIG_LINUX
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#include "qemu/futex.h"
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/* On Linux, bits 0-1 are a futex-based lock, bits 2-31 are the counter.
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* For the mutex algorithm see Ulrich Drepper's "Futexes Are Tricky" (ok,
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* this is not the most relaxing citation I could make...). It is similar
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* to mutex2 in the paper.
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*/
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#define QEMU_LOCKCNT_STATE_MASK 3
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#define QEMU_LOCKCNT_STATE_FREE 0 /* free, uncontended */
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#define QEMU_LOCKCNT_STATE_LOCKED 1 /* locked, uncontended */
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#define QEMU_LOCKCNT_STATE_WAITING 2 /* locked, contended */
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#define QEMU_LOCKCNT_COUNT_STEP 4
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#define QEMU_LOCKCNT_COUNT_SHIFT 2
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void qemu_lockcnt_init(QemuLockCnt *lockcnt)
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{
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lockcnt->count = 0;
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}
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void qemu_lockcnt_destroy(QemuLockCnt *lockcnt)
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{
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}
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/* *val is the current value of lockcnt->count.
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*
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* If the lock is free, try a cmpxchg from *val to new_if_free; return
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* true and set *val to the old value found by the cmpxchg in
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* lockcnt->count.
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*
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* If the lock is taken, wait for it to be released and return false
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* *without trying again to take the lock*. Again, set *val to the
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* new value of lockcnt->count.
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*
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* If *waited is true on return, new_if_free's bottom two bits must not
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* be QEMU_LOCKCNT_STATE_LOCKED on subsequent calls, because the caller
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* does not know if there are other waiters. Furthermore, after *waited
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* is set the caller has effectively acquired the lock. If it returns
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* with the lock not taken, it must wake another futex waiter.
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*/
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static bool qemu_lockcnt_cmpxchg_or_wait(QemuLockCnt *lockcnt, int *val,
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int new_if_free, bool *waited)
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{
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/* Fast path for when the lock is free. */
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if ((*val & QEMU_LOCKCNT_STATE_MASK) == QEMU_LOCKCNT_STATE_FREE) {
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int expected = *val;
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trace_lockcnt_fast_path_attempt(lockcnt, expected, new_if_free);
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*val = atomic_cmpxchg(&lockcnt->count, expected, new_if_free);
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if (*val == expected) {
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trace_lockcnt_fast_path_success(lockcnt, expected, new_if_free);
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*val = new_if_free;
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return true;
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}
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}
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/* The slow path moves from locked to waiting if necessary, then
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* does a futex wait. Both steps can be repeated ad nauseam,
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* only getting out of the loop if we can have another shot at the
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* fast path. Once we can, get out to compute the new destination
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* value for the fast path.
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*/
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while ((*val & QEMU_LOCKCNT_STATE_MASK) != QEMU_LOCKCNT_STATE_FREE) {
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if ((*val & QEMU_LOCKCNT_STATE_MASK) == QEMU_LOCKCNT_STATE_LOCKED) {
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int expected = *val;
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int new = expected - QEMU_LOCKCNT_STATE_LOCKED + QEMU_LOCKCNT_STATE_WAITING;
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trace_lockcnt_futex_wait_prepare(lockcnt, expected, new);
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*val = atomic_cmpxchg(&lockcnt->count, expected, new);
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if (*val == expected) {
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*val = new;
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}
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continue;
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}
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if ((*val & QEMU_LOCKCNT_STATE_MASK) == QEMU_LOCKCNT_STATE_WAITING) {
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*waited = true;
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trace_lockcnt_futex_wait(lockcnt, *val);
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qemu_futex_wait(&lockcnt->count, *val);
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*val = atomic_read(&lockcnt->count);
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trace_lockcnt_futex_wait_resume(lockcnt, *val);
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continue;
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}
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abort();
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}
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return false;
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}
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static void lockcnt_wake(QemuLockCnt *lockcnt)
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{
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trace_lockcnt_futex_wake(lockcnt);
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qemu_futex_wake(&lockcnt->count, 1);
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}
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void qemu_lockcnt_inc(QemuLockCnt *lockcnt)
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{
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int val = atomic_read(&lockcnt->count);
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bool waited = false;
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for (;;) {
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if (val >= QEMU_LOCKCNT_COUNT_STEP) {
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int expected = val;
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val = atomic_cmpxchg(&lockcnt->count, val, val + QEMU_LOCKCNT_COUNT_STEP);
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if (val == expected) {
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break;
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}
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} else {
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/* The fast path is (0, unlocked)->(1, unlocked). */
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if (qemu_lockcnt_cmpxchg_or_wait(lockcnt, &val, QEMU_LOCKCNT_COUNT_STEP,
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&waited)) {
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break;
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}
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}
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}
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/* If we were woken by another thread, we should also wake one because
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* we are effectively releasing the lock that was given to us. This is
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* the case where qemu_lockcnt_lock would leave QEMU_LOCKCNT_STATE_WAITING
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* in the low bits, and qemu_lockcnt_inc_and_unlock would find it and
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* wake someone.
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*/
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if (waited) {
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lockcnt_wake(lockcnt);
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}
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}
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void qemu_lockcnt_dec(QemuLockCnt *lockcnt)
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{
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atomic_sub(&lockcnt->count, QEMU_LOCKCNT_COUNT_STEP);
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}
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/* Decrement a counter, and return locked if it is decremented to zero.
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* If the function returns true, it is impossible for the counter to
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* become nonzero until the next qemu_lockcnt_unlock.
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*/
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bool qemu_lockcnt_dec_and_lock(QemuLockCnt *lockcnt)
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{
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int val = atomic_read(&lockcnt->count);
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int locked_state = QEMU_LOCKCNT_STATE_LOCKED;
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bool waited = false;
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for (;;) {
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if (val >= 2 * QEMU_LOCKCNT_COUNT_STEP) {
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int expected = val;
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val = atomic_cmpxchg(&lockcnt->count, val, val - QEMU_LOCKCNT_COUNT_STEP);
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if (val == expected) {
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break;
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}
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} else {
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/* If count is going 1->0, take the lock. The fast path is
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* (1, unlocked)->(0, locked) or (1, unlocked)->(0, waiting).
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*/
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if (qemu_lockcnt_cmpxchg_or_wait(lockcnt, &val, locked_state, &waited)) {
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return true;
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}
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if (waited) {
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/* At this point we do not know if there are more waiters. Assume
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* there are.
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*/
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locked_state = QEMU_LOCKCNT_STATE_WAITING;
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}
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}
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}
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/* If we were woken by another thread, but we're returning in unlocked
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* state, we should also wake a thread because we are effectively
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* releasing the lock that was given to us. This is the case where
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* qemu_lockcnt_lock would leave QEMU_LOCKCNT_STATE_WAITING in the low
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* bits, and qemu_lockcnt_unlock would find it and wake someone.
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*/
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if (waited) {
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lockcnt_wake(lockcnt);
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}
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return false;
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}
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/* If the counter is one, decrement it and return locked. Otherwise do
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* nothing.
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*
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* If the function returns true, it is impossible for the counter to
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* become nonzero until the next qemu_lockcnt_unlock.
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*/
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bool qemu_lockcnt_dec_if_lock(QemuLockCnt *lockcnt)
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{
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int val = atomic_read(&lockcnt->count);
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int locked_state = QEMU_LOCKCNT_STATE_LOCKED;
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bool waited = false;
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while (val < 2 * QEMU_LOCKCNT_COUNT_STEP) {
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/* If count is going 1->0, take the lock. The fast path is
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* (1, unlocked)->(0, locked) or (1, unlocked)->(0, waiting).
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*/
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if (qemu_lockcnt_cmpxchg_or_wait(lockcnt, &val, locked_state, &waited)) {
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return true;
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}
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if (waited) {
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/* At this point we do not know if there are more waiters. Assume
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* there are.
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*/
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locked_state = QEMU_LOCKCNT_STATE_WAITING;
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}
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}
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/* If we were woken by another thread, but we're returning in unlocked
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* state, we should also wake a thread because we are effectively
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* releasing the lock that was given to us. This is the case where
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* qemu_lockcnt_lock would leave QEMU_LOCKCNT_STATE_WAITING in the low
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* bits, and qemu_lockcnt_inc_and_unlock would find it and wake someone.
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*/
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if (waited) {
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lockcnt_wake(lockcnt);
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}
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return false;
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}
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void qemu_lockcnt_lock(QemuLockCnt *lockcnt)
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{
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int val = atomic_read(&lockcnt->count);
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int step = QEMU_LOCKCNT_STATE_LOCKED;
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bool waited = false;
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/* The third argument is only used if the low bits of val are 0
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* (QEMU_LOCKCNT_STATE_FREE), so just blindly mix in the desired
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* state.
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*/
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while (!qemu_lockcnt_cmpxchg_or_wait(lockcnt, &val, val + step, &waited)) {
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if (waited) {
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/* At this point we do not know if there are more waiters. Assume
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* there are.
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*/
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step = QEMU_LOCKCNT_STATE_WAITING;
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}
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}
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}
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void qemu_lockcnt_inc_and_unlock(QemuLockCnt *lockcnt)
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{
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int expected, new, val;
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val = atomic_read(&lockcnt->count);
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do {
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expected = val;
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new = (val + QEMU_LOCKCNT_COUNT_STEP) & ~QEMU_LOCKCNT_STATE_MASK;
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trace_lockcnt_unlock_attempt(lockcnt, val, new);
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val = atomic_cmpxchg(&lockcnt->count, val, new);
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} while (val != expected);
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trace_lockcnt_unlock_success(lockcnt, val, new);
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if (val & QEMU_LOCKCNT_STATE_WAITING) {
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lockcnt_wake(lockcnt);
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}
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}
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void qemu_lockcnt_unlock(QemuLockCnt *lockcnt)
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{
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int expected, new, val;
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val = atomic_read(&lockcnt->count);
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do {
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expected = val;
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new = val & ~QEMU_LOCKCNT_STATE_MASK;
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trace_lockcnt_unlock_attempt(lockcnt, val, new);
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val = atomic_cmpxchg(&lockcnt->count, val, new);
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} while (val != expected);
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trace_lockcnt_unlock_success(lockcnt, val, new);
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if (val & QEMU_LOCKCNT_STATE_WAITING) {
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lockcnt_wake(lockcnt);
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}
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}
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unsigned qemu_lockcnt_count(QemuLockCnt *lockcnt)
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{
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return atomic_read(&lockcnt->count) >> QEMU_LOCKCNT_COUNT_SHIFT;
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}
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#else
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void qemu_lockcnt_init(QemuLockCnt *lockcnt)
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{
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qemu_mutex_init(&lockcnt->mutex);
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@ -112,3 +394,4 @@ unsigned qemu_lockcnt_count(QemuLockCnt *lockcnt)
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{
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return atomic_read(&lockcnt->count);
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}
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#endif
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