qemu

lockcnt.c (11936B)

---

 /*
  * 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