* atomic_long_cmpxchg() will be used to obtain writer lock.
*
* There are three places where the lock handoff bit may be set or cleared.
- * 1) rwsem_mark_wake() for readers.
- * 2) rwsem_try_write_lock() for writers.
- * 3) Error path of rwsem_down_write_slowpath().
+ * 1) rwsem_mark_wake() for readers -- set, clear
+ * 2) rwsem_try_write_lock() for writers -- set, clear
+ * 3) rwsem_del_waiter() -- clear
*
* For all the above cases, wait_lock will be held. A writer must also
* be the first one in the wait_list to be eligible for setting the handoff
struct task_struct *task;
enum rwsem_waiter_type type;
unsigned long timeout;
+
+ /* Writer only, not initialized in reader */
+ bool handoff_set;
};
#define rwsem_first_waiter(sem) \
list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
};
-enum writer_wait_state {
- WRITER_NOT_FIRST, /* Writer is not first in wait list */
- WRITER_FIRST, /* Writer is first in wait list */
- WRITER_HANDOFF /* Writer is first & handoff needed */
-};
-
/*
* The typical HZ value is either 250 or 1000. So set the minimum waiting
* time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
*/
#define MAX_READERS_WAKEUP 0x100
+static inline void
+rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
+{
+ lockdep_assert_held(&sem->wait_lock);
+ list_add_tail(&waiter->list, &sem->wait_list);
+ /* caller will set RWSEM_FLAG_WAITERS */
+}
+
+/*
+ * Remove a waiter from the wait_list and clear flags.
+ *
+ * Both rwsem_mark_wake() and rwsem_try_write_lock() contain a full 'copy' of
+ * this function. Modify with care.
+ */
+static inline void
+rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
+{
+ lockdep_assert_held(&sem->wait_lock);
+ list_del(&waiter->list);
+ if (likely(!list_empty(&sem->wait_list)))
+ return;
+
+ atomic_long_andnot(RWSEM_FLAG_HANDOFF | RWSEM_FLAG_WAITERS, &sem->count);
+}
+
/*
* handle the lock release when processes blocked on it that can now run
* - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
* preferably when the wait_lock is released
* - woken process blocks are discarded from the list after having task zeroed
* - writers are only marked woken if downgrading is false
+ *
+ * Implies rwsem_del_waiter() for all woken readers.
*/
static void rwsem_mark_wake(struct rw_semaphore *sem,
enum rwsem_wake_type wake_type,
adjustment = woken * RWSEM_READER_BIAS - adjustment;
lockevent_cond_inc(rwsem_wake_reader, woken);
+
+ oldcount = atomic_long_read(&sem->count);
if (list_empty(&sem->wait_list)) {
- /* hit end of list above */
+ /*
+ * Combined with list_move_tail() above, this implies
+ * rwsem_del_waiter().
+ */
adjustment -= RWSEM_FLAG_WAITERS;
+ if (oldcount & RWSEM_FLAG_HANDOFF)
+ adjustment -= RWSEM_FLAG_HANDOFF;
+ } else if (woken) {
+ /*
+ * When we've woken a reader, we no longer need to force
+ * writers to give up the lock and we can clear HANDOFF.
+ */
+ if (oldcount & RWSEM_FLAG_HANDOFF)
+ adjustment -= RWSEM_FLAG_HANDOFF;
}
- /*
- * When we've woken a reader, we no longer need to force writers
- * to give up the lock and we can clear HANDOFF.
- */
- if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
- adjustment -= RWSEM_FLAG_HANDOFF;
-
if (adjustment)
atomic_long_add(adjustment, &sem->count);
* race conditions between checking the rwsem wait list and setting the
* sem->count accordingly.
*
- * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
- * bit is set or the lock is acquired with handoff bit cleared.
+ * Implies rwsem_del_waiter() on success.
*/
static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
- enum writer_wait_state wstate)
+ struct rwsem_waiter *waiter)
{
+ bool first = rwsem_first_waiter(sem) == waiter;
long count, new;
lockdep_assert_held(&sem->wait_lock);
do {
bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
- if (has_handoff && wstate == WRITER_NOT_FIRST)
- return false;
+ if (has_handoff) {
+ if (!first)
+ return false;
+
+ /* First waiter inherits a previously set handoff bit */
+ waiter->handoff_set = true;
+ }
new = count;
if (count & RWSEM_LOCK_MASK) {
- if (has_handoff || (wstate != WRITER_HANDOFF))
+ if (has_handoff || (!rt_task(waiter->task) &&
+ !time_after(jiffies, waiter->timeout)))
return false;
new |= RWSEM_FLAG_HANDOFF;
* We have either acquired the lock with handoff bit cleared or
* set the handoff bit.
*/
- if (new & RWSEM_FLAG_HANDOFF)
+ if (new & RWSEM_FLAG_HANDOFF) {
+ waiter->handoff_set = true;
+ lockevent_inc(rwsem_wlock_handoff);
return false;
+ }
+ /*
+ * Have rwsem_try_write_lock() fully imply rwsem_del_waiter() on
+ * success.
+ */
+ list_del(&waiter->list);
rwsem_set_owner(sem);
return true;
}
}
adjustment += RWSEM_FLAG_WAITERS;
}
- list_add_tail(&waiter.list, &sem->wait_list);
+ rwsem_add_waiter(sem, &waiter);
/* we're now waiting on the lock, but no longer actively locking */
count = atomic_long_add_return(adjustment, &sem->count);
return sem;
out_nolock:
- list_del(&waiter.list);
- if (list_empty(&sem->wait_list)) {
- atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
- &sem->count);
- }
+ rwsem_del_waiter(sem, &waiter);
raw_spin_unlock_irq(&sem->wait_lock);
__set_current_state(TASK_RUNNING);
lockevent_inc(rwsem_rlock_fail);
rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
{
long count;
- enum writer_wait_state wstate;
struct rwsem_waiter waiter;
- struct rw_semaphore *ret = sem;
DEFINE_WAKE_Q(wake_q);
/* do optimistic spinning and steal lock if possible */
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
+ waiter.handoff_set = false;
raw_spin_lock_irq(&sem->wait_lock);
-
- /* account for this before adding a new element to the list */
- wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
-
- list_add_tail(&waiter.list, &sem->wait_list);
+ rwsem_add_waiter(sem, &waiter);
/* we're now waiting on the lock */
- if (wstate == WRITER_NOT_FIRST) {
+ if (rwsem_first_waiter(sem) != &waiter) {
count = atomic_long_read(&sem->count);
/*
/* wait until we successfully acquire the lock */
set_current_state(state);
for (;;) {
- if (rwsem_try_write_lock(sem, wstate)) {
+ if (rwsem_try_write_lock(sem, &waiter)) {
/* rwsem_try_write_lock() implies ACQUIRE on success */
break;
}
raw_spin_unlock_irq(&sem->wait_lock);
+ if (signal_pending_state(state, current))
+ goto out_nolock;
+
/*
* After setting the handoff bit and failing to acquire
* the lock, attempt to spin on owner to accelerate lock
* In this case, we attempt to acquire the lock again
* without sleeping.
*/
- if (wstate == WRITER_HANDOFF) {
+ if (waiter.handoff_set) {
enum owner_state owner_state;
preempt_disable();
goto trylock_again;
}
- /* Block until there are no active lockers. */
- for (;;) {
- if (signal_pending_state(state, current))
- goto out_nolock;
-
- schedule();
- lockevent_inc(rwsem_sleep_writer);
- set_current_state(state);
- /*
- * If HANDOFF bit is set, unconditionally do
- * a trylock.
- */
- if (wstate == WRITER_HANDOFF)
- break;
-
- if ((wstate == WRITER_NOT_FIRST) &&
- (rwsem_first_waiter(sem) == &waiter))
- wstate = WRITER_FIRST;
-
- count = atomic_long_read(&sem->count);
- if (!(count & RWSEM_LOCK_MASK))
- break;
-
- /*
- * The setting of the handoff bit is deferred
- * until rwsem_try_write_lock() is called.
- */
- if ((wstate == WRITER_FIRST) && (rt_task(current) ||
- time_after(jiffies, waiter.timeout))) {
- wstate = WRITER_HANDOFF;
- lockevent_inc(rwsem_wlock_handoff);
- break;
- }
- }
+ schedule();
+ lockevent_inc(rwsem_sleep_writer);
+ set_current_state(state);
trylock_again:
raw_spin_lock_irq(&sem->wait_lock);
}
__set_current_state(TASK_RUNNING);
- list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);
lockevent_inc(rwsem_wlock);
-
- return ret;
+ return sem;
out_nolock:
__set_current_state(TASK_RUNNING);
raw_spin_lock_irq(&sem->wait_lock);
- list_del(&waiter.list);
-
- if (unlikely(wstate == WRITER_HANDOFF))
- atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count);
-
- if (list_empty(&sem->wait_list))
- atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
- else
+ rwsem_del_waiter(sem, &waiter);
+ if (!list_empty(&sem->wait_list))
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
lockevent_inc(rwsem_wlock_fail);
-
return ERR_PTR(-EINTR);
}
projects / users / hch / xfs.git / commitdiff