#include "lock_events.h"
/*
- * The least significant 3 bits of the owner value has the following
+ * The least significant 2 bits of the owner value has the following
* meanings when set.
* - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
- * - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock.
- * - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock.
+ * - Bit 1: RWSEM_NONSPINNABLE - Cannot spin on a reader-owned lock
*
- * When the rwsem is either owned by an anonymous writer, or it is
- * reader-owned, but a spinning writer has timed out, both nonspinnable
- * bits will be set to disable optimistic spinning by readers and writers.
- * In the later case, the last unlocking reader should then check the
- * writer nonspinnable bit and clear it only to give writers preference
- * to acquire the lock via optimistic spinning, but not readers. Similar
- * action is also done in the reader slowpath.
+ * When the rwsem is reader-owned and a spinning writer has timed out,
+ * the nonspinnable bit will be set to disable optimistic spinning.
* When a writer acquires a rwsem, it puts its task_struct pointer
* into the owner field. It is cleared after an unlock.
* is involved. Ideally we would like to track all the readers that own
* a rwsem, but the overhead is simply too big.
*
- * Reader optimistic spinning is helpful when the reader critical section
- * is short and there aren't that many readers around. It makes readers
- * relatively more preferred than writers. When a writer times out spinning
- * on a reader-owned lock and set the nospinnable bits, there are two main
- * reasons for that.
- *
- * 1) The reader critical section is long, perhaps the task sleeps after
- * acquiring the read lock.
- * 2) There are just too many readers contending the lock causing it to
- * take a while to service all of them.
- *
- * In the former case, long reader critical section will impede the progress
- * of writers which is usually more important for system performance. In
- * the later case, reader optimistic spinning tends to make the reader
- * groups that contain readers that acquire the lock together smaller
- * leading to more of them. That may hurt performance in some cases. In
- * other words, the setting of nonspinnable bits indicates that reader
- * optimistic spinning may not be helpful for those workloads that cause
- * it.
- *
- * Therefore, any writers that had observed the setting of the writer
- * nonspinnable bit for a given rwsem after they fail to acquire the lock
- * via optimistic spinning will set the reader nonspinnable bit once they
- * acquire the write lock. Similarly, readers that observe the setting
- * of reader nonspinnable bit at slowpath entry will set the reader
- * nonspinnable bits when they acquire the read lock via the wakeup path.
- *
- * Once the reader nonspinnable bit is on, it will only be reset when
- * a writer is able to acquire the rwsem in the fast path or somehow a
- * reader or writer in the slowpath doesn't observe the nonspinable bit.
- *
- * This is to discourage reader optmistic spinning on that particular
- * rwsem and make writers more preferred. This adaptive disabling of reader
- * optimistic spinning will alleviate the negative side effect of this
- * feature.
+ * A fast path reader optimistic lock stealing is supported when the rwsem
+ * is previously owned by a writer and the following conditions are met:
+ * - OSQ is empty
+ * - rwsem is not currently writer owned
+ * - the handoff isn't set.
*/
#define RWSEM_READER_OWNED (1UL << 0)
-#define RWSEM_RD_NONSPINNABLE (1UL << 1)
-#define RWSEM_WR_NONSPINNABLE (1UL << 2)
-#define RWSEM_NONSPINNABLE (RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE)
+#define RWSEM_NONSPINNABLE (1UL << 1)
#define RWSEM_OWNER_FLAGS_MASK (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
#ifdef CONFIG_DEBUG_RWSEMS
struct task_struct *owner)
{
unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
- (atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE);
+ (atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE);
atomic_long_set(&sem->owner, val);
}
struct task_struct *task;
enum rwsem_waiter_type type;
unsigned long timeout;
- unsigned long last_rowner;
};
#define rwsem_first_waiter(sem) \
list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
* the reader is copied over.
*/
owner = waiter->task;
- if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) {
- owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE);
- lockevent_inc(rwsem_opt_norspin);
- }
__rwsem_set_reader_owned(sem, owner);
}
}
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
-/*
- * Try to acquire read lock before the reader is put on wait queue.
- * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
- * is ongoing.
- */
-static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
-{
- long count = atomic_long_read(&sem->count);
-
- if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
- return false;
-
- count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
- if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
- rwsem_set_reader_owned(sem);
- lockevent_inc(rwsem_opt_rlock);
- return true;
- }
-
- /* Back out the change */
- atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
- return false;
-}
-
/*
* Try to acquire write lock before the writer has been put on wait queue.
*/
if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
count | RWSEM_WRITER_LOCKED)) {
rwsem_set_owner(sem);
- lockevent_inc(rwsem_opt_wlock);
+ lockevent_inc(rwsem_opt_lock);
return true;
}
}
return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
}
-static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
- unsigned long nonspinnable)
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *owner;
unsigned long flags;
/*
* Don't check the read-owner as the entry may be stale.
*/
- if ((flags & nonspinnable) ||
+ if ((flags & RWSEM_NONSPINNABLE) ||
(owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
ret = false;
rcu_read_unlock();
#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER | OWNER_READER)
static inline enum owner_state
-rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable)
+rwsem_owner_state(struct task_struct *owner, unsigned long flags)
{
- if (flags & nonspinnable)
+ if (flags & RWSEM_NONSPINNABLE)
return OWNER_NONSPINNABLE;
if (flags & RWSEM_READER_OWNED)
}
static noinline enum owner_state
-rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
+rwsem_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *new, *owner;
unsigned long flags, new_flags;
enum owner_state state;
owner = rwsem_owner_flags(sem, &flags);
- state = rwsem_owner_state(owner, flags, nonspinnable);
+ state = rwsem_owner_state(owner, flags);
if (state != OWNER_WRITER)
return state;
*/
new = rwsem_owner_flags(sem, &new_flags);
if ((new != owner) || (new_flags != flags)) {
- state = rwsem_owner_state(new, new_flags, nonspinnable);
+ state = rwsem_owner_state(new, new_flags);
break;
}
return sched_clock() + delta;
}
-static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
+static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
bool taken = false;
int prev_owner_state = OWNER_NULL;
int loop = 0;
u64 rspin_threshold = 0;
- unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
- : RWSEM_RD_NONSPINNABLE;
preempt_disable();
for (;;) {
enum owner_state owner_state;
- owner_state = rwsem_spin_on_owner(sem, nonspinnable);
+ owner_state = rwsem_spin_on_owner(sem);
if (!(owner_state & OWNER_SPINNABLE))
break;
/*
* Try to acquire the lock
*/
- taken = wlock ? rwsem_try_write_lock_unqueued(sem)
- : rwsem_try_read_lock_unqueued(sem);
+ taken = rwsem_try_write_lock_unqueued(sem);
if (taken)
break;
/*
* Time-based reader-owned rwsem optimistic spinning
*/
- if (wlock && (owner_state == OWNER_READER)) {
+ if (owner_state == OWNER_READER) {
/*
* Re-initialize rspin_threshold every time when
* the owner state changes from non-reader to reader.
* the beginning of the 2nd reader phase.
*/
if (prev_owner_state != OWNER_READER) {
- if (rwsem_test_oflags(sem, nonspinnable))
+ if (rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))
break;
rspin_threshold = rwsem_rspin_threshold(sem);
loop = 0;
}
/*
- * Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should
+ * Clear the owner's RWSEM_NONSPINNABLE bit if it is set. This should
* only be called when the reader count reaches 0.
- *
- * This give writers better chance to acquire the rwsem first before
- * readers when the rwsem was being held by readers for a relatively long
- * period of time. Race can happen that an optimistic spinner may have
- * just stolen the rwsem and set the owner, but just clearing the
- * RWSEM_WR_NONSPINNABLE bit will do no harm anyway.
- */
-static inline void clear_wr_nonspinnable(struct rw_semaphore *sem)
-{
- if (rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE))
- atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner);
-}
-
-/*
- * This function is called when the reader fails to acquire the lock via
- * optimistic spinning. In this case we will still attempt to do a trylock
- * when comparing the rwsem state right now with the state when entering
- * the slowpath indicates that the reader is still in a valid reader phase.
- * This happens when the following conditions are true:
- *
- * 1) The lock is currently reader owned, and
- * 2) The lock is previously not reader-owned or the last read owner changes.
- *
- * In the former case, we have transitioned from a writer phase to a
- * reader-phase while spinning. In the latter case, it means the reader
- * phase hasn't ended when we entered the optimistic spinning loop. In
- * both cases, the reader is eligible to acquire the lock. This is the
- * secondary path where a read lock is acquired optimistically.
- *
- * The reader non-spinnable bit wasn't set at time of entry or it will
- * not be here at all.
*/
-static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
- unsigned long last_rowner)
+static inline void clear_nonspinnable(struct rw_semaphore *sem)
{
- unsigned long owner = atomic_long_read(&sem->owner);
-
- if (!(owner & RWSEM_READER_OWNED))
- return false;
-
- if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) &&
- rwsem_try_read_lock_unqueued(sem)) {
- lockevent_inc(rwsem_opt_rlock2);
- lockevent_add(rwsem_opt_fail, -1);
- return true;
- }
- return false;
-}
-
-static inline bool rwsem_no_spinners(struct rw_semaphore *sem)
-{
- return !osq_is_locked(&sem->osq);
+ if (rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))
+ atomic_long_andnot(RWSEM_NONSPINNABLE, &sem->owner);
}
#else
-static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
- unsigned long nonspinnable)
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
return false;
}
-static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
+static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
return false;
}
-static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }
-
-static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
- unsigned long last_rowner)
-{
- return false;
-}
-
-static inline bool rwsem_no_spinners(sem)
-{
- return false;
-}
+static inline void clear_nonspinnable(struct rw_semaphore *sem) { }
static inline int
-rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
+rwsem_spin_on_owner(struct rw_semaphore *sem)
{
return 0;
}
static struct rw_semaphore __sched *
rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, int state)
{
- long owner, adjustment = -RWSEM_READER_BIAS;
+ long adjustment = -RWSEM_READER_BIAS;
long rcnt = (count >> RWSEM_READER_SHIFT);
struct rwsem_waiter waiter;
DEFINE_WAKE_Q(wake_q);
/*
* To prevent a constant stream of readers from starving a sleeping
- * waiter, don't attempt optimistic spinning if the lock is currently
- * owned by readers.
+ * waiter, don't attempt optimistic lock stealing if the lock is
+ * currently owned by readers.
*/
- owner = atomic_long_read(&sem->owner);
- if ((owner & RWSEM_READER_OWNED) && (rcnt > 1) &&
- !(count & RWSEM_WRITER_LOCKED))
+ if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
+ (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
goto queue;
/*
- * Reader optimistic lock stealing
- *
- * We can take the read lock directly without doing
- * rwsem_optimistic_spin() if the conditions are right.
- * Also wake up other readers if it is the first reader.
+ * Reader optimistic lock stealing.
*/
- if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF)) &&
- rwsem_no_spinners(sem)) {
+ if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF))) {
rwsem_set_reader_owned(sem);
lockevent_inc(rwsem_rlock_steal);
- if (rcnt == 1)
- goto wake_readers;
- return sem;
- }
- /*
- * Save the current read-owner of rwsem, if available, and the
- * reader nonspinnable bit.
- */
- waiter.last_rowner = owner;
- if (!(waiter.last_rowner & RWSEM_READER_OWNED))
- waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;
-
- if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))
- goto queue;
-
- /*
- * Undo read bias from down_read() and do optimistic spinning.
- */
- atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
- adjustment = 0;
- if (rwsem_optimistic_spin(sem, false)) {
- /* rwsem_optimistic_spin() implies ACQUIRE on success */
/*
- * Wake up other readers in the wait list if the front
- * waiter is a reader.
+ * Wake up other readers in the wait queue if it is
+ * the first reader.
*/
-wake_readers:
- if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) {
+ if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
raw_spin_lock_irq(&sem->wait_lock);
if (!list_empty(&sem->wait_list))
rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
wake_up_q(&wake_q);
}
return sem;
- } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
- /* rwsem_reader_phase_trylock() implies ACQUIRE on success */
- return sem;
}
queue:
* exit the slowpath and return immediately as its
* RWSEM_READER_BIAS has already been set in the count.
*/
- if (adjustment && !(atomic_long_read(&sem->count) &
+ if (!(atomic_long_read(&sem->count) &
(RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
/* Provide lock ACQUIRE */
smp_acquire__after_ctrl_dep();
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
- if (adjustment)
- count = atomic_long_add_return(adjustment, &sem->count);
- else
- count = atomic_long_read(&sem->count);
+ count = atomic_long_add_return(adjustment, &sem->count);
/*
* If there are no active locks, wake the front queued process(es).
* wake our own waiter to join the existing active readers !
*/
if (!(count & RWSEM_LOCK_MASK)) {
- clear_wr_nonspinnable(sem);
+ clear_nonspinnable(sem);
wake = true;
}
if (wake || (!(count & RWSEM_WRITER_MASK) &&
return ERR_PTR(-EINTR);
}
-/*
- * This function is called by the a write lock owner. So the owner value
- * won't get changed by others.
- */
-static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
- bool disable)
-{
- if (unlikely(disable)) {
- atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner);
- lockevent_inc(rwsem_opt_norspin);
- }
-}
-
/*
* Wait until we successfully acquire the write lock
*/
rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
{
long count;
- bool disable_rspin;
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 */
- if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
- rwsem_optimistic_spin(sem, true)) {
+ if (rwsem_can_spin_on_owner(sem) && rwsem_optimistic_spin(sem)) {
/* rwsem_optimistic_spin() implies ACQUIRE on success */
return sem;
}
- /*
- * Disable reader optimistic spinning for this rwsem after
- * acquiring the write lock when the setting of the nonspinnable
- * bits are observed.
- */
- disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE;
-
/*
* Optimistic spinning failed, proceed to the slowpath
* and block until we can acquire the sem.
* without sleeping.
*/
if (wstate == WRITER_HANDOFF &&
- rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
+ rwsem_spin_on_owner(sem) == OWNER_NULL)
goto trylock_again;
/* Block until there are no active lockers. */
}
__set_current_state(TASK_RUNNING);
list_del(&waiter.list);
- rwsem_disable_reader_optspin(sem, disable_rspin);
raw_spin_unlock_irq(&sem->wait_lock);
lockevent_inc(rwsem_wlock);
DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
RWSEM_FLAG_WAITERS)) {
- clear_wr_nonspinnable(sem);
+ clear_nonspinnable(sem);
rwsem_wake(sem, tmp);
}
}
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