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|
/* rwsem.c: R/W semaphores: contention handling functions
*
* Written by David Howells (dhowells@redhat.com).
* Derived from arch/i386/kernel/semaphore.c
*
* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
* and Michel Lespinasse <walken@google.com>
*
* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
*/
#include <linux/rwsem.h>
#include <linux/init.h>
#include <linux/export.h>
#include <linux/sched/signal.h>
#include <linux/sched/rt.h>
#include <linux/sched/wake_q.h>
#include <linux/sched/debug.h>
#include <linux/osq_lock.h>
#include "rwsem.h"
/*
* Guide to the rw_semaphore's count field for common values.
* (32-bit case illustrated, similar for 64-bit)
*
* 0x0000000X (1) X readers active or attempting lock, no writer waiting
* X = #active_readers + #readers attempting to lock
* (X*ACTIVE_BIAS)
*
* 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
* attempting to read lock or write lock.
*
* 0xffff000X (1) X readers active or attempting lock, with waiters for lock
* X = #active readers + # readers attempting lock
* (X*ACTIVE_BIAS + WAITING_BIAS)
* (2) 1 writer attempting lock, no waiters for lock
* X-1 = #active readers + #readers attempting lock
* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
* (3) 1 writer active, no waiters for lock
* X-1 = #active readers + #readers attempting lock
* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
*
* 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
* (WAITING_BIAS + ACTIVE_BIAS)
* (2) 1 writer active or attempting lock, no waiters for lock
* (ACTIVE_WRITE_BIAS)
*
* 0xffff0000 (1) There are writers or readers queued but none active
* or in the process of attempting lock.
* (WAITING_BIAS)
* Note: writer can attempt to steal lock for this count by adding
* ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
*
* 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
* (ACTIVE_WRITE_BIAS + WAITING_BIAS)
*
* Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
* the count becomes more than 0 for successful lock acquisition,
* i.e. the case where there are only readers or nobody has lock.
* (1st and 2nd case above).
*
* Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
* checking the count becomes ACTIVE_WRITE_BIAS for successful lock
* acquisition (i.e. nobody else has lock or attempts lock). If
* unsuccessful, in rwsem_down_write_failed, we'll check to see if there
* are only waiters but none active (5th case above), and attempt to
* steal the lock.
*
*/
/*
* Initialize an rwsem:
*/
void __init_rwsem(struct rw_semaphore *sem, const char *name,
struct lock_class_key *key)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
/*
* Make sure we are not reinitializing a held semaphore:
*/
debug_check_no_locks_freed((void *)sem, sizeof(*sem));
lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->wait_list);
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
sem->owner = NULL;
osq_lock_init(&sem->osq);
#endif
}
EXPORT_SYMBOL(__init_rwsem);
enum rwsem_waiter_type {
RWSEM_WAITING_FOR_WRITE,
RWSEM_WAITING_FOR_READ
};
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
};
enum rwsem_wake_type {
RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
RWSEM_WAKE_READERS, /* Wake readers only */
RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
};
/*
* handle the lock release when processes blocked on it that can now run
* - if we come here from up_xxxx(), then:
* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
* - there must be someone on the queue
* - the wait_lock must be held by the caller
* - tasks are marked for wakeup, the caller must later invoke wake_up_q()
* to actually wakeup the blocked task(s) and drop the reference count,
* 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
*/
static void __rwsem_mark_wake(struct rw_semaphore *sem,
enum rwsem_wake_type wake_type,
struct wake_q_head *wake_q)
{
struct rwsem_waiter *waiter, *tmp;
long oldcount, woken = 0, adjustment = 0;
/*
* Take a peek at the queue head waiter such that we can determine
* the wakeup(s) to perform.
*/
waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
if (wake_type == RWSEM_WAKE_ANY) {
/*
* Mark writer at the front of the queue for wakeup.
* Until the task is actually later awoken later by
* the caller, other writers are able to steal it.
* Readers, on the other hand, will block as they
* will notice the queued writer.
*/
wake_q_add(wake_q, waiter->task);
}
return;
}
/*
* Writers might steal the lock before we grant it to the next reader.
* We prefer to do the first reader grant before counting readers
* so we can bail out early if a writer stole the lock.
*/
if (wake_type != RWSEM_WAKE_READ_OWNED) {
adjustment = RWSEM_ACTIVE_READ_BIAS;
try_reader_grant:
oldcount = atomic_long_fetch_add(adjustment, &sem->count);
if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
/*
* If the count is still less than RWSEM_WAITING_BIAS
* after removing the adjustment, it is assumed that
* a writer has stolen the lock. We have to undo our
* reader grant.
*/
if (atomic_long_add_return(-adjustment, &sem->count) <
RWSEM_WAITING_BIAS)
return;
/* Last active locker left. Retry waking readers. */
goto try_reader_grant;
}
/*
* It is not really necessary to set it to reader-owned here,
* but it gives the spinners an early indication that the
* readers now have the lock.
*/
rwsem_set_reader_owned(sem);
}
/*
* Grant an infinite number of read locks to the readers at the front
* of the queue. We know that woken will be at least 1 as we accounted
* for above. Note we increment the 'active part' of the count by the
* number of readers before waking any processes up.
*/
list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
struct task_struct *tsk;
if (waiter->type == RWSEM_WAITING_FOR_WRITE)
break;
woken++;
tsk = waiter->task;
wake_q_add(wake_q, tsk);
list_del(&waiter->list);
/*
* Ensure that the last operation is setting the reader
* waiter to nil such that rwsem_down_read_failed() cannot
* race with do_exit() by always holding a reference count
* to the task to wakeup.
*/
smp_store_release(&waiter->task, NULL);
}
adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
if (list_empty(&sem->wait_list)) {
/* hit end of list above */
adjustment -= RWSEM_WAITING_BIAS;
}
if (adjustment)
atomic_long_add(adjustment, &sem->count);
}
/*
* Wait for the read lock to be granted
*/
static inline struct rw_semaphore __sched *
__rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
{
long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
struct rwsem_waiter waiter;
DEFINE_WAKE_Q(wake_q);
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_READ;
raw_spin_lock_irq(&sem->wait_lock);
if (list_empty(&sem->wait_list))
adjustment += RWSEM_WAITING_BIAS;
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
count = atomic_long_add_return(adjustment, &sem->count);
/*
* If there are no active locks, wake the front queued process(es).
*
* If there are no writers and we are first in the queue,
* wake our own waiter to join the existing active readers !
*/
if (count == RWSEM_WAITING_BIAS ||
(count > RWSEM_WAITING_BIAS &&
adjustment != -RWSEM_ACTIVE_READ_BIAS))
__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
/* wait to be given the lock */
while (true) {
set_current_state(state);
if (!waiter.task)
break;
if (signal_pending_state(state, current)) {
raw_spin_lock_irq(&sem->wait_lock);
if (waiter.task)
goto out_nolock;
raw_spin_unlock_irq(&sem->wait_lock);
break;
}
schedule();
}
__set_current_state(TASK_RUNNING);
return sem;
out_nolock:
list_del(&waiter.list);
if (list_empty(&sem->wait_list))
atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
raw_spin_unlock_irq(&sem->wait_lock);
__set_current_state(TASK_RUNNING);
return ERR_PTR(-EINTR);
}
__visible struct rw_semaphore * __sched
rwsem_down_read_failed(struct rw_semaphore *sem)
{
return __rwsem_down_read_failed_common(sem, TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(rwsem_down_read_failed);
__visible struct rw_semaphore * __sched
rwsem_down_read_failed_killable<
|