// SPDX-License-Identifier: GPL-2.0
/*
* Ldisc rw semaphore
*
* The ldisc semaphore is semantically a rw_semaphore but which enforces
* an alternate policy, namely:
* 1) Supports lock wait timeouts
* 2) Write waiter has priority
* 3) Downgrading is not supported
*
* Implementation notes:
* 1) Upper half of semaphore count is a wait count (differs from rwsem
* in that rwsem normalizes the upper half to the wait bias)
* 2) Lacks overflow checking
*
* The generic counting was copied and modified from include/asm-generic/rwsem.h
* by Paul Mackerras <paulus@samba.org>.
*
* The scheduling policy was copied and modified from lib/rwsem.c
* Written by David Howells (dhowells@redhat.com).
*
* This implementation incorporates the write lock stealing work of
* Michel Lespinasse <walken@google.com>.
*
* Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com>
*/
#include <linux/list.h>
#include <linux/spinlock.h>
#include <linux/atomic.h>
#include <linux/tty.h>
#include <linux/sched.h>
#include <linux/sched/debug.h>
#include <linux/sched/task.h>
#if BITS_PER_LONG == 64
# define LDSEM_ACTIVE_MASK 0xffffffffL
#else
# define LDSEM_ACTIVE_MASK 0x0000ffffL
#endif
#define LDSEM_UNLOCKED 0L
#define LDSEM_ACTIVE_BIAS 1L
#define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1)
#define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS
#define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS)
struct ldsem_waiter {
struct list_head list;
struct task_struct *task;
};
/*
* Initialize an ldsem:
*/
void __init_ldsem(struct ld_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, LDSEM_UNLOCKED);
sem->wait_readers = 0;
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->read_wait);
INIT_LIST_HEAD(&sem->write_wait);
}
static void __ldsem_wake_readers(struct ld_semaphore *sem)
{
struct ldsem_waiter *waiter, *next;
struct task_struct *tsk;
long adjust, count;
/*
* Try to grant read locks to all readers on the read wait list.
* Note the 'active part' of the count is incremented by
* the number of readers before waking any processes up.
*/
adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS);
count = atomic_long_add_return(adjust, &sem->count);
do {
if (count > 0)
break;
if (atomic_long_try_cmpxchg(&sem->count, &count, count - adjust))
return;
} while (1);
list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
tsk = waiter->task;
smp_mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
}
INIT_LIST_HEAD(&sem->read_wait);
sem->wait_readers = 0;
}
static inline int writer_trylock(struct ld_semaphore *sem)
{
/*
* Only wake this writer if the active part of the count can be
* transitioned from 0 -> 1
*/
long count = atomic_long_add_return(LDSEM_ACTIVE_BIAS, &sem->count);
do {
if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
return 1;
if (atomic_long_try_cmpxchg(&sem->count, &count, count - LDSEM_ACTIVE_BIAS))
return 0;
} while (1);
}
static void __ldsem_wake_writer(struct ld_semaphore *sem)
{
struct ldsem_waiter *waiter;
waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
wake_up_process(waiter->task);
}
/*
* 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)
* - the spinlock must be held by the caller
* - woken process blocks are discarded from the list after having task zeroed
*/
static void __ldsem_wake(struct ld_semaphore *sem)
{
if (!list_empty(&sem->write_wait))
__ldsem_wake_writer(sem);
else if (!list_empty(&sem->read_wait))
__ldsem_wake_readers(sem);
}
static void ldsem_wake(struct ld_semaphore *sem)
{
unsigned long flags;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
__ldsem_wake(sem);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
}
/*
* wait for the read lock to be granted
*/
static struct ld_semaphore __sched *
down_read_failed(struct ld_semaphore *sem, long count, long timeout)
{
struct ldsem_waiter waiter;
long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
/* set up my own style of waitqueue */
raw_spin_lock_irq(&sem->wait_lock);
/*
* Try to reverse the lock attempt but if the count has changed
* so that reversing fails, check if there are are no waiters,
* and early-out if not
*/
do {
if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust)) {
count += adjust;
break;
}
if (count > 0) {
raw_spin_unlock_irq(&sem->wait_lock);
return sem;
}
} while (1);
list_add_tail(&waiter.list, &sem->read_wait);
sem->wait_readers++;
waiter.task = current;
get_task_struct(current);
/* if there are no active locks, wake the new lock owner(s) */
if ((count & LDSEM_ACTIVE_MASK) == 0)
__ldsem_wake(sem);
raw_spin_unlock_irq(&sem->wait_lock);
/* wait to be given the lock */
for (;;) {
set_current_state(TASK_UNINTERRUPTIBLE);
if (!waiter.task)
break;
if (!timeout)
break;
timeout = schedule_timeout(timeout);
}
__set_current_state(TASK_RUNNING);
if (!timeout) {
/*
* Lock timed out but check if this task was just
* granted lock ownership - if so, pretend there
* was no timeout; otherwise, cleanup lock wait.
*/
raw_spin_lock_irq(&sem->wait_lock);
if (waiter.task) {
atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
sem->wait_readers--;
list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);
put_task_struct(waiter.task);
return NULL;
}
raw_spin_unlock_irq(&sem->wait_lock);
}
return sem;
}
/*
* wait for the write lock to be granted
*/
static struct ld_semaphore __sched *
down_write_failed(struct ld_semaphore *sem, long count, long timeout)
{
struct ldsem_waiter waiter;
long adjust = -LDSEM_ACTIVE_BIAS;
int locked = 0;
/* set up my own style of waitqueue */
raw_spin_lock_irq(&sem->wait_lock);
/*
* Try to reverse the lock attempt but if the count has changed
* so that reversing fails, check if the lock is now owned,
* and early-out if so.
*/
do {
if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust))
break;
if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) {
raw_spin_unlock_irq(&sem->wait_lock);
return sem;
}
} while (1);
list_add_tail(&waiter.list, &sem->write_wait);
waiter.task = current;
set_current_state(TASK_UNINTERRUPTIBLE);
for (;;) {
if (!timeout)
break;
raw_spin_unlock_irq(&sem->wait_lock);
timeout = schedule_timeout(timeout);
raw_spin_lock_irq(&sem->wait_lock);
set_current_state(TASK_UNINTERRUPTIBLE);
locked = writer_trylock(sem);
if (locked)
break;
}
if (!locked)
atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
list_del(&waiter.list);
/*
* In case of timeout, wake up every reader who gave the right of way
* to writer. Prevent separation readers into two groups:
* one that helds semaphore and another that sleeps.
* (in case of no contention with a writer)
*/
if (!locked && list_empty(&sem->write_wait))
__ldsem_wake_readers(sem);
raw_spin_unlock_irq(&sem->wait_lock);
__set_current_state(TASK_RUNNING);
/* lock wait may have timed out */
if (!locked)
return NULL;
return sem;
}
static int __ldsem_down_read_nested(struct ld_semaphore *sem,
int subclass, long timeout)
{
long count;
rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
count = atomic_long_add_return(LDSEM_READ_BIAS, &sem->count);
if (count <= 0) {
lock_contended(&sem->dep_map, _RET_IP_);
if (!down_read_failed(sem, count, timeout)) {
rwsem_release(&sem->dep_map, 1, _RET_IP_);
return 0;
}
}
lock_acquired(&sem->dep_map, _RET_IP_);
return 1;
}
static int __ldsem_down_write_nested(struct ld_semaphore *sem,
int subclass, long timeout)
{
long count;
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
count = atomic_long_add_return(LDSEM_WRITE_BIAS, &sem->count);
if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
lock_contended(&sem->dep_map, _RET_IP_);
if (!down_write_failed(sem, count, timeout)) {
rwsem_release(&sem->dep_map, 1, _RET_IP_);
return 0;
}
}
lock_acquired(&sem->dep_map, _RET_IP_);
return 1;
}
/*
* lock for reading -- returns 1 if successful, 0 if timed out
*/
int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
{
might_sleep();
return __ldsem_down_read_nested(sem, 0, timeout);
}
/*
* trylock for reading -- returns 1 if successful, 0 if contention
*/
int ldsem_down_read_trylock(struct ld_semaphore *sem)
{
long count = atomic_long_read(&sem->count);
while (count >= 0) {
if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_READ_BIAS)) {
rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
lock_acquired(&sem->dep_map, _RET_IP_);
return 1;
}
}
return 0;
}
/*
* lock for writing -- returns 1 if successful, 0 if timed out
*/
int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
{
might_sleep();
return __ldsem_down_write_nested(sem, 0, timeout);
}
/*
* trylock for writing -- returns 1 if successful, 0 if contention
*/
int ldsem_down_write_trylock(struct ld_semaphore *sem)
{
long count = atomic_long_read(&sem->count);
while ((count & LDSEM_ACTIVE_MASK) == 0) {
if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_WRITE_BIAS)) {
rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
lock_acquired(&sem->dep_map, _RET_IP_);
return 1;
}
}
return 0;
}
/*
* release a read lock
*/
void ldsem_up_read(struct ld_semaphore *sem)
{
long count;
rwsem_release(&sem->dep_map, 1, _RET_IP_);
count = atomic_long_add_return(-LDSEM_READ_BIAS, &sem->count);
if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
ldsem_wake(sem);
}
/*
* release a write lock
*/
void ldsem_up_write(struct ld_semaphore *sem)
{
long count;
rwsem_release(&sem->dep_map, 1, _RET_IP_);
count = atomic_long_add_return(-LDSEM_WRITE_BIAS, &sem->count);
if (count < 0)
ldsem_wake(sem);
}
#ifdef CONFIG_DEBUG_LOCK_ALLOC
int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
{
might_sleep();
return __ldsem_down_read_nested(sem, subclass, timeout);
}
int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
long timeout)
{
might_sleep();
return __ldsem_down_write_nested(sem, subclass, timeout);
}
#endif
