linux/drivers/tty/tty_ldsem.c
Greg Kroah-Hartman e5656d43dc tty: Remove redundant license text
Now that the SPDX tag is in all tty files, that identifies the license
in a specific and legally-defined manner.  So the extra GPL text wording
can be removed as it is no longer needed at all.

This is done on a quest to remove the 700+ different ways that files in
the kernel describe the GPL license text.  And there's unneeded stuff
like the address (sometimes incorrect) for the FSF which is never
needed.

No copyright headers or other non-license-description text was removed.

Cc: Jiri Slaby <jslaby@suse.com>
Cc: James Hogan <jhogan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2017-11-08 13:08:12 +01:00

456 lines
11 KiB
C

// 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>
#ifdef CONFIG_DEBUG_LOCK_ALLOC
# define __acq(l, s, t, r, c, n, i) \
lock_acquire(&(l)->dep_map, s, t, r, c, n, i)
# define __rel(l, n, i) \
lock_release(&(l)->dep_map, n, i)
#define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i)
#define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i)
#define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i)
#define lockdep_release(l, n, i) __rel(l, n, i)
#else
# define lockdep_acquire(l, s, t, i) do { } while (0)
# define lockdep_acquire_nest(l, s, t, n, i) do { } while (0)
# define lockdep_acquire_read(l, s, t, i) do { } while (0)
# define lockdep_release(l, n, i) do { } while (0)
#endif
#ifdef CONFIG_LOCK_STAT
# define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_)
#else
# define lock_stat(_lock, stat) do { } while (0)
#endif
#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;
};
static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem)
{
return atomic_long_add_return(delta, (atomic_long_t *)&sem->count);
}
/*
* ldsem_cmpxchg() updates @*old with the last-known sem->count value.
* Returns 1 if count was successfully changed; @*old will have @new value.
* Returns 0 if count was not changed; @*old will have most recent sem->count
*/
static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem)
{
long tmp = atomic_long_cmpxchg(&sem->count, *old, new);
if (tmp == *old) {
*old = new;
return 1;
} else {
*old = tmp;
return 0;
}
}
/*
* 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
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 = ldsem_atomic_update(adjust, sem);
do {
if (count > 0)
break;
if (ldsem_cmpxchg(&count, count - adjust, sem))
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 = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem);
do {
if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
return 1;
if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem))
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 (ldsem_cmpxchg(&count, count + adjust, sem))
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) {
ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
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 (ldsem_cmpxchg(&count, count + adjust, sem))
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)
ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
list_del(&waiter.list);
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;
lockdep_acquire_read(sem, subclass, 0, _RET_IP_);
count = ldsem_atomic_update(LDSEM_READ_BIAS, sem);
if (count <= 0) {
lock_stat(sem, contended);
if (!down_read_failed(sem, count, timeout)) {
lockdep_release(sem, 1, _RET_IP_);
return 0;
}
}
lock_stat(sem, acquired);
return 1;
}
static int __ldsem_down_write_nested(struct ld_semaphore *sem,
int subclass, long timeout)
{
long count;
lockdep_acquire(sem, subclass, 0, _RET_IP_);
count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem);
if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
lock_stat(sem, contended);
if (!down_write_failed(sem, count, timeout)) {
lockdep_release(sem, 1, _RET_IP_);
return 0;
}
}
lock_stat(sem, acquired);
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 = sem->count;
while (count >= 0) {
if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) {
lockdep_acquire_read(sem, 0, 1, _RET_IP_);
lock_stat(sem, acquired);
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 = sem->count;
while ((count & LDSEM_ACTIVE_MASK) == 0) {
if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) {
lockdep_acquire(sem, 0, 1, _RET_IP_);
lock_stat(sem, acquired);
return 1;
}
}
return 0;
}
/*
* release a read lock
*/
void ldsem_up_read(struct ld_semaphore *sem)
{
long count;
lockdep_release(sem, 1, _RET_IP_);
count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem);
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;
lockdep_release(sem, 1, _RET_IP_);
count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem);
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