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SubjectRe: [PATCH 1/7] atomics/tty: add missing atomic_long_t * cast
On Tue, May 29, 2018 at 07:07:40PM +0100, Mark Rutland wrote:
> In ldsem_cmpxchg a pointer to unsigned long is passed to
> atomic_long_cmpxchg(), which expects a pointer to atomic_long_t.

> - long tmp = atomic_long_cmpxchg(&sem->count, *old, new);
> + long tmp = atomic_long_cmpxchg((atomic_long_t *)&sem->count, *old, new);

Needing to cast anything to atomic types is dodgy at best, so I had me a
look at this code.

Would not the following patch be much better?

---
drivers/tty/tty_ldsem.c | 82 ++++++++++++++++++++---------------------------
include/linux/tty_ldisc.h | 4 +--
2 files changed, 37 insertions(+), 49 deletions(-)

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index 37a91b3df980..0c98d88f795a 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -74,28 +74,6 @@ struct ldsem_waiter {
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:
*/
@@ -109,7 +87,7 @@ void __init_ldsem(struct ld_semaphore *sem, const char *name,
debug_check_no_locks_freed((void *)sem, sizeof(*sem));
lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
- sem->count = LDSEM_UNLOCKED;
+ atomic_long_set(&sem->count, LDSEM_UNLOCKED);
sem->wait_readers = 0;
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->read_wait);
@@ -122,16 +100,17 @@ static void __ldsem_wake_readers(struct ld_semaphore *sem)
struct task_struct *tsk;
long adjust, count;

- /* Try to grant read locks to all readers on the read wait list.
+ /*
+ * 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);
+ count = atomic_long_add_return(adjust, &sem->count);
do {
if (count > 0)
break;
- if (ldsem_cmpxchg(&count, count - adjust, sem))
+ if (atomic_long_try_cmpxchg(&sem->count, &count, count - adjust))
return;
} while (1);

@@ -148,14 +127,15 @@ static void __ldsem_wake_readers(struct ld_semaphore *sem)

static inline int writer_trylock(struct ld_semaphore *sem)
{
- /* only wake this writer if the active part of the count can be
+ /*
+ * 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);
+ long count = atomic_long_add_return(LDSEM_ACTIVE_BIAS, &sem->count);
do {
if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
return 1;
- if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem))
+ if (atomic_long_try_cmpxchg(&sem->count, &count, count - LDSEM_ACTIVE_BIAS))
return 0;
} while (1);
}
@@ -205,12 +185,16 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
/* 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
+ /*
+ * 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 */
+ * and early-out if not
+ */
do {
- if (ldsem_cmpxchg(&count, count + adjust, sem))
+ 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;
@@ -243,12 +227,14 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
__set_current_state(TASK_RUNNING);

if (!timeout) {
- /* lock timed out but check if this task was just
+ /*
+ * Lock timed out but check if this task was just
* granted lock ownership - if so, pretend there
- * was no timeout; otherwise, cleanup lock wait */
+ * was no timeout; otherwise, cleanup lock wait.
+ */
raw_spin_lock_irq(&sem->wait_lock);
if (waiter.task) {
- ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
+ atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);
put_task_struct(waiter.task);
@@ -273,11 +259,13 @@ down_write_failed(struct ld_semaphore *sem, long count, long timeout)
/* 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
+ /*
+ * 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 */
+ * and early-out if so.
+ */
do {
- if (ldsem_cmpxchg(&count, count + adjust, sem))
+ 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);
@@ -303,7 +291,7 @@ down_write_failed(struct ld_semaphore *sem, long count, long timeout)
}

if (!locked)
- ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
+ atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);

@@ -324,7 +312,7 @@ static int __ldsem_down_read_nested(struct ld_semaphore *sem,

lockdep_acquire_read(sem, subclass, 0, _RET_IP_);

- count = ldsem_atomic_update(LDSEM_READ_BIAS, sem);
+ count = atomic_long_add_return(LDSEM_READ_BIAS, &sem->count);
if (count <= 0) {
lock_stat(sem, contended);
if (!down_read_failed(sem, count, timeout)) {
@@ -343,7 +331,7 @@ static int __ldsem_down_write_nested(struct ld_semaphore *sem,

lockdep_acquire(sem, subclass, 0, _RET_IP_);

- count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem);
+ count = atomic_long_add_return(LDSEM_WRITE_BIAS, &sem->count);
if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
lock_stat(sem, contended);
if (!down_write_failed(sem, count, timeout)) {
@@ -370,10 +358,10 @@ int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
*/
int ldsem_down_read_trylock(struct ld_semaphore *sem)
{
- long count = sem->count;
+ long count = atomic_long_read(&sem->count);

while (count >= 0) {
- if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) {
+ if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_READ_BIAS)) {
lockdep_acquire_read(sem, 0, 1, _RET_IP_);
lock_stat(sem, acquired);
return 1;
@@ -396,10 +384,10 @@ int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
*/
int ldsem_down_write_trylock(struct ld_semaphore *sem)
{
- long count = sem->count;
+ long count = atomic_long_read(&sem->count);

while ((count & LDSEM_ACTIVE_MASK) == 0) {
- if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) {
+ if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_WRITE_BIAS)) {
lockdep_acquire(sem, 0, 1, _RET_IP_);
lock_stat(sem, acquired);
return 1;
@@ -417,7 +405,7 @@ void ldsem_up_read(struct ld_semaphore *sem)

lockdep_release(sem, 1, _RET_IP_);

- count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem);
+ count = atomic_long_add_return(-LDSEM_READ_BIAS, &sem->count);
if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
ldsem_wake(sem);
}
@@ -431,7 +419,7 @@ void ldsem_up_write(struct ld_semaphore *sem)

lockdep_release(sem, 1, _RET_IP_);

- count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem);
+ count = atomic_long_add_return(-LDSEM_WRITE_BIAS, &sem->count);
if (count < 0)
ldsem_wake(sem);
}
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index 1ef64d4ad887..840894ca3fc0 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -119,13 +119,13 @@

#include <linux/fs.h>
#include <linux/wait.h>
-
+#include <linux/atomic.h>

/*
* the semaphore definition
*/
struct ld_semaphore {
- long count;
+ atomic_long_t count;
raw_spinlock_t wait_lock;
unsigned int wait_readers;
struct list_head read_wait;
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