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Message-ID: <20180605145334.GA12235@hirez.programming.kicks-ass.net>
Date: Tue, 5 Jun 2018 16:53:34 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
Boqun Feng <boqun.feng@...il.com>,
Will Deacon <will.deacon@....com>,
Jiri Slaby <jslaby@...e.com>,
Peter Hurley <peter@...leysoftware.com>
Subject: Re: [PATCH 1/7] atomics/tty: add missing atomic_long_t * cast
On Tue, Jun 05, 2018 at 04:43:04PM +0200, Peter Zijlstra wrote:
> I can make a proper patch, hold on.
---
Subject: atomic/tty: Fix up atomic abuse in ldsem
Mark found ldsem_cmpxchg() needed an (atomic_long_t *) cast to keep
working after making the atomic_long interface type safe.
Needing casts is bad form, which made me look at the code. There are no
ld_semaphore::count users outside of these functions so there is no
reason why it can not be an atomic_long_t in the first place, obviating
the need for this cast.
That also ensures the loads use atomic_long_read(), which implies (at
least) READ_ONCE() in order to guarantee single-copy-atomic loads.
When using atomic_long_try_cmpxchg() the ldsem_cmpxchg() wrapper gets
very thin (the only difference is not changing *old on success, which
most callers don't seem to care about).
So rework the whole thing to use atomic_long_t and its accessors
directly.
While there, fixup all the horrible comment styles.
Cc: Peter Hurley <peter@...leysoftware.com>
Reported-by: Mark Rutland <mark.rutland@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
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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