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SubjectRe: [PATCH 1/7] atomics/tty: add missing atomic_long_t * cast
On Tue, Jun 05, 2018 at 04:53:34PM +0200, Peter Zijlstra wrote:
> 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@hurleysoftware.com>
> Reported-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Greg, I assume you'll queue this at some point soon.

In the mean time, I've picked this up as part of this series, replacing
my patch with the atomic_long_t cast.

Thanks,
Mark.

> ---
> 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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