On Sun, Feb 10, 2019 at 09:00:50PM -0500, Waiman Long wrote:
> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
> index bad2bca..067e265 100644
> --- a/kernel/locking/rwsem.h
> +++ b/kernel/locking/rwsem.h
> @@ -32,6 +32,26 @@
> # define DEBUG_RWSEMS_WARN_ON(c)
> #endif
>
> +/*
> + * R/W semaphores originally for PPC using the stuff in lib/rwsem.c.
> + * Adapted largely from include/asm-i386/rwsem.h
> + * by Paul Mackerras <paulus@xxxxxxxxx>.
> + */
> +
> +/*
> + * the semaphore definition
> + */
> +#ifdef CONFIG_64BIT
> +# define RWSEM_ACTIVE_MASK 0xffffffffL
> +#else
> +# define RWSEM_ACTIVE_MASK 0x0000ffffL
> +#endif
> +
> +#define RWSEM_ACTIVE_BIAS 0x00000001L
> +#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
> +#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
> +#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
> +
> #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> /*
> * All writes to owner are protected by WRITE_ONCE() to make sure that
> @@ -132,3 +152,113 @@ static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
> {
> }
> #endif
> +
> +#ifdef CONFIG_RWSEM_XCHGADD_ALGORITHM
> +/*
> + * lock for reading
> + */
> +static inline void __down_read(struct rw_semaphore *sem)
> +{
> + if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0))
> + rwsem_down_read_failed(sem);
> +}
> +
> +static inline int __down_read_killable(struct rw_semaphore *sem)
> +{
> + if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0)) {
> + if (IS_ERR(rwsem_down_read_failed_killable(sem)))
> + return -EINTR;
> + }
> +
> + return 0;
> +}
> +
> +static inline int __down_read_trylock(struct rw_semaphore *sem)
> +{
> + long tmp;
> +
> + while ((tmp = atomic_long_read(&sem->count)) >= 0) {
> + if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
> + tmp + RWSEM_ACTIVE_READ_BIAS)) {
> + return 1;
That really wants to be:
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
tmp + RWSEM_ACTIVE_READ_BIAS))
> + }
> + }
> + return 0;
> +}
> +
> +/*
> + * lock for writing
> + */
> +static inline void __down_write(struct rw_semaphore *sem)
> +{
> + long tmp;
> +
> + tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
> + &sem->count);
> + if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
> + rwsem_down_write_failed(sem);
> +}
> +
> +static inline int __down_write_killable(struct rw_semaphore *sem)
> +{
> + long tmp;
> +
> + tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
> + &sem->count);
> + if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
> + if (IS_ERR(rwsem_down_write_failed_killable(sem)))
> + return -EINTR;
> + return 0;
> +}
> +
> +static inline int __down_write_trylock(struct rw_semaphore *sem)
> +{
> + long tmp;
tmp = RWSEM_UNLOCKED_VALUE;
> +
> + tmp = atomic_long_cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
> + RWSEM_ACTIVE_WRITE_BIAS);
> + return tmp == RWSEM_UNLOCKED_VALUE;
return atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
RWSEM_ACTIVE_WRITE_BIAS);
> +}
> +
> +/*
> + * unlock after reading
> + */
> +static inline void __up_read(struct rw_semaphore *sem)
> +{
> + long tmp;
> +
> + tmp = atomic_long_dec_return_release(&sem->count);
> + if (unlikely(tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0))
> + rwsem_wake(sem);
> +}
> +
> +/*
> + * unlock after writing
> + */
> +static inline void __up_write(struct rw_semaphore *sem)
> +{
> + if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
> + &sem->count) < 0))
> + rwsem_wake(sem);
> +}
> +
> +/*
> + * downgrade write lock to read lock
> + */
> +static inline void __downgrade_write(struct rw_semaphore *sem)
> +{
> + long tmp;
> +
> + /*
> + * When downgrading from exclusive to shared ownership,
> + * anything inside the write-locked region cannot leak
> + * into the read side. In contrast, anything in the
> + * read-locked region is ok to be re-ordered into the
> + * write side. As such, rely on RELEASE semantics.
> + */
> + tmp = atomic_long_add_return_release(-RWSEM_WAITING_BIAS, &sem->count);
> + if (tmp < 0)
> + rwsem_downgrade_wake(sem);
> +}
> +
> +#endif /* CONFIG_RWSEM_XCHGADD_ALGORITHM */
