Re: [PATCH 1/5] locking: Add rwsem_is_write_locked()

Waiman Long
2023-09-07T18:08:33Z

On 9/7/23 13:47, Matthew Wilcox (Oracle) wrote:
> Several places want to know whether the lock is held by a writer, instead
> of just whether it's held. We can implement this for both normal and
> rt rwsems. RWSEM_WRITER_LOCKED is declared in rwsem.c and exposing
> it outside that file might tempt other people to use it, so just use
> a comment to note that's what the 1 means, and help anybody find it if
> they're looking to change the implementation.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> include/linux/rwbase_rt.h | 5 +++++
> include/linux/rwsem.h | 10 ++++++++++
> 2 files changed, 15 insertions(+)
>
> diff --git a/include/linux/rwbase_rt.h b/include/linux/rwbase_rt.h
> index 1d264dd08625..3c25b14edc05 100644
> --- a/include/linux/rwbase_rt.h
> +++ b/include/linux/rwbase_rt.h
> @@ -31,6 +31,11 @@ static __always_inline bool rw_base_is_locked(struct rwbase_rt *rwb)
> return atomic_read(&rwb->readers) != READER_BIAS;
> }
>
> +static __always_inline bool rw_base_is_write_locked(struct rwbase_rt *rwb)
> +{
> + return atomic_read(&rwb->readers) == WRITER_BIAS;
> +}
> +
> static __always_inline bool rw_base_is_contended(struct rwbase_rt *rwb)
> {
> return atomic_read(&rwb->readers) > 0;
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 1dd530ce8b45..0f78b8d2e653 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -72,6 +72,11 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
> return atomic_long_read(&sem->count) != 0;
> }
>
> +static inline int rwsem_is_write_locked(struct rw_semaphore *sem)
> +{
> + return atomic_long_read(&sem->count) & 1 /* RWSEM_WRITER_LOCKED */;
> +}

I would prefer you move the various RWSEM_* count bit macros from
kernel/locking/rwsem.c to under the !PREEMPT_RT block and directly use
RWSEM_WRITER_LOCKED instead of hardcoding a value of 1.

Cheers,
Longman