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

[Waiman Long <longman@redhat.com>]

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