Re: [PATCH 1/2] rwsem: add rwsem_is_contended V2

[Peter Hurley <peter@hurleysoftware.com>]

On 09/19/2013 11:48 AM, Josef Bacik wrote:
> Btrfs needs a simple way to know if it needs to let go of it's read lock on a
> rwsem.  Introduce rwsem_is_contended to check to see if there are any waiters on
> this rwsem currently.  This is just a hueristic, it is meant to be light and not
> 100% accurate and called by somebody already holding on to the rwsem in either
> read or write.  Thanks,
>
> Signed-off-by: Josef Bacik <jbacik@fusionio.com>
> ---
> V1->V2: took everybodys suggestions and simplified it to just one function in
> rwsem.h so it works for both the spinlock case and non-spinlock case.
>
>   include/linux/rwsem.h | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 0616ffe..c340493 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -75,6 +75,19 @@ do {								\
>   } while (0)
>
>   /*
> + * This is the same regardless of which rwsem implementation that is being used.
> + * It is just a heuristic meant to be called by somebody alreadying holding the
> + * rwsem to see if somebody from  the opposite type is wanting access to the
                                         ^^^^^^^^^^^^^

Readers can infer that at least one writer is waiting if the wait_list is
!empty; however, writers cannot infer anything other than some other
thread is waiting -- it could be a reader or a writer or multiples of either.


> + * lock.
> + */
> +static inline int rwsem_is_contended(struct rw_semaphore *sem)
> +{
> +	if (!list_empty(&sem->wait_list))
> +		return 1;
> +	return 0;

How about

         return !list_empty(&sem->wait_list);

?

> +}
> +
> +/*
>    * lock for reading
>    */
>   extern void down_read(struct rw_semaphore *sem);
>