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

[Josef Bacik <jbacik@fb.com>]

On 01/13/2014 02:02 PM, Ingo Molnar wrote:
> * Josef Bacik <jbacik@fb.com> wrote:
>
>> From: Josef Bacik <jbacik@fusionio.com>
>>
>> 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>
>> ---
>>   include/linux/rwsem.h | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
>> index 0616ffe..03f3b05 100644
>> --- a/include/linux/rwsem.h
>> +++ b/include/linux/rwsem.h
>> @@ -75,6 +75,17 @@ 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 an incompatible type is wanting access to the
>> + * lock.
>> + */
>> +static inline int rwsem_is_contended(struct rw_semaphore *sem)
>> +{
>> +	return !list_empty(&sem->wait_list);
>> +}
>> +
>> +/*
>>    * lock for reading
>>    */
>>   extern void down_read(struct rw_semaphore *sem);
> Looks good to me. To make it easier to merge 2/2 feel free to add this
> to the BTRFS tree, with my Acked-by:
>
>    Acked-by: Ingo Molnar <mingo@kernel.org>
Great thanks Ingo,

Josef