Messages in this thread | | | Date | Thu, 19 Sep 2013 18:57:27 -0400 | From | Peter Hurley <> | Subject | Re: [PATCH 1/2] rwsem: add rwsem_is_contended V2 |
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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); >
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