On Thu, Sep 19, 2013 at 06:57:27PM -0400, Peter Hurley wrote:
> 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 <jba...@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.
>
Right duh, I'll fix that up.
>
> >+ * 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);
>
> ?
>
Another duh, thanks I'll wait for any other input and then fix this up and
resend. Thanks,
Josef
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