From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from dkim2.fusionio.com ([66.114.96.54]:50975 "EHLO dkim2.fusionio.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752566Ab3ISX1d (ORCPT ); Thu, 19 Sep 2013 19:27:33 -0400 Received: from mx2.fusionio.com (unknown [10.101.1.160]) by dkim2.fusionio.com (Postfix) with ESMTP id 925FB9A0681 for ; Thu, 19 Sep 2013 17:27:33 -0600 (MDT) Date: Thu, 19 Sep 2013 19:27:32 -0400 From: Josef Bacik To: Peter Hurley CC: Josef Bacik , , , , , Subject: Re: [PATCH 1/2] rwsem: add rwsem_is_contended V2 Message-ID: <20130919232732.GC1615@localhost.localdomain> References: <1379605688-987-1-git-send-email-jbacik@fusionio.com> <523B8157.4050508@hurleysoftware.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" In-Reply-To: <523B8157.4050508@hurleysoftware.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: 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 > >--- > >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