From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756785Ab0EXUto (ORCPT ); Mon, 24 May 2010 16:49:44 -0400 Received: from fifo99.com ([67.223.236.141]:37428 "EHLO fifo99.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755617Ab0EXUtn (ORCPT ); Mon, 24 May 2010 16:49:43 -0400 Subject: Re: [PATCH 04/11] rwsem: lighter active count checks when waking up readers From: Daniel Walker To: Michel Lespinasse Cc: Linus Torvalds , David Howells , Ingo Molnar , Thomas Gleixner , LKML , Andrew Morton , Mike Waychison , Suleiman Souhlal , Ying Han In-Reply-To: <1274733081-4623-5-git-send-email-walken@google.com> References: <1274733081-4623-1-git-send-email-walken@google.com> <1274733081-4623-5-git-send-email-walken@google.com> Content-Type: text/plain; charset="UTF-8" Date: Mon, 24 May 2010 13:49:16 -0700 Message-ID: <1274734156.11372.50.camel@c-dwalke-linux.qualcomm.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2010-05-24 at 13:31 -0700, Michel Lespinasse wrote: > In __rwsem_do_wake(), we can skip the active count check unless we come > there from up_xxxx(). Also when checking the active count, it is not > actually necessary to increment it; this allows us to get rid of the > read side undo code and simplify the calculation of the final rwsem count > adjustment once we've counted the reader threads to wake. > > The basic observation is the following. When there are waiter threads > on a rwsem and the spinlock is held, other threads can only increment the > active count by trying to grab the rwsem in down_xxxx(). However down_xxxx() > will notice there are waiter threads and take the down_failed path, > blocking to acquire the spinlock on the way there. Therefore, a thread > observing an active count of zero with waiters queued and the spinlock held, > is protected against other threads acquiring the rwsem until it wakes the > last waiter or releases the spinlock. > > Signed-off-by: Michel Lespinasse > --- > lib/rwsem.c | 57 ++++++++++++++++++++++++++++++++------------------------- > 1 files changed, 32 insertions(+), 25 deletions(-) > > diff --git a/lib/rwsem.c b/lib/rwsem.c > index 917fd94..94f2d7a 100644 > --- a/lib/rwsem.c > +++ b/lib/rwsem.c > @@ -36,6 +36,14 @@ struct rwsem_waiter { > #define RWSEM_WAITING_FOR_WRITE 0x00000002 > }; > > +/* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and > + * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held > + * since the rwsem value was observed. > + */ > +#define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */ > +#define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */ > +#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */ > + > /* > * handle the lock release when processes blocked on it that can now run > * - if we come here from up_xxxx(), then: > @@ -46,8 +54,8 @@ struct rwsem_waiter { > * - woken process blocks are discarded from the list after having task zeroed > * - writers are only woken if downgrading is false > */ > -static inline struct rw_semaphore * > -__rwsem_do_wake(struct rw_semaphore *sem, int downgrading) > +static struct rw_semaphore * > +__rwsem_do_wake(struct rw_semaphore *sem, int wake_type) You could convert the "wake_type" into an enum along with the defines above. Daniel