From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757673Ab0KOOwS (ORCPT ); Mon, 15 Nov 2010 09:52:18 -0500 Received: from mail-wy0-f174.google.com ([74.125.82.174]:58962 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754435Ab0KOOwP (ORCPT ); Mon, 15 Nov 2010 09:52:15 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=Rub5cbeJ/5rbf3FF70MCLI3lUmKGATO2u/pcWq6NsppdGsyDTa4koUn8XIvSbLrRYD Zy/IwtZblmBKAAjblZZBfZla8HMKSvtBlFLgnmyg1wpOADEjFOojJcPfsJnjSiTSTgG4 m1gQkR0bOzFcWm4Get+QSlp+YZQJGMZFQSWWI= Subject: Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don't block new readers From: Eric Dumazet To: Chris Metcalf Cc: linux-kernel@vger.kernel.org, =?ISO-8859-1?Q?Am=E9rico?= Wang , netdev , Cypher Wu In-Reply-To: <201011151425.oAFEPU3W005682@farm-0010.internal.tilera.com> References: <1289489007.17691.1310.camel@edumazet-laptop> <4CDF1945.8090101@tilera.com> <201011151425.oAFEPU3W005682@farm-0010.internal.tilera.com> Content-Type: text/plain; charset="UTF-8" Date: Mon, 15 Nov 2010 15:52:10 +0100 Message-ID: <1289832730.2607.87.camel@edumazet-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le lundi 15 novembre 2010 à 09:18 -0500, Chris Metcalf a écrit : > This avoids a deadlock in the IGMP code where one core gets a read > lock, another core starts trying to get a write lock (thus blocking > new readers), and then the first core tries to recursively re-acquire > the read lock. > > We still try to preserve some degree of balance by giving priority > to additional write lockers that come along while the lock is held > for write, so they can all complete quickly and return the lock to > the readers. > > Signed-off-by: Chris Metcalf > --- > This should apply relatively cleanly to 2.6.26.7 source code too. > > arch/tile/lib/spinlock_32.c | 29 ++++++++++++++++++----------- > 1 files changed, 18 insertions(+), 11 deletions(-) > > diff --git a/arch/tile/lib/spinlock_32.c b/arch/tile/lib/spinlock_32.c > index 485e24d..5cd1c40 100644 > --- a/arch/tile/lib/spinlock_32.c > +++ b/arch/tile/lib/spinlock_32.c > @@ -167,23 +167,30 @@ void arch_write_lock_slow(arch_rwlock_t *rwlock, u32 val) > * when we compare them. > */ > u32 my_ticket_; > + u32 iterations = 0; > > - /* Take out the next ticket; this will also stop would-be readers. */ > - if (val & 1) > - val = get_rwlock(rwlock); > - rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT); > + /* > + * Wait until there are no readers, then bump up the next > + * field and capture the ticket value. > + */ > + for (;;) { > + if (!(val & 1)) { > + if ((val >> RD_COUNT_SHIFT) == 0) > + break; > + rwlock->lock = val; > + } > + delay_backoff(iterations++); Are you sure a writer should have a growing delay_backoff() ? It seems to me this only allow new readers to come (so adding more unfairness to the rwlock, that already favor readers against writer[s]) Maybe allow one cpu to spin, and eventually other 'writers' be queued ? > + val = __insn_tns((int *)&rwlock->lock); > + } > > - /* Extract my ticket value from the original word. */ > + /* Take out the next ticket and extract my ticket value. */ > + rwlock->lock = __insn_addb(val, 1 << WR_NEXT_SHIFT); > my_ticket_ = val >> WR_NEXT_SHIFT; > > - /* > - * Wait until the "current" field matches our ticket, and > - * there are no remaining readers. > - */ > + /* Wait until the "current" field matches our ticket. */ > for (;;) { > u32 curr_ = val >> WR_CURR_SHIFT; > - u32 readers = val >> RD_COUNT_SHIFT; > - u32 delta = ((my_ticket_ - curr_) & WR_MASK) + !!readers; > + u32 delta = ((my_ticket_ - curr_) & WR_MASK); > if (likely(delta == 0)) > break; >