From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754005AbcGUTZF (ORCPT ); Thu, 21 Jul 2016 15:25:05 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:34600 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753833AbcGUTZB (ORCPT ); Thu, 21 Jul 2016 15:25:01 -0400 From: Nicolai Stange To: John Stultz Cc: Nicolai Stange , Thomas Gleixner , lkml Subject: Re: [RFC v3 2/3] kernel/time/clockevents: make setting of ->mult and ->mult_mono atomic In-Reply-To: (John Stultz's message of "Thu, 21 Jul 2016 11:16:10 -0700") References: <20160713130017.8202-1-nicstange@gmail.com> <20160713130017.8202-3-nicstange@gmail.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.95 (gnu/linux) Date: Thu, 21 Jul 2016 21:24:57 +0200 Message-ID: <87lh0urdyu.fsf@gmail.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org John Stultz writes: > On Wed, Jul 13, 2016 at 6:00 AM, Nicolai Stange wrote: >> In order to avoid races between setting a struct clock_event_device's >> ->mult_mono in clockevents_update_freq() and yet to be implemented updates >> triggered from the timekeeping core, the setting of ->mult and ->mult_mono >> should be made atomic. >> >> Protect the update in clockevents_update_freq() by locking the >> clockevents_lock spinlock. Frequency updates are expected to be done >> seldomly and thus, taking this subsystem lock should not have any impact >> on performance. >> >> Use a raw_spin_lock_irq_save()/raw_spin_unlock_irq_restore() pair for >> locking/unlocking the clockevents_lock spinlock. >> Purge the now redundant local_irq_save()/local_irq_restore() pair from >> clockevents_update_freq(). Since the call to tick_broadcast_update_freq() >> isn't done with interrupts disabled anymore, its >> raw_spin_lock()/raw_spin_unlock() pair must be converted to >> raw_spin_lock_irq_save()/raw_spin_unlock_irq_restore(). >> >> Signed-off-by: Nicolai Stange >> --- >> kernel/time/clockevents.c | 7 ++++--- >> kernel/time/tick-broadcast.c | 5 +++-- >> 2 files changed, 7 insertions(+), 5 deletions(-) >> >> diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c >> index ba7fea4..ec01375 100644 >> --- a/kernel/time/clockevents.c >> +++ b/kernel/time/clockevents.c >> @@ -589,11 +589,12 @@ int clockevents_update_freq(struct clock_event_device *dev, u32 freq) >> unsigned long flags; >> int ret; >> >> - local_irq_save(flags); >> ret = tick_broadcast_update_freq(dev, freq); >> - if (ret == -ENODEV) >> + if (ret == -ENODEV) { >> + raw_spin_lock_irqsave(&clockevents_lock, flags); >> ret = __clockevents_update_freq(dev, freq); >> - local_irq_restore(flags); >> + raw_spin_unlock_irqrestore(&clockevents_lock, flags); >> + } >> return ret; >> } >> >> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c >> index f6aae79..9c94c41 100644 >> --- a/kernel/time/tick-broadcast.c >> +++ b/kernel/time/tick-broadcast.c >> @@ -125,11 +125,12 @@ int tick_is_broadcast_device(struct clock_event_device *dev) >> int tick_broadcast_update_freq(struct clock_event_device *dev, u32 freq) >> { >> int ret = -ENODEV; >> + unsigned long flags; >> >> if (tick_is_broadcast_device(dev)) { >> - raw_spin_lock(&tick_broadcast_lock); >> + raw_spin_lock_irqsave(&tick_broadcast_lock, flags); >> ret = __clockevents_update_freq(dev, freq); >> - raw_spin_unlock(&tick_broadcast_lock); >> + raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags); >> } > > > So not necessarily part of your change, but this makes using > tick_broadcast_update_freq() seem strange. > > We call it and if dev is a broadcast_device we call > __clockevents_update_freq(), and if not, it fails and we then just > call __clockevents_update_freq() again? Yes, but the first call is made under a different lock than the second one. > > Why bother calling tick_broadcast_update_freq here, and instead just > call __clockevents_update_freq() directly the first time? Thanks, Nicolai