From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 94CFBC32753 for ; Wed, 31 Jul 2019 15:49:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 76B00206B8 for ; Wed, 31 Jul 2019 15:49:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730144AbfGaPtF (ORCPT ); Wed, 31 Jul 2019 11:49:05 -0400 Received: from mx2.suse.de ([195.135.220.15]:50746 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726755AbfGaPtF (ORCPT ); Wed, 31 Jul 2019 11:49:05 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id C38D7AF95; Wed, 31 Jul 2019 15:49:02 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 007DA1E3F4D; Wed, 31 Jul 2019 17:48:59 +0200 (CEST) Date: Wed, 31 Jul 2019 17:48:59 +0200 From: Jan Kara To: Thomas Gleixner Cc: LKML , Steven Rostedt , Peter Zijlstra , Matthew Wilcox , Ingo Molnar , Anna-Maria Gleixner , Sebastian Siewior , Theodore Tso , Julia Cartwright , Jan Kara , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, Alexander Viro Subject: Re: [patch 4/4] fs: jbd/jbd2: Substitute BH locks for RT and lock debugging Message-ID: <20190731154859.GI15806@quack2.suse.cz> References: <20190730112452.871257694@linutronix.de> <20190730120321.489374435@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190730120321.489374435@linutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 30-07-19 13:24:56, Thomas Gleixner wrote: > Bit spinlocks are problematic if PREEMPT_RT is enabled. They disable > preemption, which is undesired for latency reasons and breaks when regular > spinlocks are taken within the bit_spinlock locked region because regular > spinlocks are converted to 'sleeping spinlocks' on RT. > > Substitute the BH_State and BH_JournalHead bit spinlocks with regular > spinlock for PREEMPT_RT enabled kernels. Is there a real need for substitution for BH_JournalHead bit spinlock? The critical sections are pretty tiny, all located within fs/jbd2/journal.c. Maybe only the one around __journal_remove_journal_head() would need a bit of refactoring so that journal_free_journal_head() doesn't get called under the bit-spinlock. BH_State lock is definitely worth it. In fact, if you placed the spinlock inside struct journal_head (which is the structure whose members are in fact protected by it), I'd be even fine with just using the spinlock always instead of the bit spinlock. journal_head is pretty big anyway (and there's even 4-byte hole in it for 64-bit archs) and these structures are pretty rare (only for actively changed metadata buffers). Honza > > Bit spinlocks are also not covered by lock debugging, e.g. lockdep. With > the spinlock substitution in place, they can be exposed via > CONFIG_DEBUG_BIT_SPINLOCKS. > > Originally-by: Steven Rostedt > Signed-off-by: Thomas Gleixner > Cc: linux-ext4@vger.kernel.org > Cc: "Theodore Ts'o" > Cc: Matthew Wilcox > Cc: Jan Kara > -- > include/linux/buffer_head.h | 8 ++++++++ > include/linux/jbd2.h | 36 ++++++++++++++++++++++++++++++++++++ > 2 files changed, 44 insertions(+) > > --- a/include/linux/buffer_head.h > +++ b/include/linux/buffer_head.h > @@ -79,6 +79,10 @@ struct buffer_head { > > #if defined(CONFIG_PREEMPT_RT) || defined(CONFIG_DEBUG_BIT_SPINLOCKS) > spinlock_t b_uptodate_lock; > +# if IS_ENABLED(CONFIG_JBD2) > + spinlock_t b_state_lock; > + spinlock_t b_journal_head_lock; > +# endif > #endif > }; > > @@ -101,6 +105,10 @@ bh_uptodate_unlock_irqrestore(struct buf > static inline void buffer_head_init_locks(struct buffer_head *bh) > { > spin_lock_init(&bh->b_uptodate_lock); > +#if IS_ENABLED(CONFIG_JBD2) > + spin_lock_init(&bh->b_state_lock); > + spin_lock_init(&bh->b_journal_head_lock); > +#endif > } > > #else /* PREEMPT_RT || DEBUG_BIT_SPINLOCKS */ > --- a/include/linux/jbd2.h > +++ b/include/linux/jbd2.h > @@ -342,6 +342,40 @@ static inline struct journal_head *bh2jh > return bh->b_private; > } > > +#if defined(CONFIG_PREEMPT_RT) || defined(CONFIG_DEBUG_BIT_SPINLOCKS) > + > +static inline void jbd_lock_bh_state(struct buffer_head *bh) > +{ > + spin_lock(&bh->b_state_lock); > +} > + > +static inline int jbd_trylock_bh_state(struct buffer_head *bh) > +{ > + return spin_trylock(&bh->b_state_lock); > +} > + > +static inline int jbd_is_locked_bh_state(struct buffer_head *bh) > +{ > + return spin_is_locked(&bh->b_state_lock); > +} > + > +static inline void jbd_unlock_bh_state(struct buffer_head *bh) > +{ > + spin_unlock(&bh->b_state_lock); > +} > + > +static inline void jbd_lock_bh_journal_head(struct buffer_head *bh) > +{ > + spin_lock(&bh->b_journal_head_lock); > +} > + > +static inline void jbd_unlock_bh_journal_head(struct buffer_head *bh) > +{ > + spin_unlock(&bh->b_journal_head_lock); > +} > + > +#else /* PREEMPT_RT || DEBUG_BIT_SPINLOCKS */ > + > static inline void jbd_lock_bh_state(struct buffer_head *bh) > { > bit_spin_lock(BH_State, &bh->b_state); > @@ -372,6 +406,8 @@ static inline void jbd_unlock_bh_journal > bit_spin_unlock(BH_JournalHead, &bh->b_state); > } > > +#endif /* !PREEMPT_RT && !DEBUG_BIT_SPINLOCKS */ > + > #define J_ASSERT(assert) BUG_ON(!(assert)) > > #define J_ASSERT_BH(bh, expr) J_ASSERT(expr) > > > -- Jan Kara SUSE Labs, CR