linux-kernel - Re: [PATCH] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a regular spinlock

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Message-ID: <20190820171721.GA4949@bombadil.infradead.org>
Date:   Tue, 20 Aug 2019 10:17:21 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Sebastian Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Julia Cartwright <julia@...com>, Jan Kara <jack@...e.cz>,
        Theodore Tso <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.com>, Mark Fasheh <mark@...heh.com>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Christoph Hellwig <hch@...radead.org>,
        Joel Becker <jlbec@...lplan.org>
Subject: Re: [PATCH] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a regular
 spinlock_t

On Tue, Aug 20, 2019 at 07:08:18PM +0200, Sebastian Siewior wrote:
> Bit spinlocks are problematic if PREEMPT_RT is enabled, because 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. So RT
> replaces the bit spinlocks with regular spinlocks to avoid this problem.
> Bit spinlocks are also not covered by lock debugging, e.g. lockdep.
> 
> Substitute the BH_Uptodate_Lock bit spinlock with a regular spinlock.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> [bigeasy: remove the wrapper and use always spinlock_t]

Uhh ... always grow the buffer_head, even for non-PREEMPT_RT?  Why?