Re: [PATCH] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a regular spinlock_t

From: Jan Kara <jack@suse.cz>
To: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Matthew Wilcox <willy@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Anna-Maria Gleixner <anna-maria@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Julia Cartwright <julia@ni.com>, Jan Kara <jack@suse.cz>,
	Theodore Tso <tytso@mit.edu>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jan Kara <jack@suse.com>, Mark Fasheh <mark@fasheh.com>,
	Joseph Qi <joseph.qi@linux.alibaba.com>,
	Christoph Hellwig <hch@infradead.org>,
	Joel Becker <jlbec@evilplan.org>
Subject: Re: [PATCH] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a regular spinlock_t
Date: Fri, 15 Nov 2019 15:56:38 +0100	[thread overview]
Message-ID: <20191115145638.GA5461@quack2.suse.cz> (raw)
In-Reply-To: <20191011112525.7dksg6ixb5c3hxn5@linutronix.de>

On Fri 11-10-19 13:25:25, Sebastian Siewior wrote:
> On 2019-08-20 20:01:14 [+0200], Thomas Gleixner wrote:
> > On Tue, 20 Aug 2019, Matthew Wilcox wrote:
> > > 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@linutronix.de>
> > > > [bigeasy: remove the wrapper and use always spinlock_t]
> > > 
> > > Uhh ... always grow the buffer_head, even for non-PREEMPT_RT?  Why?
> > 
> > Christoph requested that:
> > 
> >   https://lkml.kernel.org/r/20190802075612.GA20962@infradead.org
> 
> What do we do about this one?

I was thinking about this for quite some time. In the end I think the patch
is almost fine but I'd name the lock b_update_lock and put it just after
b_size element in struct buffer_head to use the hole there. That way we
don't grow struct buffer_head.

With some effort, we could even shrink struct buffer_head from 104 bytes
(on x86_64) to 96 bytes but I don't think that effort is worth it (I'd find
it better use of time to actually work on getting rid of buffer heads
completely).

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR