Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging

From: Thomas Gleixner <tglx@linutronix.de>
To: Christoph Hellwig <hch@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Sebastian Siewior <bigeasy@linutronix.de>,
	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>,
	Matthew Wilcox <willy@infradead.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	Jan Kara <jack@suse.com>, Mark Fasheh <mark@fasheh.com>,
	Joseph Qi <joseph.qi@linux.alibaba.com>,
	Joel Becker <jlbec@evilplan.org>
Subject: Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
Date: Thu, 8 Aug 2019 09:02:47 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1908080858460.2882@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20190806061119.GA17492@infradead.org>

On Mon, 5 Aug 2019, Christoph Hellwig wrote:
> On Fri, Aug 02, 2019 at 11:07:53AM +0200, Thomas Gleixner wrote:
> > Last time I did, there was resistance :)
> 
> Do you have a pointer?  Note that in the buffer head case maybe
> a hash lock based on the page address is even better, as we only
> ever use the lock in the first buffer head of a page anyway..

I need to search my archives, but I'm on a spotty and slow connection right
now. Will do so when back home.

> > What about the page lock?
> > 
> >   mm/slub.c:      bit_spin_lock(PG_locked, &page->flags);
> 
> One caller ouf of a gazillion that spins on the page lock instead of
> sleepign on it like everyone else.  That should not have passed your
> smell test to start with :)

I surely stared at it, but that cannot sleep. It's in the middle of a
preempt and interrupt disabled region and used on architectures which do
not support CMPXCHG_DOUBLE and ALIGNED_STRUCT_PAGE ...

Thanks,

	tglx