linux-ext4 - Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT

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Date:   Thu, 8 Aug 2019 09:02:47 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Christoph Hellwig <hch@...radead.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        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>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        Jan Kara <jack@...e.com>, Mark Fasheh <mark@...heh.com>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Joel Becker <jlbec@...lplan.org>
Subject: Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and
 debugging

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