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

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Message-ID: <20190806061119.GA17492@infradead.org>
Date:   Mon, 5 Aug 2019 23:11:19 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 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..

> 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 :)