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

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Date:   Sat, 10 Aug 2019 18:22:20 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        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 Sat, Aug 10, 2019 at 01:18:34AM -0700, Christoph Hellwig wrote:
> On Thu, Aug 08, 2019 at 09:54:03AM +0200, Thomas Gleixner wrote:
> > > I know.  But the problem here is that normally PG_locked is used together 
> > > with wait_on_page_bit_*, but this one instances uses the bit spinlock
> > > helpers.  This is the equivalent of calling spin_lock on a struct mutex
> > > rather than having a mutex_lock_spin helper for this case.
> > 
> > Yes, I know :(
> 
> But this means we should exclude slub from the bit_spin_lock removal.
> It really should use it's own version of it anyhow insted of pretending
> that the page lock is a bit spinlock.

But PG_locked isn't used as a mutex _when the page is allocated by slab_.
Yes, every other user uses PG_locked as a mutex, but I don't see why that
should constrain slub's usage of PG_locked.