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

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Date:   Thu, 8 Aug 2019 00:28:07 -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 Thu, Aug 08, 2019 at 09:02:47AM +0200, Thomas Gleixner wrote:
> > >   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 ...

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.  Does SLUB
work on -rt at all?