linux-kernel - Re: [patch 2/4] fs/buffer: Move BH_Uptodate

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Date:   Thu, 1 Aug 2019 00:27:08 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jan Kara <jack@...e.cz>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Theodore Tso <tytso@....edu>, Julia Cartwright <julia@...com>,
        Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [patch 2/4] fs/buffer: Move BH_Uptodate_Lock locking into wrapper
 functions

On Wed, 31 Jul 2019, Jan Kara wrote:
> On Tue 30-07-19 13:24:54, Thomas Gleixner wrote:
> > Bit spinlocks are problematic if PREEMPT_RT is enabled, because they
> > disable preemption, which is undesired for latency reasons and breaks when
> > regular spinlocks are taken within the bit_spinlock locked region because
> > regular spinlocks are converted to 'sleeping spinlocks' on RT. So RT
> > replaces the bit spinlocks with regular spinlocks to avoid this problem.
> > 
> > To avoid ifdeffery at the source level, wrap all BH_Uptodate_Lock bitlock
> > operations with inline functions, so the spinlock substitution can be done
> > at one place.
> > 
> > Using regular spinlocks can also be enabled for lock debugging purposes so
> > the lock operations become visible to lockdep.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > Cc: "Theodore Ts'o" <tytso@....edu>
> > Cc: Matthew Wilcox <willy@...radead.org>
> > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > Cc: linux-fsdevel@...r.kernel.org
> 
> Looks good to me. You can add:
> 
> Reviewed-by: Jan Kara <jack@...e.cz>
> 
> BTW, it should be possible to get rid of BH_Uptodate_Lock altogether using
> bio chaining (which was non-existent when this bh code was written) to make
> sure IO completion function gets called only once all bios used to fill in
> / write out the page are done. It would be also more efficient. But I guess
> that's an interesting cleanup project for some other time...

While 'possible cleanup' is something which triggers a certain nerve, that
particular project certainly goes beyond my basic understanding of that
whole fs/block conglomerate. I rather leave that to people who actually
have a clue. :)

Thanks,

	tglx