linux-kernel - Re: shared/298 lockdep splat?

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Date:   Tue, 26 Sep 2017 16:04:21 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Dave Chinner <david@...morbit.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        xfs <linux-xfs@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: shared/298 lockdep splat?

On Tue, Sep 26, 2017 at 01:51:49PM +1000, Dave Chinner wrote:
> On Thu, Sep 21, 2017 at 05:47:14PM +0900, Byungchul Park wrote:
> > On Thu, Sep 21, 2017 at 08:22:56AM +1000, Dave Chinner wrote:
> > > Peter, this is the sort of false positive I mentioned were likely to
> > > occur without some serious work to annotate the IO stack to prevent
> > > them.  We can nest multiple layers of IO completions and locking in
> > > the IO stack via things like loop and RAID devices.  They can be
> > > nested to arbitrary depths, too (e.g. loop on fs on loop on fs on
> > > dm-raid on n * (loop on fs) on bdev) so this new completion lockdep
> > > checking is going to be a source of false positives until there is
> > > an effective (and simple!) way of providing context based completion
> > > annotations to avoid them...
> > 
> > Hello,
> > 
> > It looks caused by that &ret.event in submit_bio_wait() is initialized
> > with the same class for all layers. I mean that completion variables in
> > different layers should be initialized with different classes, as you do
> > for typical locks in xfs.
> 
> Except that submit_bio_wait() is generic block layer functionality
> and can be used by anyone. Whatever solution you decide on, it has
> to be generic. And keep in mind that any code that submits a bio
> themselves and waits on a completion event from the bio is going to
> have to do their own annotations, which makes this a real PITA.

Right. Agree. Let me think it more. As you said, it should be generic.

> > I am not sure if I understand how xfs works correctly. Right? If yes,
> > how can we distinguish between independent 'bio's in submit_bio_wait()?
> > You or I can make it work with the answer. No?
> 
> Has nothing to do with XFS - it has no clue where it sits in the
> block device stack and has no business screwing with bio internals
> and stack layering to handle issues with stacked block devices....

Ok. Thank you for replying.

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com