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Date:   Fri, 12 May 2023 16:49:51 -0400
From:   Kent Overstreet <kent.overstreet@linux.dev>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-bcachefs@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH 03/32] locking/lockdep: lockdep_set_no_check_recursion()
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On Wed, May 10, 2023 at 10:59:05AM +0200, Peter Zijlstra wrote:
> On Tue, May 09, 2023 at 04:18:59PM -0400, Kent Overstreet wrote:
> > On Tue, May 09, 2023 at 09:31:47PM +0200, Peter Zijlstra wrote:
> > > On Tue, May 09, 2023 at 12:56:28PM -0400, Kent Overstreet wrote:
> > > > This adds a method to tell lockdep not to check lock ordering within a
> > > > lock class - but to still check lock ordering w.r.t. other lock types.
> > > > 
> > > > This is for bcachefs, where for btree node locks we have our own
> > > > deadlock avoidance strategy w.r.t. other btree node locks (cycle
> > > > detection), but we still want lockdep to check lock ordering w.r.t.
> > > > other lock types.
> > > > 
> > > 
> > > ISTR you had a much nicer version of this where you gave a custom order
> > > function -- what happend to that?
> > 
> > Actually, I spoke too soon; this patch and the other series with the
> > comparison function solve different problems.
> > 
> > For bcachefs btree node locks, we don't have a defined lock ordering at
> > all - we do full runtime cycle detection, so we don't want lockdep
> > checking for self deadlock because we're handling that but we _do_ want
> > lockdep checking lock ordering of btree node locks w.r.t. other locks in
> > the system.
> 
> Have you read the ww_mutex code? If not, please do so, it does similar
> things.
> 
> The way it gets around the self-nesting check is by using the nest_lock
> annotation, the acquire context itself also has a dep_map for this
> purpose.

So, spent some time plumbing this through the six lock code and seeing
how it'd work.

I like it in theory, it's got the right semantics and it would allow for
lockdep to check that we're not taking locks with more than one
btree_trans in the same thread. Unfortunately, we've got code paths that
are meant to be called from contexts that may or may not have a
btree_trans - and this is fine right now, because they just use
trylock(), but having to plumb nest_lock through is going to make a mess
of things.

(The relevant codepaths include shrinker callbacks, where we definitely
can not just init a new btree_trans, and also the btree node write path
which can be kicked off from all sorts of places).

Can we go with lockdep_set_no_check_recursion() for now? It's a pretty
small addition to the lockdep code.