linux-kernel - Re: [PATCH 03/32] locking/lockdep: lockdep_set_no_check

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Date:   Tue, 9 May 2023 16:18:59 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 03/32] locking/lockdep: lockdep_set_no_check_recursion()

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.