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

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Message-ID: <20230510085905.GJ4253@hirez.programming.kicks-ass.net>
Date:   Wed, 10 May 2023 10:59:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
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 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.