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

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Message-ID: <20230511082544.GS4253@hirez.programming.kicks-ass.net>
Date:   Thu, 11 May 2023 10:25:44 +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 Wed, May 10, 2023 at 04:38:15PM -0400, Kent Overstreet wrote:
> On Wed, May 10, 2023 at 10:59:05AM +0200, Peter Zijlstra wrote:

> > 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.
> 
> This might work.
> 
> I was confused for a good bit when reading tho code to figure out how
> it works - nest_lock seems to be a pretty bad name, it's really not a
> lock. acquire_ctx?

That's just how ww_mutex uses it, the annotation itself comes from
mm_take_all_locks() where mm->mmap_lock (the lock formerly known as
mmap_sem) is used to serialize multi acquisition of vma locks.

That is, no other code takes multiple vma locks (be it i_mmap_rwsem or
anonvma->root->rwsem) in any order. These locks nest inside mmap_lock
and therefore by holding mmap_lock you serialize the whole thing and can
take them in any order you like.

Perhaps, now, all these many years later another name would've made more
sense, but I don't think it's worth the hassle of the tree-wide rename
(there's a few other users since).