Re: [PATCH 03/32] locking/lockdep: lockdep_set_no_check_recursion()

Kent Overstreet
2023-05-11T09:32:42Z

On Thu, May 11, 2023 at 10:25:44AM +0200, Peter Zijlstra wrote:
> 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).

Thanks for the history lesson :)