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

Peter Zijlstra
2023-05-11T08:26:04Z

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).