Re: [PATCH v4 07/35] lockdep: Annotate lockdep assertions for context analysis

On Fri, 12 Dec 2025 at 10:59, Peter Zijlstra <[email protected]> wrote:
>
> On Thu, Dec 11, 2025 at 02:24:57PM +0100, Marco Elver wrote:
>
> > > It is *NOT* (as the clang naming suggests) an assertion of holding the
> > > lock (which is requires_ctx), but rather an annotation that forces the
> > > ctx to be considered held.
> >
> > Noted. I'll add some appropriate wording above the
> > __assumes_ctx_guard() attribute, so this is not lost in the commit
> > logs.
>
> On IRC you stated:
>
> <melver> peterz: 'assume' just forces the compiler to think something is
>   held, whether or not it is then becomes the programmer's problem. we
>   need it in 2 places at least: for the runtime assertions (to help
>   patterns beyond the compiler's static reasoning abilities), and for
>   initialization (so we can access guarded variables right after
>   initialization; nobody should hold the lock yet)
>
> I'm really not much a fan of that init hack either ;-)
>
> Once we get the scope crap working sanely, I would much rather we move
> to something like:
>
>         scoped_guard (spinlock_init, &foo->lock) {
>                 // init foo fields
>         }
>
> or perhaps:
>
>         guard(mutex_init)(&bar->lock);
>         // init until end of current scope
>
> Where this latter form is very similar to the current semantics where
> mutex_init() will implicitly 'leak' the holding of the lock. But the
> former gives more control where we need it.

I like it. It would also more clearly denote where initialization
start+ends if not confined to a dedicated function.