Re: [PATCH bpf-next v6 3/6] locking/local_lock: Introduce local_trylock_t and local_trylock_irqsave()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

PeterZ, may I summon you.

On 2025-01-28 10:50:33 [-0800], Alexei Starovoitov wrote:
> On Tue, Jan 28, 2025 at 9:21 AM Sebastian Andrzej Siewior
> <bigeasy@linutronix.de> wrote:
> >
> > On 2025-01-23 19:56:52 [-0800], Alexei Starovoitov wrote:
> > > Usage:
> > >
> > > local_lock_t lock;                     // sizeof(lock) == 0 in !RT
> > > local_lock_irqsave(&lock, ...);        // irqsave as before
> > > if (local_trylock_irqsave(&lock, ...)) // compilation error
> > >
> > > local_trylock_t lock;                  // sizeof(lock) == 4 in !RT
> > > local_lock_irqsave(&lock, ...);        // irqsave and active = 1
> > > if (local_trylock_irqsave(&lock, ...)) // if (!active) irqsave
> >
> > so I've been looking at this for a while and I don't like the part where
> > the type is hidden away. It is then casted back. So I tried something
> > with _Generics but then the existing guard implementation complained.
> > Then I asked myself why do we want to hide much of the implementation
> > and not make it obvious.
> 
> Well, the idea of hiding extra field with _Generic is to avoid
> the churn:
> 
> git grep -E 'local_.*lock_irq'|wc -l
> 42

This could be also hidden with a macro defining the general body and
having a place holder for "lock primitive".

> I think the api is clean enough and _Generic part is not exposed
> to users.
> Misuse or accidental usage is not possible either.
> See the point:
> if (local_trylock_irqsave(&lock, ...)) // compilation error
> 
> So imo it's a better tradeoff.
> 
> > is this anywhere near possible to accept?
> 
> Other than churn it's fine.
> I can go with it if you insist,
> but casting and _Generic() I think is cleaner.
> Certainly a bit unusual pattern.
> Could you sleep on it?

The cast there is somehow… We could have BUILD_BUG_ON() to ensure a
stable the layout of the structs… However all this is not my call.

PeterZ, do you have any preferences or an outline what you would like to
see here?

> I can do s/local_trylock_t/localtry_lock_t/.
> That part is trivial.

Sebastian