linux-kernel - Re: [RFC 1/2] refcount: introduce generic lockptr funcs

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Message-ID: <CAK-6q+iwqjkuxVqt6hMw3uoq0UsZxvx_x2rdLDtsBSdURpjv-g@mail.gmail.com>
Date:   Mon, 6 Nov 2023 10:12:39 -0500
From:   Alexander Aring <aahringo@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     will@...nel.org, gfs2@...ts.linux.dev, boqun.feng@...il.com,
        mark.rutland@....com, linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/2] refcount: introduce generic lockptr funcs

Hi,

On Mon, Nov 6, 2023 at 6:11 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Nov 03, 2023 at 03:20:08PM -0400, Alexander Aring wrote:
> > Hi,
> >
> > On Fri, Nov 3, 2023 at 2:54 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Fri, Nov 03, 2023 at 12:16:34PM -0400, Alexander Aring wrote:
> > >
> > > > diff --git a/lib/refcount.c b/lib/refcount.c
> > > > index a207a8f22b3c..e28678f0f473 100644
> > > > --- a/lib/refcount.c
> > > > +++ b/lib/refcount.c
> > > > @@ -94,6 +94,34 @@ bool refcount_dec_not_one(refcount_t *r)
> > > >  }
> > > >  EXPORT_SYMBOL(refcount_dec_not_one);
> > > >
> > > > +bool refcount_dec_and_lockptr(refcount_t *r, void (*lock)(void *lockptr),
> > > > +                           void (*unlock)(void *lockptr),  void *lockptr)
> > > > +{
> > > > +     if (refcount_dec_not_one(r))
> > > > +             return false;
> > > > +
> > > > +     lock(lockptr);
> > > > +     if (!refcount_dec_and_test(r)) {
> > > > +             unlock(lockptr);
> > > > +             return false;
> > > > +     }
> > > > +
> > > > +     return true;
> > > > +}
> > > > +EXPORT_SYMBOL(refcount_dec_and_lockptr);
> > >
> > > This is terrible, you're forcing indirect calls on everything.
> > >
> >
> > Okay, I see. How about introducing a macro producing all the code at
> > preprocessor time?
>
> __always_inline should work, then you get constant propagation for the
> function pointer.
>

Thanks, it is always good to learn something new.

> But indeed, perhaps a macro is more convenient vs the irq flags
> argument. You'll then end up with something like:
>
> #define __refcount_dec_and_lock(_ref, _lock, _unlock) \
> ({      bool _ret = false; \
>         if (!refcount_dec_not_one(_ref)) { \
>                 _lock; \
>                 if (!refcount_dec_and_test(_ref)) { \
>                         _unlock; \
>                 } else { \
>                         _ret = true; \
>                 } \
>         } \
>         _ret; \
> })
>
>
> bool refcount_dec_and_spinlock_irqsave(refcount_t *r, spinlock_t *lock,
>                                        unsigned long *flags)
> {
>         return __refcount_dec_and_lock(r, spin_lock_irqsave(*lock, *flags),
>                                           spin_unlock_irqrestore(*lock, *flags));
> }

I was thinking of solving the additional flags parameter with
prototype and argos macros e.g. TRACE_EVENT() is doing it, but this
version looks much better.
I will send a patch and do similar things with _kref_put_lock().

- Alex