linux-kernel - Re: [PATCHv2 01/11] atomic/tty: Fix up atomic abuse in ldsem

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Message-ID: <20180626193040.GO2494@hirez.programming.kicks-ass.net>
Date:   Tue, 26 Jun 2018 21:30:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Peter Hurley <peter@...leysoftware.com>
Subject: Re: [PATCHv2 01/11] atomic/tty: Fix up atomic abuse in ldsem

On Tue, Jun 26, 2018 at 09:53:18PM +0300, Andy Shevchenko wrote:
> On Mon, Jun 25, 2018 at 1:59 PM, Mark Rutland <mark.rutland@....com> wrote:
> > From: Peter Zijlstra <peterz@...radead.org>
> >
> > Mark found ldsem_cmpxchg() needed an (atomic_long_t *) cast to keep
> > working after making the atomic_long interface type safe.
> >
> > Needing casts is bad form, which made me look at the code. There are no
> > ld_semaphore::count users outside of these functions so there is no
> > reason why it can not be an atomic_long_t in the first place, obviating
> > the need for this cast.
> >
> > That also ensures the loads use atomic_long_read(), which implies (at
> > least) READ_ONCE() in order to guarantee single-copy-atomic loads.
> >
> > When using atomic_long_try_cmpxchg() the ldsem_cmpxchg() wrapper gets
> > very thin (the only difference is not changing *old on success, which
> > most callers don't seem to care about).
> >
> > So rework the whole thing to use atomic_long_t and its accessors
> > directly.
> >
> > While there, fixup all the horrible comment styles.
> 
> 
> > -               ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
> > +               atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
> 
> I suppose it's simple atomic_long_add() here?

Different ordering rules for those two. I didn't look hard enough to see
if that mattered here.