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

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Date:   Tue, 26 Jun 2018 21:53:18 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        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 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?

Good fix!
Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>

-- 
With Best Regards,
Andy Shevchenko