linux-kernel - Re: [PATCHv4 01/12] atomic/tty: Fix up atomic abuse in ldsem

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Date:   Tue, 24 Jul 2018 09:15:18 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     mingo@...hat.com, andy.shevchenko@...il.com, arnd@...db.de,
        aryabinin@...tuozzo.com, boqun.feng@...il.com,
        catalin.marinas@....com, dvyukov@...gle.com, glider@...gle.com,
        hpa@...or.com, linux-kernel@...r.kernel.org,
        parri.andrea@...il.com, peter@...leysoftware.com,
        peterz@...radead.org, tglx@...utronix.de, will.deacon@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv4 01/12] atomic/tty: Fix up atomic abuse in ldsem


* 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.
> 
> Cc: Peter Hurley <peter@...leysoftware.com>
> Acked-by: Will Deacon <will.deacon@....com>
> Reported-by: Mark Rutland <mark.rutland@....com>
> Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Ingo Molnar <mingo@...nel.org>
> ---
>  drivers/tty/tty_ldsem.c   | 82 ++++++++++++++++++++---------------------------
>  include/linux/tty_ldisc.h |  4 +--
>  2 files changed, 37 insertions(+), 49 deletions(-)
> 
> Note: Greg has queued this via the in the tty tree for v4.19, which can be seen at: 
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?h=tty-next&id=5fd691afdf929061c391d897fa627822c3b2fd5a

Can this patch be skipped, or do the others depend on it?

Thanks,

	Ingo