linux-kernel - Re: [PATCH 15/44] kdbus: Simplify bitwise expression in kdbus_meta_get

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Message-ID: <CANq1E4QQcRO=A11htHrheKPK-fM0gd=34ri3OiDhSDCW0anYjQ@mail.gmail.com>
Date:	Thu, 8 Oct 2015 16:24:30 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Sergei Zviagintsev <sergei@...v.net>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Daniel Mack <daniel@...que.org>,
	Djalal Harouni <tixxdz@...ndz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 15/44] kdbus: Simplify bitwise expression in kdbus_meta_get_mask()

Hi

On Thu, Oct 8, 2015 at 1:31 PM, Sergei Zviagintsev <sergei@...v.net> wrote:
> Replace the expression with more concise and readable equivalent. It can
> be proven by opening parentheses:
>
>     r & ~((p | i) & r) == r & (~(p | i) | ~r) ==
>         (r & ~(p | i)) | (r & ~r) == r & ~(p | i) == r & ~p & ~i

But why? The current code follows the description, and does exactly
the same. It shows that it merges the "provided" and "implied" masks,
and then extracts the flags that are missing compared to the required
mask.

I cannot follow why your code is more obvious?
David

> Signed-off-by: Sergei Zviagintsev <sergei@...v.net>
> ---
>  ipc/kdbus/metadata.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/ipc/kdbus/metadata.c b/ipc/kdbus/metadata.c
> index 788b4d9c7ecb..61215a078359 100644
> --- a/ipc/kdbus/metadata.c
> +++ b/ipc/kdbus/metadata.c
> @@ -1321,7 +1321,7 @@ static u64 kdbus_meta_get_mask(struct pid *prv_pid, u64 prv_mask,
>          * the sender, but still requested by the receiver. If any are left,
>          * perform rather expensive /proc access checks for them.
>          */
> -       missing = req_mask & ~((prv_mask | impl_mask) & req_mask);
> +       missing = req_mask & ~prv_mask & ~impl_mask;
>         if (missing)
>                 proc_mask = kdbus_meta_proc_mask(prv_pid, req_pid, req_cred,
>                                                  missing);
> --
> 1.8.3.1
>
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