Re: [PATCH 15/44] kdbus: Simplify bitwise expression in kdbus_meta_get_mask()

[Sergei Zviagintsev]

Hi David,

On Thu, Oct 08, 2015 at 04:24:30PM +0200, David Herrmann wrote:
> Hi
> 
> On Thu, Oct 8, 2015 at 1:31 PM, Sergei Zviagintsev <sergei@xxxxxxxx> 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?

The variant I propose has one to one correspondence to the description,
but is shorter and has no multi levels of parentheses, thus easier to
read, IMO. The comment says "... set of metadata that is not granted
implicitly" (which is ~impl_mask), "... nor by the sender" (~prv_mask),
"... but still requested by the receiver" (req_mask).

We can leave parentheses, i.e. 'req_mask & ~(prv_mask | impl_mask)', but
even in this case the original code does one extra bitwise AND.

> David
> 
> > Signed-off-by: Sergei Zviagintsev <sergei@xxxxxxxx>
> > ---
> >  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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