linux-kernel - Re: [PATCH] locking: type cleanup when accessing fast_read

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Date:	Sat, 23 May 2015 11:23:36 +0200
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Mc Guire <hofrat@...dl.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking: type cleanup when accessing fast_read_ctr

On Wed, 20 May 2015, Oleg Nesterov wrote:

> On 05/19, Nicholas Mc Guire wrote:
> >
> > I assumed it would not matter but did not see a simple way of getting it
> > type clean with unsigned either mainly due to the atomic_t being int and
> > val in update_fast_ctr() being passed as -1.
> 
> Perhaps clear_fast_ctr() should have a comment to explain why it returns
> "int"... even if "unsigned" should work too.
>
Might not be into c99 standard far enough but from reviewing 6.5/J.2
I do not see this argument here.

The "well defined modulo 2**n" behavior for unsigned int can be
found stated in a few places - but not in the c99 standard for 
arithmetic overflow.

The "well defined overflow behavior" as far as I understand c99, 
*only* applies to left shift operations when overflowing - see 6.5.7 "
Bitwise shift operators" -> Semantics -> 4) -  further for the counter
problem this well defined behavior is of little help as the sum would 
be wrong in both cases.

I still do not see the point in the implicit/automatic type conversion
here and why that should be an advantage - could somone point me to 
the right c99 clauses ?

thx!
hofrat
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