linux-kernel - Re: [PATCH 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily

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Date:	Sun, 4 Nov 2012 16:52:04 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	George Spelvin <linux@...izon.com>
Cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] percpu_rw_semaphore: reimplement to not block the
	readers unnecessarily

On 11/04, George Spelvin wrote:
>
> Grand poo-bah Linus wrote:
> > Now, I doubt you'll find an architecture or C compiler where this will
> > actually ever make a difference, but the fact remains that you
> > shouldn't use signed integers for counters like this. You should use
> > unsigned, and you should rely on the well-defined modulo-2**n
> > semantics.
>
> Actually, this is another C standard undefined case that recent versions of
> GCC exploit for optimization.
                  ^^^^^^^^^^^^

This is another thing,

> For example, the loop:
> 	for (i = 1; i; i++)
> 		/* Code */
> will never terminate!  Feed the following to gcc -O2 and see for yourself:

Yes, because ...

> Notice the lack of test in the "jmp .L2" loop.

Exactly.

But if we have like

	int A, B;

	int sum(void)
	{
		return A + B;
	}

then I doubt there is any architecture (at least supported by linux)
which can generate the different code if you do s/int/unsigned/.


Anyway I agree, unsigned makes more sense, and I changed this patch
accordingly..

Oleg.

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