Date:   Fri, 10 Dec 2021 11:57:11 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] block: switch to atomic_t for request references
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On Wed, Dec 08, 2021 at 10:50:10AM -0800, Linus Torvalds wrote:
> On Wed, Dec 8, 2021 at 10:44 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > From testing xadd had different flags from add; I've not yet looked at
> > the SDM to see what it said on the matter.
> 
> That should not be the case. Just checked, and it just says
> 
>   "The CF, PF, AF, SF, ZF, and OF flags are set according to the
> result of the addition, which is stored in the destination operand"
> 
> which shows that I was confused about 'xadd' - I thought it returned
> the old value in the register ("fetch_add"). It doesn't. It returns
> the new one ("add_fetch"). And then 'fetch_add' ends up undoing it by
> doing a sub or whatever.
> 
> So the actual returned value and the flags should match on x86.
> 
> Other architectures have the "return old value" model, which does mean
> that my "different architectures can have different preferences for
> which one to test" argument was right, even if I got xadd wrong.

I think XADD does return old too; SDM states:

"Exchanges the first operand (destination operand) with the second
operand (source operand), then loads the sum of the two values into the
destination operand. The destination operand can be a register or a
memory location; the source operand is a register."

So it first exchanges and then adds. Which is why the flags are set for
add, not exchange.