linux-kernel - Re: [PATCH] block: switch to atomic

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Date:   Tue, 7 Dec 2021 15:23:02 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: switch to atomic_t for request references

On Tue, Dec 7, 2021 at 12:28 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Argh.. __atomic_add_fetch() != __atomic_fetch_add(); much confusion for
> GCC having both. With the right primitive it becomes:
>
>         movl    $1, %eax
>         lock xaddl      %eax, (%rdi)
>         testl   %eax, %eax
>         je      .L5
>         js      .L6
>
> Which makes a whole lot more sense.

Note that the above misses the case where the old value was MAX_INT
and the result now became negative.

That isn't a _problem_, of course. I think it's fine. But if you cared
about it, you'd have to do something like

>         movl    $1, %eax
>         lock xaddl      %eax, (%rdi)
>         jl      .L6
>         testl   %eax, %eax
>         je      .L5

instead (I might have gotten that "jl" wrong, needs more testing.

But if you don't care about the MAX_INT overflow and make the overflow
boundary be the next increment, then just make it be one error case:

>         movl    $1, %eax
>         lock xaddl      %eax, (%rdi)
>         testl   %eax, %eax
>         jle      .L5

and then (if you absolutely have to distinguish them) you can test eax
again in the slow path.

                     Linus