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

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Message-ID: <CAHk-=wh6NyZeMEfw8YDyqCeoev7X319DDF41p0YSJFVNEkVQfw@mail.gmail.com>
Date:   Tue, 7 Dec 2021 08:03:20 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.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 1:34 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Now, it could be GCC generates atrociously bad code simply because it
> doesn't know it can use the flags from the XADD in which case we can
> look at doing an arch asm implementation.

That may help.

This thread started because of alleged performance problems with
refcount_t.  No numbers, and maybe it was wrong, but they have been
seen before, so I wouldn't dismiss the issue.

What I've seen personally is the horrendous "multiple different calls
to overflow functions with different arguments", which makes
__refcount_add() do stupid things and blow up the code. All for
entirely pointless debugging code.

                  Linus