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

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Message-ID: <202112061247.C5CD07E3C@keescook>
Date:   Mon, 6 Dec 2021 12:51:21 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Mon, Dec 06, 2021 at 11:13:20AM -0700, Jens Axboe wrote:
> On 12/6/21 10:35 AM, Linus Torvalds wrote:
> > On Mon, Dec 6, 2021 at 12:31 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >>
> >> Quite; and for something that pretends to be about performance, it also
> >> lacks any actual numbers to back that claim.
> >>
> >> The proposed implementation also doesn't do nearly as much as the
> >> refcount_t one does.
> > 
> > Stop pretending refcoutn_t is that great.
> > 
> > It's horrid. The code it generators is disgusting. It should never
> > have been inlines in the first place, and the design decsisions were
> > questionable to begin with.
> > 
> > There's a reason core stuff (like the page counters) DO NOT USE REFCOUNT_T.
> > 
> > I seriously believe that refcount_t should be used for things like
> > device reference counting or similar issues, and not for _any_ truly
> > core code.

I'd like core code to be safe too, though. :)

> Maybe we just need to embrace it generically, took a quick stab at it
> which is attached. Totally untested...

As long as we have an API that can't end up in a pathological state, I'm
happy. The problem with prior atomic_t use was that it never noticed
when it was entering a condition that could be used to confuse system
state (use-after-free, etc). Depending on people to "use it correctly"
or never make mistakes is not sufficient: we need an API that protects
itself. We have to assume there are, and will continue to be, bugs with
refcounting.

-- 
Kees Cook