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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 7 Dec 2021 08:13:38 -0800
Message-ID: <CAHk-=wjtvUDbbcXw0iqAPn3dmZK+RnqVMFrU9i53HzvPtWx_vw@mail.gmail.com>
Subject: Re: [PATCH] block: switch to atomic_t for request references
To:     Peter Zijlstra <peterz@infradead.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>
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On Tue, Dec 7, 2021 at 5:28 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> +#define refcount_dec_and_test refcount_dec_and_test
> +static inline bool refcount_dec_and_test(refcount_t *r)
> +{
> +       asm_volatile_goto (LOCK_PREFIX "decl %[var]\n\t"
> +                          "jz %l[cc_zero]\n\t"
> +                          "jl %l[cc_error]"
> +                          : : [var] "m" (r->refs.counter)
> +                          : "memory"
> +                          : cc_zero, cc_error);
> +       return false;
> +
> +cc_zero:
> +       return true;
> +
> +cc_error:
> +       refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
> +       return false;
> +}

Please don't add broken arch-specific helpers that are useless for
anything else than the broken refcount_t use.

Make it return three return values, call it atomic_dec_and_test_ref()
or something like that, and now people can use it without having to
use a refcount_t.

Nobody really wants two different error cases anyway. The fact that
refcount_warn_saturate() has different cases is only an annoyance.

             Linus