linux-kernel - Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*

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Message-ID: <CAHk-=whStLWvUzmz3SmQxy1PPyNDjf1O-7z1mq5=WZ-+EABa7w@mail.gmail.com>
Date:   Fri, 10 Dec 2021 09:27:48 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marco Elver <elver@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme

On Fri, Dec 10, 2021 at 8:53 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> That said - it may not matter - I'm not sure a plain "dec" is even a
> valid operation on a ref in the first place. How could you ever
> validly decrement a ref without checking for it being the last entry?

I should have checked the users - it seems to be a pattern at least in
networking where people have extra references and do

        refcount_dec(&skb->users);
        dev_kfree_skb_any(skb);

because there's no way to tell dev_kfree_skb*() to decrement more than once.

So I guess it's all good, but yes, I still think you can just do "lock
dec .. js" for this operation.

             Linus