linux-kernel - Re: [PATCH v2 1/3] jump_label: Add static_key_fast

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Message-ID: <Y2Q1laQoklgDtVg9@hirez.programming.kicks-ass.net>
Date:   Thu, 3 Nov 2022 22:41:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     linux-kernel@...r.kernel.org, David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Bob Gilligan <gilligan@...sta.com>,
        "David S. Miller" <davem@...emloft.net>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Francesco Ruggeri <fruggeri@...sta.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Salam Noureddine <noureddine@...sta.com>,
        netdev@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Jason Baron <jbaron@...mai.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 1/3] jump_label: Add static_key_fast_inc()

On Thu, Nov 03, 2022 at 09:25:22PM +0000, Dmitry Safonov wrote:
> A helper to add another user for an existing enabled static key.

Utter lack of clue what for.

> +/***
> + * static_key_fast_inc - adds a user for a static key
> + * @key: static key that must be already enabled
> + *
> + * The caller must make sure that the static key can't get disabled while
> + * in this function. It doesn't patch jump labels, only adds a user to
> + * an already enabled static key.
> + */
> +static inline void static_key_fast_inc(struct static_key *key)
> +{
> +	STATIC_KEY_CHECK_USE(key);
> +	WARN_ON_ONCE(atomic_read(&key->enabled) < 1);
> +	atomic_inc(&key->enabled);
> +}

And no, that's racy as heck. We have things like atomic_inc_not_zero(),
I suggest looking into it.