linux-kernel - Re: [PATCH v6 1/2] ftrace: disable preemption when recursion locked

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Date:   Tue, 26 Oct 2021 22:55:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     王贇 <yun.wang@...ux.alibaba.com>
Cc:     Guo Ren <guoren@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Jisheng Zhang <jszhang@...nel.org>, linux-csky@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v6 1/2] ftrace: disable preemption when recursion locked

On Wed, 27 Oct 2021 10:34:13 +0800
王贇 <yun.wang@...ux.alibaba.com> wrote:

> +/*
> + * Preemption will be enabled (if it was previously enabled).
> + */
>  static __always_inline void trace_clear_recursion(int bit)
>  {
> +	WARN_ON_ONCE(bit < 0);

Can you send a v7 without the WARN_ON.

This is an extremely hot path, and this will cause noticeable overhead.

If something were to call this with bit < 0, then it would crash and
burn rather quickly.

-- Steve


> +
> +	preempt_enable_notrace();
>  	barrier();
>  	trace_recursion_clear(bit);
>  }