linux-kernel - Re: [PATCH v5 1/2] ftrace: disable preemption when recursion locked

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Message-ID: <20211026080117.366137a5@gandalf.local.home>
Date:   Tue, 26 Oct 2021 08:01:17 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     王贇 <yun.wang@...ux.alibaba.com>
Cc:     Miroslav Benes <mbenes@...e.cz>, 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>, 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 v5 1/2] ftrace: disable preemption when recursion locked

On Tue, 26 Oct 2021 17:48:10 +0800
王贇 <yun.wang@...ux.alibaba.com> wrote:

> > The two comments should be updated too since Steven removed the "bit == 0" 
> > trick.  
> 
> Could you please give more hint on how will it be correct?
> 
> I get the point that bit will no longer be 0, there are only -1 or > 0 now
> so trace_test_and_set_recursion() will disable preemption on bit > 0 and
> trace_clear_recursion() will enabled it since it should only be called when
> bit > 0 (I remember we could use a WARN_ON here now :-P).
> 
> >   
> >> @@ -178,7 +187,7 @@ static __always_inline void trace_clear_recursion(int bit)
> >>   * tracing recursed in the same context (normal vs interrupt),
> >>   *
> >>   * Returns: -1 if a recursion happened.
> >> - *           >= 0 if no recursion
> >> + *           > 0 if no recursion.
> >>   */
> >>  static __always_inline int ftrace_test_recursion_trylock(unsigned long ip,
> >>  							 unsigned long parent_ip)  
> > 
> > And this change would not be correct now.  
> 
> I thought it will no longer return 0 so I change it to > 0, isn't that correct?

No it is not. I removed the bit + 1 return value, which means it returns the
actual bit now. Which is 0 or more.

-- Steve