linux-kernel - Re: [PATCH 1/2] ftrace: disable preemption on the testing of recursion

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Message-ID: <74090798-7d93-0713-982c-6f0247118d20@linux.alibaba.com>
Date:   Wed, 13 Oct 2021 09:50:17 +0800
From:   王贇 <yun.wang@...ux.alibaba.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Guo Ren <guoren@...nel.org>, Steven Rostedt <rostedt@...dmis.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>,
        Colin Ian King <colin.king@...onical.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 1/2] ftrace: disable preemption on the testing of
 recursion



On 2021/10/12 下午8:24, Miroslav Benes wrote:
[snip]
>>
>>  	func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
>>  				      stack_node);
>> @@ -120,7 +115,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
>>  	klp_arch_set_pc(fregs, (unsigned long)func->new_func);
>>
>>  unlock:
>> -	preempt_enable_notrace();
>>  	ftrace_test_recursion_unlock(bit);
>>  }
> 
> I don't like this change much. We have preempt_disable there not because 
> of ftrace_test_recursion, but because of RCU. ftrace_test_recursion was 
> added later. Yes, it would work with the change, but it would also hide 
> things which should not be hidden in my opinion.

Not very sure about the backgroup stories, but just found this in
'Documentation/trace/ftrace-uses.rst':

  Note, on success,
  ftrace_test_recursion_trylock() will disable preemption, and the
  ftrace_test_recursion_unlock() will enable it again (if it was previously
  enabled).

Seems like this lock pair was supposed to take care the preemtion itself?

Regards,
Michael Wang

> 
> Miroslav
>