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

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Date:   Wed, 13 Oct 2021 09:52:16 +0800
From:   王贇 <yun.wang@...ux.alibaba.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Miroslav Benes <mbenes@...e.cz>
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>, 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8:29, Steven Rostedt wrote:
> On Tue, 12 Oct 2021 14:24:43 +0200 (CEST)
> Miroslav Benes <mbenes@...e.cz> wrote:
> 
>>> +++ b/kernel/livepatch/patch.c
>>> @@ -52,11 +52,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
>>>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>>>  	if (WARN_ON_ONCE(bit < 0))
>>>  		return;
>>> -	/*
>>> -	 * A variant of synchronize_rcu() is used to allow patching functions
>>> -	 * where RCU is not watching, see klp_synchronize_transition().
>>> -	 */
>>> -	preempt_disable_notrace();
>>>
>>>  	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.
> 
> Agreed, but I believe the change is fine, but requires a nice comment to
> explain what you said above.
> 
> Thus, before the "ftrace_test_recursion_trylock()" we need:
> 
> 	/*
> 	 * The ftrace_test_recursion_trylock() will disable preemption,
> 	 * which is required for the variant of synchronize_rcu() that is
> 	 * used to allow patching functions where RCU is not watching.
> 	 * See klp_synchronize_transition() for more details.
> 	 */

Will be in v2 too :-)

Regards,
Michael Wang

> 
> -- Steve
>