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Subject: Re: [PATCH 1/2] ftrace: disable preemption on the testing of
 recursion
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Miroslav Benes <mbenes@suse.cz>, Guo Ren <guoren@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
        Helge Deller <deller@gmx.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>, Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Colin Ian King <colin.king@canonical.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Jisheng Zhang <jszhang@kernel.org>, linux-csky@vger.kernel.org,
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        linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
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From:   =?UTF-8?B?546L6LSH?= <yun.wang@linux.alibaba.com>
Message-ID: <4b4be164-0095-90bc-a193-faa7100558d2@linux.alibaba.com>
Date:   Wed, 13 Oct 2021 10:36:16 +0800
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On 2021/10/13 上午10:27, Steven Rostedt wrote:
> On Wed, 13 Oct 2021 09:50:17 +0800
> 王贇 <yun.wang@linux.alibaba.com> wrote:
> 
>>>> -	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).
> 
> Right that part is to be fixed by what you are adding here.
> 
> The point that Miroslav is complaining about is that the preemption
> disabling is special in this case, and not just from the recursion
> point of view, which is why the comment is still required.

My bad... the title do confusing people, will rewrite it.

Regards,
Michael Wang

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