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Subject: Re: [PATCH 1/2] ftrace: disable preemption on the testing of
 recursion
To:     Miroslav Benes <mbenes@suse.cz>
Cc:     Guo Ren <guoren@kernel.org>, Steven Rostedt <rostedt@goodmis.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,
        linux-kernel@vger.kernel.org, linux-parisc@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
        live-patching@vger.kernel.org
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Date:   Wed, 13 Oct 2021 09:50:17 +0800
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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
>