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Subject: Re: [RESEND PATCH v2 1/2] ftrace: disable preemption between
 ftrace_test_recursion_trylock/unlock()
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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 <alpine.LSU.2.21.2110130948120.5647@pobox.suse.cz>
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Message-ID: <d5fbd49a-55c5-a9f5-6600-707c8d749312@linux.alibaba.com>
Date:   Wed, 13 Oct 2021 16:11:10 +0800
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On 2021/10/13 下午3:55, Miroslav Benes wrote:
>> diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
>> index a9f9c57..101e1fb 100644
>> --- a/include/linux/trace_recursion.h
>> +++ b/include/linux/trace_recursion.h
>> @@ -208,13 +208,29 @@ static __always_inline void trace_clear_recursion(int bit)
>>   * Use this for ftrace callbacks. This will detect if the function
>>   * tracing recursed in the same context (normal vs interrupt),
>>   *
>> + * 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.
>> + *
> 
> I think that you misunderstood. Steven proposed to put the comment before 
> ftrace_test_recursion_trylock() call site in klp_ftrace_handler().

Oh, I see... thanks for pointing out :-)

> 
>>   * Returns: -1 if a recursion happened.
[snip]
>>  }
> 
> Side note... the comment will eventually conflict with peterz's 
> https://lore.kernel.org/all/20210929152429.125997206@infradead.org/.

Steven, would you like to share your opinion on this patch?

If klp_synchronize_transition() will be removed anyway, the comments
will be meaningless and we can just drop it :-P

Regards,
Michael Wang


> 
> Miroslav
>