linux-kernel - Re: [PATCH v5 1/2] ftrace: disable preemption when recursion locked

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Date:   Tue, 26 Oct 2021 17:48:10 +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>,
        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 v5 1/2] ftrace: disable preemption when recursion locked

Hi, Miroslav

On 2021/10/26 下午5:35, Miroslav Benes wrote:
> Hi,
> 
>> diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
>> index abe1a50..2bc1522 100644
>> --- a/include/linux/trace_recursion.h
>> +++ b/include/linux/trace_recursion.h
>> @@ -135,6 +135,9 @@ static __always_inline int trace_get_context_bit(void)
>>  # define do_ftrace_record_recursion(ip, pip)	do { } while (0)
>>  #endif
>>
>> +/*
>> + * Preemption is promised to be disabled when return bit > 0.
>> + */
>>  static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsigned long pip,
>>  							int start)
>>  {
>> @@ -162,11 +165,17 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
>>  	current->trace_recursion = val;
>>  	barrier();
>>
>> +	preempt_disable_notrace();
>> +
>>  	return bit;
>>  }
>>
>> +/*
>> + * Preemption will be enabled (if it was previously enabled).
>> + */
>>  static __always_inline void trace_clear_recursion(int bit)
>>  {
>> +	preempt_enable_notrace();
>>  	barrier();
>>  	trace_recursion_clear(bit);
>>  }
> 
> The two comments should be updated too since Steven removed the "bit == 0" 
> trick.

Could you please give more hint on how will it be correct?

I get the point that bit will no longer be 0, there are only -1 or > 0 now
so trace_test_and_set_recursion() will disable preemption on bit > 0 and
trace_clear_recursion() will enabled it since it should only be called when
bit > 0 (I remember we could use a WARN_ON here now :-P).

> 
>> @@ -178,7 +187,7 @@ static __always_inline void trace_clear_recursion(int bit)
>>   * tracing recursed in the same context (normal vs interrupt),
>>   *
>>   * Returns: -1 if a recursion happened.
>> - *           >= 0 if no recursion
>> + *           > 0 if no recursion.
>>   */
>>  static __always_inline int ftrace_test_recursion_trylock(unsigned long ip,
>>  							 unsigned long parent_ip)
> 
> And this change would not be correct now.

I thought it will no longer return 0 so I change it to > 0, isn't that correct?

Regards,
Michael Wang

> 
> Regards
> Miroslav
>