linux-kernel - Re: [PATCH v3 1/2] ftrace: disable preemption between ftrace_test_recursion

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Message-ID: <a5bae9a5-9ab7-2660-caa6-facecb1dba32@linux.alibaba.com>
Date:   Fri, 15 Oct 2021 12:45:27 +0800
From:   王贇 <yun.wang@...ux.alibaba.com>
To:     Petr Mladek <pmladek@...e.com>
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>,
        Miroslav Benes <mbenes@...e.cz>,
        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 v3 1/2] ftrace: disable preemption between
 ftrace_test_recursion_trylock/unlock()



On 2021/10/15 上午11:13, 王贇 wrote:
[snip]
>>  # define do_ftrace_record_recursion(ip, pip)	do { } while (0)
>>  #endif
>>  
>> +/*
>> + * trace_test_and_set_recursion() is called on several layers
>> + * of the ftrace code when handling the same ftrace entry.
>> + * These calls might be nested/recursive.
>> + *
>> + * It uses TRACE_LIST_*BITs to distinguish between this
>> + * internal recursion and recursion caused by calling
>> + * the traced function by the ftrace code.
>> + *
>> + * Returns: > 0 when no recursion
>> + *          0 when called recursively internally (safe)
> 
> The 0 can also happened when ftrace handler recursively called trylock()
> under the same context, or not?
> 

Never mind... you're right about this.

Regards,
Michael Wang

> Regards,
> Michael Wang
> 
>> + *	    -1 when the traced function was called recursively from
>> + *             the ftrace handler (unsafe)
>> + */
>>  static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsigned long pip,
>>  							int start, int max)
>>  {
>>  	unsigned int val = READ_ONCE(current->trace_recursion);
>>  	int bit;
>>  
>> -	/* A previous recursion check was made */
>> +	/* Called recursively internally by different ftrace code layers? */
>>  	if ((val & TRACE_CONTEXT_MASK) > max)
>>  		return 0;
> 
>>  
>>