linux-kernel - Re: [PATCH] ftrace: fix race between ftrace and insmod

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Date:	Thu, 17 Dec 2015 14:48:52 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	"Qiu, PeiyangX" <peiyangx.qiu@...el.com>,
	linux-kernel@...r.kernel.org, mingo@...hat.com,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] ftrace: fix race between ftrace and insmod

On 2015/12/16 22:28, Steven Rostedt wrote:
> On Wed, 16 Dec 2015 18:28:35 +0800
> "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com> wrote:
>
>>> +	/*
>>> +	 * If the tracing is enabled, go ahead and enable the record.
>>> +	 *
>>> +	 * The reason not to enable the record immediatelly is the
>>> +	 * inherent check of ftrace_make_nop/ftrace_make_call for
>>> +	 * correct previous instructions.  Making first the NOP
>>> +	 * conversion puts the module to the correct state, thus
>>> +	 * passing the ftrace_make_call check.
>>> +	 *
>>> +	 * We also delay this to after the module code already set the
>>> +	 * text to read-only, as we now need to set it back to read-write
>>> +	 * so that we can modify the text.
>>> +	 */
>>> +	if (ftrace_start_up)
>>> +		ftrace_arch_code_modify_prepare();
>>> +
>>> +	do_for_each_ftrace_rec(pg, rec) {
>>> +		int cnt;
>>> +		/*
>>> +		 * do_for_each_ftrace_rec() is a double loop.
>>> +		 * module text shares the pg. If a record is
>>> +		 * not part of this module, then skip this pg,
>>> +		 * which the "break" will do.
>>> +		 */
>>> +		if (!within_module_core(rec->ip, mod))
>>> +			break;
>>> +
>>> +		cnt = 0;
>>> +
>>> +		/*
>>> +		 * When adding a module, we need to check if tracers are
>>> +		 * currently enabled and if they are, and can trace this record,
>>> +		 * we need to enable the module functions as well as update the
>>> +		 * reference counts for those function records.
>>> +		 */
>>> +		if (ftrace_start_up)
>>> +			cnt += referenced_filters(rec);
>>> +
>>> +		/* This clears FTRACE_FL_DISABLED */
>>> +		rec->flags = cnt;
>>> +
>>> +		if (ftrace_start_up && cnt) {
>>> +			int failed = __ftrace_replace_code(rec, 1);  
>> If we choose to call ftrace_module_enable when receiving module notification
>>   MODULE_STATE_COMING, TEXT section of the module is already changed to RO.
> And that's why we call ftrace_arch_code_modify_prepare(). That should
> change all text to RW.

Thanks for the kind pointer. 
We would add codes into your patch based on notifier and send patch to you by
private email.

Yanmin

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