linux-kernel - Re: [PATCH] tracing/uprobes: Support ftrace_event

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Date:	Fri, 21 Jun 2013 16:17:20 +0800
From:	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/uprobes: Support ftrace_event_file base multibuffer

On 2013/6/21 0:43, Oleg Nesterov wrote:
> Hi,
> 
> Now that we finished the discussion of the similar code in kprobes,
> let me summarize.
> 
> On 06/14, Oleg Nesterov wrote:
>>
>> On 06/14, zhangwei(Jovi) wrote:
>>>
>>> +		rcu_assign_pointer(tu->files, new);
>>> +		tu->flags |= TP_FLAG_TRACE;
>>> +
>>> +		if (old) {
>>> +			/* Make sure the probe is done with old files */
>>> +			synchronize_sched();
>>> +			kfree(old);
>>> +		}
>>> +	} else
>>> +		tu->flags |= TP_FLAG_PROFILE;
>>
>> So it can set both TP_FLAG_TRACE and TP_FLAG_PROFILE, yes?
>>
>> If yes, this is not right. Until we change the pre-filtering at least.
>> Currently TP_FLAG_TRACE/TP_FLAG_PROFILE are mutually exclusive.
> 
> Yes. So please update this patch so that TP_FLAG_PROFILE can't be
> set if TP_FLAG_TRACE is set and vice versa.
> 
> Once again, this limitation is artificial. But it was always here,
> and we need more changes to remove it. I'll try to do this later.
> (but if you want to play with this code - welcome ;)
> 
> Don't add the mutex, and do not use array-of-pointers (I hope you
> noticed the recent discussion).
> 
> Locking. Oh, OK. Let it be rcu for now (but please do not forget
> that you need rcu_read_lock, uprobe handlers run in the sleepable
> context). This is sub-optimal, but this is just another indication
> that uprobes API is not perfect, we can't use uprobe->register_sem.
> 
> Also. When I was reading trace_kprobes.c I notice the new (and imho
> useful) feature, soft disable/enable. Perhaps you can make a 2nd
> patch which adds the FTRACE_EVENT_FL_SOFT_DISABLED_BIT check?
> 
> Oleg.
> 
Thanks Oleg, I will send v2 patch soon.

.jovi




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