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Date: Wed, 27 May 2009 09:59:12 +0800
From: Li Zefan <lizf@cn.fujitsu.com>
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To: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@elte.hu>, Jens Axboe <jens.axboe@oracle.com>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Tom Zanussi <tzanussi@gmail.com>, "Theodore Ts'o" <tytso@mit.edu>,
       Steven Whitehouse <swhiteho@redhat.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Jeff Moyer <jmoyer@redhat.com>, Christoph Hellwig <hch@infradead.org>,
       FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 1/2] tracing/events: make __string() more general
References: <4A1B6A2F.2040703@cn.fujitsu.com> <4A1B6A4C.3070407@cn.fujitsu.com> <alpine.DEB.2.00.0905261655120.26705@gandalf.stny.rr.com> <4A1C9645.7010107@cn.fujitsu.com> <alpine.DEB.2.00.0905262147470.27902@gandalf.stny.rr.com>
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Steven Rostedt wrote:
> On Wed, 27 May 2009, Li Zefan wrote:
> 
>> Steven Rostedt wrote:
>>> On Tue, 26 May 2009, Li Zefan wrote:
>>>
>>>> This changes __string(..., src) to __string(..., len), thus makes it more
>>>> general, and then block TRACE_EVENT() can use it.
>>>>
>>>> Also introduce __fetch_str(), which is used in TP_assign() and returns
>>>> the address of the string, while __get_str() is used in TP_print().
>>>>
>>>> [ Impact: TRACE_EVENT api change ]
>>> What's the reason for the change?
>>>
>>> Is it to handle NULL pointers? If so, lets change the ftrace.h to handle
>>> it instead.
>>>
>> No, not for this purpose.
>>
>> With __string(..., src), we need to have a source string, but sometimes no
>> source string is available, and what we know is how big the string will be.
>>
>> Take block trace events for example, I want to convert an unsigned char array
>> into a string, after this patch, this can be done this way:
>>
>> TP_STRUCT_entry(
>> 	__string( cmd, rq->cmd_len * 3 )
>> )
>>
>> TP_fast_assign(
>> 	construct_str( cmd )
> 
> What does the construct_str do?
> 

Ah, it's pseudo code.. Should be construct_str(__entry->cmd, rq),
this helper function convert rq->cmd (unsigned char[]) to a string of
hex dump.

No source string is available, so I'm not calling assign_str() in
TP_fast_assign.

> 
>> )
>>
>> TP_prink("%s\n", get_str(cmd))
>>
>> Seems Christoph also runs into this problem while doing his XFS tracing.
>> (and NULL str ptr that I'm not suffering)
>>
>>> This is why I like the Impact line really be a "rational" line.
>>>  "TRACE_EVENT api change" is meaningless to me.
>>>
>> So what should this impact be. :(
> 
> That's the point. I don't know the purpose of the patch except that it 
> changes the API. But changing the API is the means to the end. I'm still 
> confused by what the end is.
>