linux-kernel - Re: [PATCH v2 1/2] tracing/events: make _

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Date:	Wed, 27 May 2009 09:24:21 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tom Zanussi <tzanussi@...il.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Steven Whitehouse <swhiteho@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] tracing/events: make __string() more general

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 )
)

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. :(
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