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

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Date:	Wed, 27 May 2009 16:12:34 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	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>,
	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

Christoph Hellwig wrote:
> On Wed, May 27, 2009 at 09:43:32AM +0800, Li Zefan wrote:
>> I'm not suffering from NULL str, so I'm not quite sure how you want __string()
>> deal with it.
> 
> The internals of __string is still magic to me, need to finish my
> advance cpp abuse degree first ;-)
> 

It's no longer magic to me. ;)

> But the expected outcome would be that when we get a NULL pointer as
> input we get a NULL pointer as output again.
> 

The following trick should meet your needs (based on this 1/2 patch):

(Not even compiled!)

diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
index d1c0cc5..c5911d0 100644
--- a/include/trace/ftrace.h
+++ b/include/trace/ftrace.h
@@ -117,7 +117,10 @@
 #define TP_printk(fmt, args...) fmt "\n", args
 
 #undef __get_str
-#define __get_str(field)	((char *)__entry + __entry->__str_loc_##field)
+#define __get_str(field)						\
+	(__entry->__str_lock_##field != -1) ?				\
+		((char *)__entry + __entry->__str_loc_##field) :	\
+		NULL
 
 #undef TRACE_EVENT
 #define TRACE_EVENT(call, proto, args, tstruct, assign, print)		\
@@ -412,8 +415,11 @@ static void ftrace_profile_disable_##call(struct ftrace_event_call *call) \
 
 #undef __string
 #define __string(item, len)						 \
-	__str_offsets.item = __str_size +				 \
-			     offsetof(typeof(*entry), __str_data);	 \
+	if (len)							 \
+		__str_offsets.item = __str_size +			 \
+				     offsetof(typeof(*entry), __str_data);\
+	else								 \
+		__str_offsets.item = -1;				 \
 	__str_size += (len) + 1;
 
 #undef __fetch_str

> What I currently do is:
> 
> TRACE_EVENT(xfs_dir2,
> 	TP_PROTO(struct xfs_da_args *args, int i, int j, int count),
> 	TP_ARGS(args, i, j, count),
> 
> 	TP_STRUCT__entry(
> 		__field(xfs_ino_t, ino)
> 		__array(char, name, TRACE_MAXNAMELEN)
> 		__field(int, namelen)
> 		...
> 	),
> 
>         TP_fast_assign(
> 		__entry->ino = args->dp->i_ino;
> 		if (args->namelen)
> 			memcpy(__entry->name, args->name,
> 			       min(args->namelen, TRACE_MAXNAMELEN));

This string is not NULL-terminated?

> 		__entry->namelen = args->namelen;
> 		...
> 	),
> 
> 	TP_printk("ino 0x%lld %pF name %.*s namelen %d hashval 0x%x "
> 		  "inumber 0x%llx op_flags %s i %d j %d count %d",
> 		  __entry->ino,
> 		  (void *)__entry->caller_ip,
> 		  min(__entry->namelen, TRACE_MAXNAMELEN),
> 		  __entry->namelen ? __entry->name : NULL,
> 		  __entry->namelen,
> 		  ...)
> );
> 

Then the above code can be rewrote:

TRACE_EVENT(xfs_dir2,
	TP_PROTO(struct xfs_da_args *args, int i, int j, int count),
	TP_ARGS(args, i, j, count),

	TP_STRUCT__entry(
		__field(xfs_ino_t, ino)
		__string(name, args->namelen)
		__field(int, namelen)
		...
	),

        TP_fast_assign(
		__entry->ino = args->dp->i_ino;
		memcpy(__fetch_str(name), args->name, args->namelen);
		__entry->namelen = args->namelen;
		...
	),

	TP_printk("ino 0x%lld %pF name %.*s namelen %d hashval 0x%x "
		  "inumber 0x%llx op_flags %s i %d j %d count %d",
		  __entry->ino,
		  (void *)__entry->caller_ip,
		  __entry->namelen,
		  __get_str(name),
		  __entry->namelen,
		  ...)
);

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