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SubjectRe: [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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