tracing: Rework __assign_str() and __string() to not duplicate getting the string

The TRACE_EVENT() macro handles dynamic strings by having:

TP_PROTO(struct some_struct *s),
TP_ARGS(s),
TP_STRUCT__entry(
__string(my_string, s->string)
),
TP_fast_assign(
__assign_str(my_string, s->string);
)
TP_printk("%s", __get_str(my_string))

There's even some code that may call a function helper to find the
s->string value. The problem with the above is that the work to get the
s->string is done twice. Once at the __string() and again in the
__assign_str().

But the __string() uses dynamic_array() which has a helper structure that
is created holding the offsets and length of the string fields. Instead of
finding the string twice, just save it off in another field from that
helper structure, and have __assign_str() use that instead.

Note, this also means that the second parameter of __assign_str() isn't
even used anymore, and may be removed in the future.

Link: https://lore.kernel.org/linux-trace-kernel/[e-mail]

Cc: Masami Hiramatsu <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Ville Syrjälä <[e-mail]>
Cc: Rodrigo Vivi <[e-mail]>
Cc: Chuck Lever <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

include/trace/stages/stage2_data_offsets.h

···
 #define __array(type, item, len)
 
 #undef __dynamic_array
-#define __dynamic_array(type, item, len)	u32 item;
+#define __dynamic_array(type, item, len)	u32 item; const void *item##_ptr_;
 
 #undef __string
 #define __string(item, src) __dynamic_array(char, item, -1)
···
 #define __sockaddr(field, len) __dynamic_array(u8, field, len)
 
 #undef __rel_dynamic_array
-#define __rel_dynamic_array(type, item, len)	u32 item;
+#define __rel_dynamic_array(type, item, len)	u32 item; const void *item##_ptr_;
 
 #undef __rel_string
 #define __rel_string(item, src) __rel_dynamic_array(char, item, -1)

include/trace/stages/stage5_get_offsets.h

···
 
 #undef __string
 #define __string(item, src) __dynamic_array(char, item,			\
-		    strlen((src) ? (const char *)(src) : "(null)") + 1)
+		    strlen((src) ? (const char *)(src) : "(null)") + 1)	\
+	__data_offsets->item##_ptr_ = src;
 
 #undef __string_len
-#define __string_len(item, src, len) __dynamic_array(char, item, (len) + 1)
+#define __string_len(item, src, len) __dynamic_array(char, item, (len) + 1)\
+	__data_offsets->item##_ptr_ = src;
 
 #undef __vstring
 #define __vstring(item, fmt, ap) __dynamic_array(char, item,		\
···
 	__data_size += __item_length;
 
 #undef __rel_string
-#define __rel_string(item, src) __rel_dynamic_array(char, item,			\
-		    strlen((src) ? (const char *)(src) : "(null)") + 1)
+#define __rel_string(item, src) __rel_dynamic_array(char, item,		\
+		    strlen((src) ? (const char *)(src) : "(null)") + 1)	\
+	__data_offsets->item##_ptr_ = src;
 
 #undef __rel_string_len
-#define __rel_string_len(item, src, len) __rel_dynamic_array(char, item, (len) + 1)
+#define __rel_string_len(item, src, len) __rel_dynamic_array(char, item, (len) + 1)\
+	__data_offsets->item##_ptr_ = src;
+
 /*
  * __bitmask_size_in_bytes_raw is the number of bytes needed to hold
  * num_possible_cpus().

include/trace/stages/stage6_event_callback.h

···
 
 #undef __assign_str
 #define __assign_str(dst, src)						\
-	strcpy(__get_str(dst), (src) ? (const char *)(src) : "(null)");
+	strcpy(__get_str(dst), __data_offsets.dst##_ptr_ ?		\
+	       __data_offsets.dst##_ptr_ : "(null)")
 
 #undef __assign_str_len
 #define __assign_str_len(dst, src, len)					\
 	do {								\
-		memcpy(__get_str(dst), (src), (len));			\
+		memcpy(__get_str(dst), __data_offsets.dst##_ptr_ ?	\
+		       __data_offsets.dst##_ptr_ : "(null)", len);	\
 		__get_str(dst)[len] = '\0';				\
 	} while(0)
 
···
 
 #undef __assign_rel_str
 #define __assign_rel_str(dst, src)					\
-	strcpy(__get_rel_str(dst), (src) ? (const char *)(src) : "(null)");
+	strcpy(__get_rel_str(dst), __data_offsets.dst##_ptr_ ?		\
+	       __data_offsets.dst##_ptr_ : "(null)")
 
 #undef __assign_rel_str_len
 #define __assign_rel_str_len(dst, src, len)				\
 	do {								\
-		memcpy(__get_rel_str(dst), (src), (len));		\
+		memcpy(__get_rel_str(dst), __data_offsets.dst##_ptr_ ?	\
+		       __data_offsets.dst##_ptr_ : "(null)", len);	\
 		__get_rel_str(dst)[len] = '\0';				\
 	} while (0)