perf probe: Use the last field name as the argument name

Set the last field name to the argument name when the argument
is refering a data-structure member.

e.g.
 ./perf probe --add 'vfs_read file->f_mode'
 Add new event:
   probe:vfs_read       (on vfs_read with f_mode=file->f_mode)

 This probe records file->f_mode, but the argument name becomes "f_mode".

This enables perf-trace command to parse trace event format correctly.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20100412171700.3790.72961.stgit@localhost6.localdomain6>
Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-probe.txt

@@ -87,7 +87,7 @@ Each probe argument follows below syntax.
 
  [NAME=]LOCALVAR|$retval|%REG|@SYMBOL
 
-'NAME' specifies the name of this argument (optional). You can use the name of local variable, local data structure member (e.g. var->field, var.field2), or kprobe-tracer argument format (e.g. $retval, %ax, etc).
+'NAME' specifies the name of this argument (optional). You can use the name of local variable, local data structure member (e.g. var->field, var.field2), or kprobe-tracer argument format (e.g. $retval, %ax, etc). Note that the name of this argument will be set as the last member name if you specify a local data structure member (e.g. field2 for 'var->field1.field2'.)
 
 LINE SYNTAX
 -----------

tools/perf/util/probe-event.c

@@ -481,6 +481,10 @@ static void parse_perf_probe_arg(const char *str, struct perf_probe_arg *arg)
 	} while (tmp);
 	(*fieldp)->name = xstrdup(str);
 	pr_debug("%s(%d)\n", (*fieldp)->name, (*fieldp)->ref);
+
+	/* If no name is specified, set the last field name */
+	if (!arg->name)
+		arg->name = xstrdup((*fieldp)->name);
 }
 
 /* Parse perf-probe event command */