ftrace: Don't manipulate @pos in t_start()

It's rather confusing that in t_start(), in some cases @pos is
incremented, and in some cases it's decremented and then incremented.

This patch rewrites t_start() in a much more general way.

Thus we fix a bug that if ftrace_filtered == 1, functions have tracer
hooks won't be printed, because the branch is always unreachable:

static void *t_start(...)
{
	...
	if (!p)
		return t_hash_start(m, pos);
	return p;
}

Before:
  # echo 'sys_open' > /mnt/tracing/set_ftrace_filter
  # echo 'sys_write:traceon:4' >> /mnt/tracing/set_ftrace_filter
  sys_open

After:
  # echo 'sys_open' > /mnt/tracing/set_ftrace_filter
  # echo 'sys_write:traceon:4' >> /mnt/tracing/set_ftrace_filter
  sys_open
  sys_write:traceon:count=4

Reviewed-by: Liming Wang <liming.wang@windriver.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4A41874B.4090507@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index cde74b9..dc81020 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1467,8 +1467,6 @@ t_next(struct seq_file *m, void *v, loff_t *pos)
                        iter->pg = iter->pg->next;
                        iter->idx = 0;
                        goto retry;
-               } else {
-                       iter->idx = -1;
                }
        } else {
                rec = &iter->pg->records[iter->idx++];
@@ -1497,6 +1495,7 @@ static void *t_start(struct seq_file *m, loff_t *pos)
 {
        struct ftrace_iterator *iter = m->private;
        void *p = NULL;
+       loff_t l;
 
        mutex_lock(&ftrace_lock);
        /*
@@ -1508,23 +1507,21 @@ static void *t_start(struct seq_file *m, loff_t *pos)
                if (*pos > 0)
                        return t_hash_start(m, pos);
                iter->flags |= FTRACE_ITER_PRINTALL;
-               (*pos)++;
                return iter;
        }
 
        if (iter->flags & FTRACE_ITER_HASH)
                return t_hash_start(m, pos);
 
-       if (*pos > 0) {
-               if (iter->idx < 0)
-                       return p;
-               (*pos)--;
-               iter->idx--;
+       iter->pg = ftrace_pages_start;
+       iter->idx = 0;
+       for (l = 0; l <= *pos; ) {
+               p = t_next(m, p, &l);
+               if (!p)
+                       break;
        }
 
-       p = t_next(m, p, pos);
-
-       if (!p)
+       if (!p && iter->flags & FTRACE_ITER_FILTER)
                return t_hash_start(m, pos);
 
        return p;