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>

1 files changed, 8 insertions, 11 deletions

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index cde74b9973b7..dc810208edde 100644
--- 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;