ftrace: Check for empty hash and comment the race with registering probes

The race between adding a function probe and reading the probes that exist
is very subtle. It needs a comment. Also, the issue can also happen if the
probe has has the EMPTY_HASH as its func_hash.

Cc: stable@vger.kernel.org
Fixes: 7b60f3d876156 ("ftrace: Dynamically create the probe ftrace_ops for the trace_array")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 80beed2..6200a6f 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3096,7 +3096,11 @@ t_probe_next(struct seq_file *m, loff_t *pos)
 
        hash = iter->probe->ops.func_hash->filter_hash;
 
-       if (!hash)
+       /*
+        * A probe being registered may temporarily have an empty hash
+        * and it's at the end of the func_probes list.
+        */
+       if (!hash || hash == EMPTY_HASH)
                return NULL;
 
        size = 1 << hash->size_bits;
@@ -4324,6 +4328,10 @@ register_ftrace_function_probe(char *glob, struct trace_array *tr,
 
        mutex_unlock(&ftrace_lock);
 
+       /*
+        * Note, there's a small window here that the func_hash->filter_hash
+        * may be NULL or empty. Need to be carefule when reading the loop.
+        */
        mutex_lock(&probe->ops.func_hash->regex_lock);
 
        orig_hash = &probe->ops.func_hash->filter_hash;