tracing: kdb: Fix kernel livelock with empty buffers

Currently kdb's ftdump command will livelock by constantly printk'ing
the empty string at KERN_EMERG level if it run when the ftrace system is
not in use. This occurs because trace_empty() never returns false when
the ring buffers are left at the start of a non-consuming read [launched
by ring_buffer_read_start()].

This patch changes the loop exit condition to use the result of
trace_find_next_entry_inc(). Effectively this switches the non-consuming
kdb dumper to follow the approach of the non-consuming userspace
interface [s_next()] rather than the consuming ftrace_dump().

Link: http://lkml.kernel.org/r/1415277716-19419-3-git-send-email-daniel.thompson@linaro.org
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 8faa7ce..b0b1c44 100644 (file)
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -59,19 +59,19 @@ static void ftrace_dump_buf(int skip_lines, long cpu_file)
                ring_buffer_read_start(iter.buffer_iter[cpu_file]);
                tracing_iter_reset(&iter, cpu_file);
        }
-       if (!trace_empty(&iter))
-               trace_find_next_entry_inc(&iter);
-       while (!trace_empty(&iter)) {
+
+       while (trace_find_next_entry_inc(&iter)) {
                if (!cnt)
                        kdb_printf("---------------------------------\n");
                cnt++;
 
-               if (trace_find_next_entry_inc(&iter) != NULL && !skip_lines)
+               if (!skip_lines) {
                        print_trace_line(&iter);
-               if (!skip_lines)
                        trace_printk_seq(&iter.seq);
-               else
+               } else {
                        skip_lines--;
+               }
+
                if (KDB_FLAG(CMD_INTERRUPT))
                        goto out;
        }