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>

1 files changed, 6 insertions, 6 deletions

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 8faa7ce58814..b0b1c44e923a 100644
--- 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;
         }