[PATCH 3.17-rc4 v2 2/2] trace: kdb: Fix kernel livelock with empty buffers

From: Daniel Thompson
Date: Wed Sep 24 2014 - 06:33:56 EST

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().

Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
Cc: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/trace/trace_kdb.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 8faa7ce..b0b1c44 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)
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)

- if (trace_find_next_entry_inc(&iter) != NULL && !skip_lines)
+ if (!skip_lines) {
- if (!skip_lines)
- else
+ } else {
+ }
goto out;

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