Commit ee5e51f5 authored by Jiri Olsa's avatar Jiri Olsa Committed by Steven Rostedt
Browse files

tracing: Avoid soft lockup in trace_pipe

running following commands:

  # enable the binary option
  echo 1 > ./options/bin
  # disable context info option
  echo 0 > ./options/context-info
  # tracing only events
  echo 1 > ./events/enable
  cat trace_pipe

plus forcing system to generate many tracing events,
is causing lockup (in NON preemptive kernels) inside
tracing_read_pipe function.

The issue is also easily reproduced by running ltp stress test.

The reasons are:
 - bin/hex/raw output functions for events are set to
   trace_nop_print function, which prints nothing and
   returns TRACE_TYPE_HANDLED value
 - LOST EVENT trace do not handle trace_seq overflow

These reasons force the while loop in tracing_read_pipe
function never to break.

The attached patch fixies handling of lost event trace, and
changes trace_nop_print to print minimal info, which is needed
for the correct tracing_read_pipe processing.

v2 changes:
 - omit the cond_resched changes by trace_nop_print changes
 - WARN changed to WARN_ONCE and added info to be able
   to find out the culprit

v3 changes:
 - make more accurate patch comment
Signed-off-by: default avatarJiri Olsa <>
LKML-Reference: <>
Signed-off-by: default avatarSteven Rostedt <>
parent 1813dc37
......@@ -2013,9 +2013,10 @@ enum print_line_t print_trace_line(struct trace_iterator *iter)
enum print_line_t ret;
if (iter->lost_events)
trace_seq_printf(&iter->seq, "CPU:%d [LOST %lu EVENTS]\n",
iter->cpu, iter->lost_events);
if (iter->lost_events &&
!trace_seq_printf(&iter->seq, "CPU:%d [LOST %lu EVENTS]\n",
iter->cpu, iter->lost_events))
if (iter->trace && iter->trace->print_line) {
ret = iter->trace->print_line(iter);
......@@ -3229,6 +3230,14 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
if (iter->seq.len >= cnt)
* Setting the full flag means we reached the trace_seq buffer
* size and we should leave by partial output condition above.
* One of the trace_seq_* functions is not used properly.
WARN_ONCE(iter->seq.full, "full flag set for trace type %d",
......@@ -830,6 +830,9 @@ EXPORT_SYMBOL_GPL(unregister_ftrace_event);
enum print_line_t trace_nop_print(struct trace_iterator *iter, int flags,
struct trace_event *event)
if (!trace_seq_printf(&iter->seq, "type: %d\n", iter->ent->type))
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