Commit ee5e51f5 authored Mar 25, 2011 by Jiri Olsa Committed by Steven Rostedt Apr 04, 2011

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.
(ftrace_stress_test.sh)

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: Jiri Olsa <jolsa@redhat.com>
LKML-Reference: <20110325110518.GC1922@jolsa.brq.redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

parent 1813dc37

kernel/trace/trace.c

+12 −3

Original line number	Diff line number	Diff line
		@@ -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))
		return TRACE_TYPE_PARTIAL_LINE;

		if (iter->trace && iter->trace->print_line) {
		ret = iter->trace->print_line(iter);
		@@ -3229,6 +3230,14 @@ waitagain:

		if (iter->seq.len >= cnt)
		break;

		/*
		* 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",
		iter->ent->type);
		}
		trace_access_unlock(iter->cpu_file);
		trace_event_read_unlock();

kernel/trace/trace_output.c

+3 −0

Original line number	Diff line number	Diff line
		@@ -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))
		return TRACE_TYPE_PARTIAL_LINE;

		return TRACE_TYPE_HANDLED;
		}

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