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>

2 files changed, 15 insertions, 3 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 9541c27c1cf2..5af42f478c06 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -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();
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 456be9063c2d..cf535ccedc86 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -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;
 }