tracing: Fix race with max_tr and changing tracers

There's a race condition between the setting of a new tracer and
the update of the max trace buffers (the swap). When a new tracer
is added, it sets current_trace to nop_trace before disabling
the old tracer. At this moment, if the old tracer uses update_max_tr(),
the update may trigger the warning against !current_trace->use_max-tr,
as nop_trace doesn't have that set.

As update_max_tr() requires that interrupts be disabled, we can
add a check to see if current_trace == nop_trace and bail if it
does. Then when disabling the current_trace, set it to nop_trace
and run synchronize_sched(). This will make sure all calls to
update_max_tr() have completed (it was called with interrupts disabled).

As a clean up, this commit also removes shrinking and recreating
the max_tr buffer if the old and new tracers both have use_max_tr set.
The old way use to always shrink the buffer, and then expand it
for the next tracer. This is a waste of time.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 22 insertions, 7 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a387bd271e71..d2a658349ca1 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -709,10 +709,14 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
                 return;
 
         WARN_ON_ONCE(!irqs_disabled());
-        if (!current_trace->use_max_tr) {
-                WARN_ON_ONCE(1);
+
+        /* If we disabled the tracer, stop now */
+        if (current_trace == &nop_trace)
                 return;
-        }
+
+        if (WARN_ON_ONCE(!current_trace->use_max_tr))
+                return;
+
         arch_spin_lock(&ftrace_max_lock);
 
         tr->buffer = max_tr.buffer;
@@ -3185,6 +3189,7 @@ static int tracing_set_tracer(const char *buf)
         static struct trace_option_dentry *topts;
         struct trace_array *tr = &global_trace;
         struct tracer *t;
+        bool had_max_tr;
         int ret = 0;
 
         mutex_lock(&trace_types_lock);
@@ -3211,7 +3216,19 @@ static int tracing_set_tracer(const char *buf)
         trace_branch_disable();
         if (current_trace && current_trace->reset)
                 current_trace->reset(tr);
-        if (current_trace && current_trace->use_max_tr) {
+
+        had_max_tr = current_trace && current_trace->use_max_tr;
+        current_trace = &nop_trace;
+
+        if (had_max_tr && !t->use_max_tr) {
+                /*
+                 * We need to make sure that the update_max_tr sees that
+                 * current_trace changed to nop_trace to keep it from
+                 * swapping the buffers after we resize it.
+                 * The update_max_tr is called from interrupts disabled
+                 * so a synchronized_sched() is sufficient.
+                 */
+                synchronize_sched();
                 /*
                  * We don't free the ring buffer. instead, resize it because
                  * The max_tr ring buffer has some state (e.g. ring->clock) and
@@ -3222,10 +3239,8 @@ static int tracing_set_tracer(const char *buf)
         }
         destroy_trace_option_files(topts);
 
-        current_trace = &nop_trace;
-
         topts = create_trace_option_files(t);
-        if (t->use_max_tr) {
+        if (t->use_max_tr && !had_max_tr) {
                 /* we need to make per cpu buffer sizes equivalent */
                 ret = resize_buffer_duplicate_size(&max_tr, &global_trace,
                                                    RING_BUFFER_ALL_CPUS);