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>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a387bd271e711484670b855d0fc5deac643f4743..d2a658349ca16a7ce76087445152220e3a312042 100644 (file)
--- 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);