Commit-ID: 34600f0e9c33c9cd48ae87448205f51332b7d5a0
Gitweb: http://git.kernel.org/tip/34600f0e9c33c9cd48ae87448205f51332b7d5a0
Author: Steven Rostedt <srostedt@xxxxxxxxxx>
AuthorDate: Tue, 22 Jan 2013 13:35:11 -0500
Committer: Steven Rostedt <rostedt@xxxxxxxxxxx>
CommitDate: Tue, 22 Jan 2013 23:33:07 -0500
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@xxxxxxxxxxx>
---
kernel/trace/trace.c | 29 ++++++++++++++++++++++-------
1 file changed, 22 insertions(+), 7 deletions(-)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a387bd2..d2a6583 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);
--
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