[PATCH 0/2] tracing: Fix snapshot accounting

[PATCH 0/2] tracing: Fix snapshot accounting

[Steven Rostedt]

The ring buffer mapping test failed after running the ftrace tests.
This was due to some mismatched snapshot accounting that left the
snapshot counter enabled when it was not, which prevents the ring buffer
from being mapped.

Steven Rostedt (Google) (2):
      tracing: Fix snapshot counter going between two tracers that use it
      tracing: Decrement the snapshot if the snapshot trigger fails to register

----
 kernel/trace/trace.c                | 2 +-
 kernel/trace/trace_events_trigger.c | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

[PATCH 1/2] tracing: Fix snapshot counter going between two tracers that use it

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

Running the ftrace selftests caused the ring buffer mapping test to fail.
Investigating, I found that the snapshot counter would be incremented
every time a tracer that uses the snapshot is enabled even if the snapshot
was used by the previous tracer.

That is:

 # cd /sys/kernel/tracing
 # echo wakeup_rt > current_tracer
 # echo wakeup_dl > current_tracer
 # echo nop > current_tracer

would leave the snapshot counter at 1 and not zero. That's because the
enabling of wakeup_dl would increment the counter again but the setting
the tracer to nop would only decrement it once.

Do not arm the snapshot for a tracer if the previous tracer already had it
armed.

Fixes: 16f7e48ffc53a ("tracing: Add snapshot refcount")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b7a870c8ae2a..480201c3b36e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6164,7 +6164,7 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
 		tracing_disarm_snapshot(tr);
 	}
 
-	if (t->use_max_tr) {
+	if (!had_max_tr && t->use_max_tr) {
 		ret = tracing_arm_snapshot_locked(tr);
 		if (ret)
 			goto out;
-- 
2.43.0

[PATCH 2/2] tracing: Decrement the snapshot if the snapshot trigger fails to register

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

Running the ftrace selftests caused the ring buffer mapping test to fail.
Investigating, I found that the snapshot counter would be incremented
every time a snapshot trigger was added, even if that snapshot trigger
failed.

 # cd /sys/kernel/tracing
 # echo "snapshot" > events/sched/sched_process_fork/trigger
 # echo "snapshot" > events/sched/sched_process_fork/trigger
 -bash: echo: write error: File exists

That second one that fails increments the snapshot counter but doesn't
decrement it. It needs to be decremented when the snapshot fails.

Fixes: 16f7e48ffc53a ("tracing: Add snapshot refcount")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace_events_trigger.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index 62e4f58b8671..4bec043c8690 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -1491,7 +1491,10 @@ register_snapshot_trigger(char *glob,
 	if (ret < 0)
 		return ret;
 
-	return register_trigger(glob, data, file);
+	ret = register_trigger(glob, data, file);
+	if (ret < 0)
+		tracing_disarm_snapshot(file->tr);
+	return ret;
 }
 
 static void unregister_snapshot_trigger(char *glob,
-- 
2.43.0