Commit 1c347a94 authored Feb 14, 2019 by Steven Rostedt (VMware)

tracing: Comment why cond_snapshot is checked outside of max_lock protection

Before setting tr->cond_snapshot, it must be NULL before it can be updated.
It can go to NULL when a trace event hist trigger is created or removed, and
can only be modified under the max_lock spin lock. But because it can only
be set to something other than NULL under both the max_lock spin lock as
well as the trace_types_lock, we can perform the check if it is not NULL
only under the trace_types_lock and fail out without having to grab the
max_lock spin lock.

This is very subtle, and deserves a comment.

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

parent a3d86a4a

kernel/trace/trace.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -1116,6 +1116,14 @@ int tracing_snapshot_cond_enable(struct trace_array tr, void cond_data,
		goto fail_unlock;
		}

		/*
		* The cond_snapshot can only change to NULL without the
		* trace_types_lock. We don't care if we race with it going
		* to NULL, but we want to make sure that it's not set to
		* something other than NULL when we get here, which we can
		* do safely with only holding the trace_types_lock and not
		* having to take the max_lock.
		*/
		if (tr->cond_snapshot) {
		ret = -EBUSY;
		goto fail_unlock;