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authorSteven Rostedt (VMware) <rostedt@goodmis.org>2019-02-14 18:45:21 -0500
committerSteven Rostedt (VMware) <rostedt@goodmis.org>2019-02-20 13:51:08 -0500
commit1c347a94ca79ef89156c7ad5d3a44bb2320a7047 (patch)
treed66b06113205b4c23da74548f15dc13c2e129d1b /kernel/trace
parenta3d86a4ad7e4cef6f04f722028916680b4f635cb (diff)
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>
Diffstat (limited to 'kernel/trace')
-rw-r--r--kernel/trace/trace.c8
1 files changed, 8 insertions, 0 deletions
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 0460cc0f28fd..2cf3c747a357 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -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;