Messages in this thread Patch in this message | | | Date | Wed, 20 Feb 2019 13:38:06 -0500 | From | Steven Rostedt <> | Subject | [for-next][PATCH 29/29] tracing: Comment why cond_snapshot is checked outside of max_lock protection |
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From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
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> --- kernel/trace/trace.c | 8 ++++++++ 1 file changed, 8 insertions(+)
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; -- 2.20.1
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