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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>
This commit is contained in:
Steven Rostedt (VMware) 2019-02-14 18:45:21 -05:00
parent a3d86a4ad7
commit 1c347a94ca
1 changed files with 8 additions and 0 deletions
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8
kernel/trace/trace.c
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@ -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;