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The buffer hold is a measure of contention, but it is tracked per cpu where the lock is a globabl resource. On some systems (eg. real time) there is no guarantee that the code will be on the same cpu pre, and post spinlock acquisition, nor that the buffer will be put back to the same percpu cache when we are done with it. Because of this the hold value can move asynchronous to the buffers on the cache, meaning it is possible to underflow, and potentially in really pathelogical cases overflow. Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com> Signed-off-by: John Johansen <john.johansen@canonical.com>