With the exception of QRCU and several of the ``toy'' RCU implementations
described in
Section ,
RCU grace periods extend for multiple milliseconds.
Although there are a number of techniques to render such long delays
harmless, including use of the asynchronous interfaces where available
(call_rcu() and call_rcu_bh()), this situation
is a major reason for the rule of thumb that RCU be used in read-mostly
situations.