On 08/17/2011 03:41 AM, Mathieu Desnoyers wrote:
> MB rcu_read_unlock = QSBR rcu_thread_offline + nesting count
> MB rcu_read_lock = QSBR rcu_thread_online + nesting count
The statement above applies to all flavors of URCU. There is a clear
link between offline/online and nested read lock/unlock. We can see them
as the two sides of the same counter: one counts the reasons why a
thread is within a rcu critical section, while the other keeps track of
the reasons why a thread is not in a rcu critical section.
Nicely put!
But MB/MEMBARRIER and SIGNAL schemes have all been derived from the same
2-phase grace-period scheme, based on a lock/unlock nesting count, while
the QSBR implementation is a different beast that requires periodic
invocation of rcu_quiescent_state() by each application thread, which
makes it unsuitable for use of RCU within libraries.
As an occasional contributor, I am glad what I wasn't saying total BS.
:) Just one thing: rcu_quiescent_state is just an optimization of
rcu_thread_offline();
rcu_thread_online();
where the store of 0 to the gp_ctr is optimized away. That was the
basis of my observation; but as far as the write-side is concerned, MB
and QSBR are basically the same thing.
Paolo
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