[lttng-dev] Question about lock in synchronize_rcu implementation of URCU

[paulmck@linux.vnet.ibm.com (Paul E. McKenney)]

On Wed, Apr 27, 2016 at 09:34:16PM -0400, Yuxin Ren wrote:
> Hi,
> 
> I am learning the URCU code.
> 
> Why do we need rcu_gp_lock in synchronize_rcu?
> https://github.com/urcu/userspace-rcu/blob/master/urcu.c#L401
> 
> In the comment, it says this lock ensures mutual exclusion between
> threads calling synchronize_rcu().
> But only the first thread added to waiter queue can proceed to detect
> grace period.
> How can multiple threads currently perform the grace thread?

They don't concurrently perform grace periods, and it would be wasteful
for them to do so.  Instead, the first one performs the grace period,
and all that were waiting at the time it started get the benefit of that
same grace period.

Any that arrived after the first grace period performs the first
grace period are served by whichever of them performs the second
grace period.

							Thanx, Paul