Re: [PATCH RFC tip/core/rcu 22/41] rcu: Simplify unboosting checks

From: Paul E. McKenney
Date: Thu Feb 02 2012 - 12:53:59 EST

On Wed, Feb 01, 2012 at 06:38:47PM -0800, Josh Triplett wrote:
> On Wed, Feb 01, 2012 at 11:41:40AM -0800, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mckenney@xxxxxxxxxx>
> >
> > This is a port of commit #82e78d80 from TREE_PREEMPT_RCU to
> >
> > This commit uses the fact that current->rcu_boost_mutex is set
> > any time that the RCU_READ_UNLOCK_BOOSTED flag is set in the
> > current->rcu_read_unlock_special bitmask. This allows tests of
> > the bit to be changed to tests of the pointer, which in turn allows
> > the RCU_READ_UNLOCK_BOOSTED flag to be eliminated.
> Does this change affect rcu_read_unlock()'s logic to trigger the
> slowpath only when special flags get set?

Interestingly enough, it does not. The only way a task can be subjected
to RCU priority boosting is for that task to block sometime in its
current RCU read-side critical section. When the task blocks, the
RCU_READ_UNLOCK_BLOCKED flag will be set. Therefore, any time that the
current->rcu_boost_mutex pointer is non-NULL, the RCU_READ_UNLOCK_BLOCKED
flag will be set, so the current test of current->rcu_read_unlock_special
against zero continues to work correctly.

OK, OK, I will update the commit message with words to this effect. ;-)

Thanx, Paul

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