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Commit 0a0ba1c9 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Adjust ->lock acquisition for tasks no longer migrating 



Tasks are no longer migrated away from a given rcu_node structure
when all CPUs corresponding to that rcu_node structure have gone offline.
This means that rcu_read_unlock_special() no longer needs to loop
retrying rcu_node ->lock acquisition because the current task is
guaranteed to stay put.

This commit takes a small and paranoid step towards relying on this
guarantee by placing a WARN_ON_ONCE() just after the early exit from
the lock-acquisition loop.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent ea46351c

kernel/rcu/tree_plugin.h

+6 −3
Original line number Diff line number Diff line
@@ -307,9 +307,11 @@ void rcu_read_unlock_special(struct task_struct *t) 
		t->rcu_read_unlock_special.b.blocked = false;



		/*

		 * Remove this task from the list it blocked on.  The

		 * task can migrate while we acquire the lock, but at

		 * most one time.  So at most two passes through loop.

		 * Remove this task from the list it blocked on.  The task

		 * now remains queued on the rcu_node corresponding to

		 * the CPU it first blocked on, so the first attempt to

		 * acquire the task's rcu_node's ->lock will succeed.

		 * Keep the loop and add a WARN_ON() out of sheer paranoia.

		 */

		for (;;) {

			rnp = t->rcu_blocked_node;
@@ -317,6 +319,7 @@ void rcu_read_unlock_special(struct task_struct *t) 
			smp_mb__after_unlock_lock();

			if (rnp == t->rcu_blocked_node)

				break;

			WARN_ON_ONCE(1);

			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */

		}

		empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);