rcu: Adjust ->lock acquisition for tasks no longer migrating (0a0ba1c9) · Commits · 戴 / test

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:

Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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);

Admin message