From 0a0ba1c93f8a0ff28bacec0d1d018081e762e2f0 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 8 Mar 2015 14:20:30 -0700 Subject: [PATCH] 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: Paul E. McKenney --- kernel/rcu/tree_plugin.h | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 58b1ebdc4387..c8340e929eb4 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -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); -- 2.20.1