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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2015-03-08 14:20:30 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2015-05-27 12:58:37 -0700
commit0a0ba1c93f8a0ff28bacec0d1d018081e762e2f0 (patch)
treeb76fd4bf7358f5a0cdeea2a97ee73485a5c4f720 /kernel/rcu
parentea46351cea79f54729d8546e5bd7f091a2e6484b (diff)
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 <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/rcu')
-rw-r--r--kernel/rcu/tree_plugin.h9
1 files 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);