Commit 79a62f95 authored Oct 30, 2013 by Lai Jiangshan Committed by Paul E. McKenney Dec 12, 2013

rcu: Warn on allegedly impossible rcu_read_unlock_special() from irq



After commit #10f39bb1 (rcu: protect __rcu_read_unlock() against
scheduler-using irq handlers), it is no longer possible to enter
the main body of rcu_read_lock_special() from an NMI, interrupt, or
softirq handler.  In theory, this implies that the check for "in_irq()
|| in_serving_softirq()" must always fail, so that in theory this check
could be removed entirely.

In practice, this commit wraps this condition with a WARN_ON_ONCE().
If this warning never triggers, then the condition will be removed
entirely.

[ paulmck: And one way of triggering the WARN_ON() is if a scheduling
  clock interrupt occurs in an RCU read-side critical section, setting
  RCU_READ_UNLOCK_NEED_QS, which is handled by rcu_read_unlock_special().
  Updated this commit to return if only that bit was set. ]

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

parent 24ef659a

kernel/rcu/tree_plugin.h

+6 −2

Original line number	Diff line number	Diff line
		@@ -361,10 +361,14 @@ void rcu_read_unlock_special(struct task_struct *t)
		special = t->rcu_read_unlock_special;
		if (special & RCU_READ_UNLOCK_NEED_QS) {
		rcu_preempt_qs(smp_processor_id());
		if (!t->rcu_read_unlock_special) {
		local_irq_restore(flags);
		return;
		}
		}

		/* Hardware IRQ handlers cannot block. */
		if (in_irq() \|\| in_serving_softirq()) {
		/* Hardware IRQ handlers cannot block, complain if they get here. */
		if (WARN_ON_ONCE(in_irq() \|\| in_serving_softirq())) {
		local_irq_restore(flags);
		return;
		}