From: Lai Jiangshan <la...@cn.fujitsu.com>
After commit #10f39bb1b2c1 (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 <la...@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Merge with 14b8cc88f674 (rcu: Warn on allegedly impossible
rcu_read_unlock_special() from irq)
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
---
kernel/rcu/tree_plugin.h | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 59a3a17a0c57..29335faf96e7 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -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;
}
--
1.8.1.5
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