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Subject[PATCH RFC tip/core/rcu 21/41] rcu: Inform RCU of irq_exit() activity
Date
From: "Paul E. McKenney" <paul.mckenney@linaro.org>

This is a port to TINY_RCU of Peter Zijlstra's commit #ec433f0c5

The rcu_read_unlock_special() function relies on in_irq() to exclude
scheduler activity from interrupt level. This fails because exit_irq()
can invoke the scheduler after clearing the preempt_count() bits that
in_irq() uses to determine that it is at interrupt level. This situation
can result in failures as follows:

$task IRQ SoftIRQ

rcu_read_lock()

/* do stuff */
<preempt> |= UNLOCK_BLOCKED
rcu_read_unlock()
--t->rcu_read_lock_nesting

irq_enter();
/* do stuff, don't use RCU */
irq_exit();
sub_preempt_count(IRQ_EXIT_OFFSET);
invoke_softirq()
ttwu();
spin_lock_irq(&pi->lock)
rcu_read_lock();
/* do stuff */
rcu_read_unlock();
rcu_read_unlock_special()
rcu_report_exp_rnp()
ttwu()
spin_lock_irq(&pi->lock) /* deadlock */
rcu_read_unlock_special(t);

This can be triggered 'easily' because invoke_softirq() immediately does
a ttwu() of ksoftirqd/# instead of doing the in-place softirq stuff first,
but even without that the above happens.

Cure this by also excluding softirqs from the rcu_read_unlock_special()
handler and ensuring the force_irqthreads ksoftirqd/# wakeup is done
from full softirq context.

[ Alternatively, delaying the ->rcu_read_lock_nesting decrement
until after the special handling would make the thing more robust
in the face of interrupts as well. And there is a separate patch
for that. ]

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutiny_plugin.h | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
index 3f6c07e..9d7d985 100644
--- a/kernel/rcutiny_plugin.h
+++ b/kernel/rcutiny_plugin.h
@@ -570,7 +570,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
rcu_preempt_cpu_qs();

/* Hardware IRQ handlers cannot block. */
- if (in_irq()) {
+ if (in_irq() || in_serving_softirq()) {
local_irq_restore(flags);
return;
}
--
1.7.8


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