From: Ankur Arora <ankur.a.ar...@oracle.com>
[ Upstream commit 83b28cfe796464ebbde1cf7916c126da6d572685 ]
With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
states for read-side critical sections via rcu_all_qs().
One reason why this was needed: lacking preempt-count, the tick
handler has no way of knowing whether it is executing in a
read-side critical section or not.
With (PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n), we get (PREEMPT_COUNT=y,
PREEMPT_RCU=n). In this configuration cond_resched() is a stub and
does not provide quiescent states via rcu_all_qs().
(PREEMPT_RCU=y provides this information via rcu_read_unlock() and
its nesting counter.)
So, use the availability of preempt_count() to report quiescent states
in rcu_flavor_sched_clock_irq().
Suggested-by: Paul E. McKenney <paul...@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
Signed-off-by: Ankur Arora <ankur.a.ar...@oracle.com>
Reviewed-by: Frederic Weisbecker <frede...@kernel.org>
Signed-off-by: Paul E. McKenney <paul...@kernel.org>
Signed-off-by: Boqun Feng <boqun.f...@gmail.com>
Signed-off-by: Sasha Levin <sas...@kernel.org>
---
kernel/rcu/tree_plugin.h | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index f5ba0740f9b50..c07a841971739 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -919,13 +919,16 @@ static void rcu_preempt_check_blocked_tasks(struct
rcu_node *rnp)
*/
static void rcu_flavor_sched_clock_irq(int user)
{
- if (user || rcu_is_cpu_rrupt_from_idle()) {
+ if (user || rcu_is_cpu_rrupt_from_idle() ||
+ (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
+ (preempt_count() == HARDIRQ_OFFSET))) {
/*
* Get here if this CPU took its interrupt from user
- * mode or from the idle loop, and if this is not a
- * nested interrupt. In this case, the CPU is in
- * a quiescent state, so note it.
+ * mode, from the idle loop without this being a nested
+ * interrupt, or while not holding the task preempt count
+ * (with PREEMPT_COUNT=y). In this case, the CPU is in a
+ * quiescent state, so note it.
*
* No memory barrier is required here because rcu_qs()
* references only CPU-local variables that other CPUs
--
2.39.5
