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-rw-r--r--kernel/context_tracking.c52
1 files changed, 33 insertions, 19 deletions
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 1f47119c5b09..7b095de356c5 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -54,17 +54,31 @@ void user_enter(void)
WARN_ON_ONCE(!current->mm);
local_irq_save(flags);
- if (__this_cpu_read(context_tracking.active) &&
- __this_cpu_read(context_tracking.state) != IN_USER) {
+ if ( __this_cpu_read(context_tracking.state) != IN_USER) {
+ if (__this_cpu_read(context_tracking.active)) {
+ /*
+ * At this stage, only low level arch entry code remains and
+ * then we'll run in userspace. We can assume there won't be
+ * any RCU read-side critical section until the next call to
+ * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
+ * on the tick.
+ */
+ vtime_user_enter(current);
+ rcu_user_enter();
+ }
/*
- * At this stage, only low level arch entry code remains and
- * then we'll run in userspace. We can assume there won't be
- * any RCU read-side critical section until the next call to
- * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
- * on the tick.
+ * Even if context tracking is disabled on this CPU, because it's outside
+ * the full dynticks mask for example, we still have to keep track of the
+ * context transitions and states to prevent inconsistency on those of
+ * other CPUs.
+ * If a task triggers an exception in userspace, sleep on the exception
+ * handler and then migrate to another CPU, that new CPU must know where
+ * the exception returns by the time we call exception_exit().
+ * This information can only be provided by the previous CPU when it called
+ * exception_enter().
+ * OTOH we can spare the calls to vtime and RCU when context_tracking.active
+ * is false because we know that CPU is not tickless.
*/
- vtime_user_enter(current);
- rcu_user_enter();
__this_cpu_write(context_tracking.state, IN_USER);
}
local_irq_restore(flags);
@@ -130,12 +144,14 @@ void user_exit(void)
local_irq_save(flags);
if (__this_cpu_read(context_tracking.state) == IN_USER) {
- /*
- * We are going to run code that may use RCU. Inform
- * RCU core about that (ie: we may need the tick again).
- */
- rcu_user_exit();
- vtime_user_exit(current);
+ if (__this_cpu_read(context_tracking.active)) {
+ /*
+ * We are going to run code that may use RCU. Inform
+ * RCU core about that (ie: we may need the tick again).
+ */
+ rcu_user_exit();
+ vtime_user_exit(current);
+ }
__this_cpu_write(context_tracking.state, IN_KERNEL);
}
local_irq_restore(flags);
@@ -178,8 +194,6 @@ EXPORT_SYMBOL_GPL(guest_exit);
void context_tracking_task_switch(struct task_struct *prev,
struct task_struct *next)
{
- if (__this_cpu_read(context_tracking.active)) {
- clear_tsk_thread_flag(prev, TIF_NOHZ);
- set_tsk_thread_flag(next, TIF_NOHZ);
- }
+ clear_tsk_thread_flag(prev, TIF_NOHZ);
+ set_tsk_thread_flag(next, TIF_NOHZ);
}