linux-kernel - Re: [PATCH RT 4/2] hrtimer: Don't lose state in cpu

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Date:   Tue, 26 Feb 2019 12:38:25 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Mike Galbraith <efault@....de>
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RT 4/2] hrtimer: Don't lose state in cpu_chill()

On 2019-02-25 18:40:36 [+0100], Mike Galbraith wrote:
> Ah.  Yup, box says you're right.

Thank you. Added:

Subject: [PATCH] hrtimer: cpu_chill(): save task state in ->saved_state()

In the previous change I saved the current task state on stack. This was
bad because while the task is scheduled-out it might receive a wake-up.
The wake up changes the task state and we must not destroy it.

Save the task-state in ->saved_state under a PI-lock to unsure that
state changes during are not missed while the task temporary scheduled
out.

Reported-by: Mike Galbraith <efault@....de>
Tested-by: Mike Galbraith <efault@....de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/time/hrtimer.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 6f2736ec4b8ef..e1040b80362c9 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1900,20 +1900,28 @@ COMPAT_SYSCALL_DEFINE2(nanosleep, struct compat_timespec __user *, rqtp,
  */
 void cpu_chill(void)
 {
-	ktime_t chill_time;
 	unsigned int freeze_flag = current->flags & PF_NOFREEZE;
-	long saved_state;
+	struct task_struct *self = current;
+	ktime_t chill_time;
 
-	saved_state = current->state;
-	chill_time = ktime_set(0, NSEC_PER_MSEC);
+	raw_spin_lock_irq(&self->pi_lock);
+	self->saved_state = self->state;
 	__set_current_state_no_track(TASK_UNINTERRUPTIBLE);
+	raw_spin_unlock_irq(&self->pi_lock);
+
+	chill_time = ktime_set(0, NSEC_PER_MSEC);
+
 	current->flags |= PF_NOFREEZE;
 	sleeping_lock_inc();
 	schedule_hrtimeout(&chill_time, HRTIMER_MODE_REL_HARD);
 	sleeping_lock_dec();
 	if (!freeze_flag)
 		current->flags &= ~PF_NOFREEZE;
-	__set_current_state_no_track(saved_state);
+
+	raw_spin_lock_irq(&self->pi_lock);
+	__set_current_state_no_track(self->saved_state);
+	self->saved_state = TASK_RUNNING;
+	raw_spin_unlock_irq(&self->pi_lock);
 }
 EXPORT_SYMBOL(cpu_chill);
 #endif
-- 
2.20.1


Sebastian