linux-kernel - [RT PATCH 1/2] kernel/cpu_chill: use schedule

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Date:   Fri, 16 Mar 2018 15:21:34 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-rt-users@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [RT PATCH 1/2] kernel/cpu_chill: use schedule_hrtimeout()

cpu_chill() uses __hrtimer_nanosleep() with a hack to get
TASK_UNINTERRUPTIBLE state. It seems to work but it does not.
The problem is that we may have a signal pending and receive a sporadic
wake up _before_ the timer expires. At that point the code will look at
->restart_block and use it based on its value from the previous syscall
resulting in a possible copy-to-user.
Instead all this trouble, we could use schedule_hrtimeout() which is not
an user-space interface and does not have this side effects.

Cc: stable-rt@...r.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/time/hrtimer.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 6c77643eaf02..c0efc22ba635 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1873,14 +1873,13 @@ COMPAT_SYSCALL_DEFINE2(nanosleep, struct compat_timespec __user *, rqtp,
  */
 void cpu_chill(void)
 {
-	struct timespec64 tu = {
-		.tv_nsec = NSEC_PER_MSEC,
-	};
+	ktime_t chill_time;
 	unsigned int freeze_flag = current->flags & PF_NOFREEZE;
 
+	chill_time = ktime_set(0, NSEC_PER_MSEC);
+	set_current_state(TASK_UNINTERRUPTIBLE);
 	current->flags |= PF_NOFREEZE;
-	__hrtimer_nanosleep(&tu, HRTIMER_MODE_REL_HARD, CLOCK_MONOTONIC,
-			    TASK_UNINTERRUPTIBLE);
+	schedule_hrtimeout(&chill_time, HRTIMER_MODE_REL_HARD);
 	if (!freeze_flag)
 		current->flags &= ~PF_NOFREEZE;
 }
-- 
2.16.2