[PATCH v5 1/2] timers: Fix usleep_range() in the context of wake_up_process()

[Douglas Anderson]

Users of usleep_range() expect that it will _never_ return in less time
than the minimum passed parameter.  However, nothing in any of the code
ensures this.  Specifically:

usleep_range() => do_usleep_range() => schedule_hrtimeout_range() =>
schedule_hrtimeout_range_clock() just ends up calling schedule() with an
appropriate timeout set using the hrtimer.  If someone else happens to
wake up our task then we'll happily return from usleep_range() early.

msleep() already has code to handle this case since it will loop as long
as there was still time left.  usleep_range() had no such loop.

The problem is is easily demonstrated with a small bit of test code:

  static int usleep_test_task(void *data)
  {
    atomic_t *done = data;
    ktime_t start, end;

    start = ktime_get();
    usleep_range(50000, 100000);
    end = ktime_get();
    pr_info("Requested 50000 - 100000 us.  Actually slept for %llu us\n",
      (unsigned long long)ktime_to_us(ktime_sub(end, start)));
    atomic_set(done, 1);

    return 0;
  }

  static void run_usleep_test(void)
  {
    struct task_struct *t;
    atomic_t done;

    atomic_set(&done, 0);

    t = kthread_run(usleep_test_task, &done, "usleep_test_task");
    while (!atomic_read(&done)) {
      wake_up_process(t);
      udelay(1000);
    }
    kthread_stop(t);
  }

If you run the above code without this patch you get things like:
  Requested 50000 - 100000 us.  Actually slept for 967 us

If you run the above code _with_ this patch, you get:
  Requested 50000 - 100000 us.  Actually slept for 98896 us

Presumably this problem was not detected before because:
- It's not terribly common to use wake_up_process() directly.
- Other ways for processes to wake up are not typically mixed with
  usleep_range().
- There aren't lots of places that use usleep_range(), since many people
  call either msleep() or udelay().

NOTES:
- An effort was made to look for users relying on the old behavior by
  looking for usleep_range() in the same file as wake_up_process().
  No problems was found by this search, though it is conceivable that
  someone could have put the sleep and wakeup in two different files.
- An effort was made to ask several upstream maintainers if they were
  aware of people relying on wake_up_process() to wake up
  usleep_range().  No maintainers were aware of that but they were aware
  of many people relying on usleep_range() never returning before the
  minimum.

Reported-by: Tao Huang <huangtao@xxxxxxxxxxxxxx>
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---
Changes in v5:
- Don't accidentally busy wait after first wakeup (Thomas Gleixner)
- Removed Reviewed-by tags

Changes in v4: None
Changes in v3:
- Add Reviewed-by tags
- Add notes about validation

Changes in v2:
- Fixed stupid bug that snuck in before posting
- Use ktime_before
- Remove delta from the loop

 kernel/time/timer.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 32bf6f75a8fe..ab65e7bcc2c2 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1896,16 +1896,6 @@ unsigned long msleep_interruptible(unsigned int msecs)
 
 EXPORT_SYMBOL(msleep_interruptible);
 
-static void __sched do_usleep_range(unsigned long min, unsigned long max)
-{
-	ktime_t kmin;
-	u64 delta;
-
-	kmin = ktime_set(0, min * NSEC_PER_USEC);
-	delta = (u64)(max - min) * NSEC_PER_USEC;
-	schedule_hrtimeout_range(&kmin, delta, HRTIMER_MODE_REL);
-}
-
 /**
  * usleep_range - Sleep for an approximate time
  * @min: Minimum time in usecs to sleep
@@ -1919,7 +1909,15 @@ static void __sched do_usleep_range(unsigned long min, unsigned long max)
  */
 void __sched usleep_range(unsigned long min, unsigned long max)
 {
-	__set_current_state(TASK_UNINTERRUPTIBLE);
-	do_usleep_range(min, max);
+	ktime_t expires = ktime_add_us(ktime_get(), min);
+	u64 delta = (u64)(max - min) * NSEC_PER_USEC;
+
+	for (;;) {
+		__set_current_state(TASK_UNINTERRUPTIBLE);
+		/* Do not return before the requested sleep time has elapsed */
+		if (!schedule_hrtimeout_range(&expires, delta,
+					      HRTIMER_MODE_ABS))
+			break;
+	}
 }
 EXPORT_SYMBOL(usleep_range);
-- 
2.8.0.rc3.226.g39d4020