[PATCH v3 2/2] timers: Fix documentation for schedule_timeout() and similar

Douglas Anderson
2016-10-20T20:34:24Z

The documentatoin for schedule_timeout(), schedule_hrtimeout(), and
schedule_hrtimeout_range() all claimed that the routines couldn't
possibly return early if the task state was TASK_UNINTERRUPTIBLE. This
was simply not true, since anyone calling wake_up_process() would cause
those routines to exit early.

As some evidence that the documentation was broken (not the code):
- If we changed the code to match the documentation, msleep() would be
identical to schedule_timeout_uninterruptible() and
msleep_interruptible() would be identical to
schedule_timeout_interruptible(). That doesn't seem likely to have
been the intention.
- The schedule() function sleeps until a task is woken up. Logically,
one would expect that the schedule_timeout() function would sleep
until a task is woken up or a timeout occurrs.

As part of the above observation, it can be seen that
schedule_hrtimeout() and schedule_hrtimeout_range() might return -EINTR
even if the task state was TASK_UNINTERRUPTIBLE. This isn't terrible
behavior so we'll document it and keep it as-is. After all, trying to
match schedule_timeout() and return the time left would incure a bunch
of extra calculation cost that isn't needed in all cases.

Suggested-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---
Changes in v3:
- Documentation fix new for v3.

kernel/time/hrtimer.c | 20 ++++++++++++++------
kernel/time/timer.c | 11 +++++++----
2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index bb5ec425dfe0..707ecf47a5fd 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1742,15 +1742,19 @@ schedule_hrtimeout_range_clock(ktime_t *expires, u64 delta,
* You can set the task state as follows -
*
* %TASK_UNINTERRUPTIBLE - at least @timeout time is guaranteed to
- * pass before the routine returns.
+ * pass before the routine returns unless the current task is explicitly
+ * woken up, (e.g. by wake_up_process())".
*
* %TASK_INTERRUPTIBLE - the routine may return early if a signal is
- * delivered to the current task.
+ * delivered to the current task or the current task is explicitly woken
+ * up.
*
* The current task state is guaranteed to be TASK_RUNNING when this
* routine returns.
*
- * Returns 0 when the timer has expired otherwise -EINTR
+ * Returns 0 when the timer has expired otherwise -EINTR. Note that
+ * -EINTR can still be returned even if the task state is
+ * TASK_UNINTERRUPTIBLE if the current task is explicitly woken up.
*/
int __sched schedule_hrtimeout_range(ktime_t *expires, u64 delta,
const enum hrtimer_mode mode)
@@ -1772,15 +1776,19 @@ EXPORT_SYMBOL_GPL(schedule_hrtimeout_range);
* You can set the task state as follows -
*
* %TASK_UNINTERRUPTIBLE - at least @timeout time is guaranteed to
- * pass before the routine returns.
+ * pass before the routine returns unless the current task is explicitly
+ * woken up, (e.g. by wake_up_process())".
*
* %TASK_INTERRUPTIBLE - the routine may return early if a signal is
- * delivered to the current task.
+ * delivered to the current task or the current task is explicitly woken
+ * up.
*
* The current task state is guaranteed to be TASK_RUNNING when this
* routine returns.
*
- * Returns 0 when the timer has expired otherwise -EINTR
+ * Returns 0 when the timer has expired otherwise -EINTR. Note that
+ * -EINTR can still be returned even if the task state is
+ * TASK_UNINTERRUPTIBLE if the current task is explicitly woken up.
*/
int __sched schedule_hrtimeout(ktime_t *expires,
const enum hrtimer_mode mode)
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 219439efd56a..b2ca2a6bc4d2 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1691,11 +1691,12 @@ static void process_timeout(unsigned long __data)
* You can set the task state as follows -
*
* %TASK_UNINTERRUPTIBLE - at least @timeout jiffies are guaranteed to
- * pass before the routine returns. The routine will return 0
+ * pass before the routine returns unless the current task is explicitly
+ * woken up, (e.g. by wake_up_process())".
*
* %TASK_INTERRUPTIBLE - the routine may return early if a signal is
- * delivered to the current task. In this case the remaining time
- * in jiffies will be returned, or 0 if the timer expired in time
+ * delivered to the current task or the current task is explicitly woken
+ * up.
*
* The current task state is guaranteed to be TASK_RUNNING when this
* routine returns.
@@ -1704,7 +1705,9 @@ static void process_timeout(unsigned long __data)
* the CPU away without a bound on the timeout. In this case the return
* value will be %MAX_SCHEDULE_TIMEOUT.
*
- * In all cases the return value is guaranteed to be non-negative.
+ * Returns 0 when the timer has expired otherwise the remaining time in
+ * jiffies will be returned. In all cases the return value is guaranteed
+ * to be non-negative.
*/
signed long __sched schedule_timeout(signed long timeout)
{
--
2.8.0.rc3.226.g39d4020