Re: [PATCH v2] timers: Fix usleep_range() in the context of wake_up_process()

[Doug Anderson]

Dan,

On Tue, Oct 18, 2016 at 6:44 AM, Daniel Kurtz <djkurtz@xxxxxxxxxxxx> wrote:
> Hi Doug,
>
> On Tue, Oct 11, 2016 at 5:04 AM, Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
>> 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.
>
> I think this change works, and fixes a real issue, however, I don't
> think you are fixing this at the right layer.
> The comment for schedule_hrtimeout_range says:
>
> /**
>  * schedule_hrtimeout_range - sleep until timeout
>  * @expires:    timeout value (ktime_t)
>  * @delta:      slack in expires timeout (ktime_t)
>  * @mode:       timer mode, HRTIMER_MODE_ABS or HRTIMER_MODE_REL
>  *
>  * Make the current task sleep until the given expiry time has
>  * elapsed. The routine will return immediately unless
>  * the current task state has been set (see set_current_state()).
>  *
>  * The @delta argument gives the kernel the freedom to schedule the
>  * actual wakeup to a time that is both power and performance friendly.
>  * The kernel give the normal best effort behavior for "@expires+@delta",
>  * but may decide to fire the timer earlier, but no earlier than @expires.
>  *
>  * You can set the task state as follows -
>  *
>  * %TASK_UNINTERRUPTIBLE - at least @timeout time is guaranteed to
>  * pass before the routine returns.
>  *
>  * %TASK_INTERRUPTIBLE - the routine may return early if a signal is
>  * delivered to the current task.
>  *
>  * The current task state is guaranteed to be TASK_RUNNING when this
>  * routine returns.
>  *
>  * Returns 0 when the timer has expired otherwise -EINTR
>  */
>
> The behavior as specified for this function "at least @timeout time is
> guaranteed to pass before the routine returns" already guarantees the
> behavior you are adding to do_usleep_range() whenever the current task
> state is (pre-)set to TASK_UNINTERRUPTIBLE.
>
> Thus, I think the loop around 'schedule()' should be moved to
> schedule_hrtimeout_range() itself.
> This would also fix direct callers of schedule_hrtimeout_range() that
> use TASK_UNINTERRUPTIBLE, although, I could only find one:
>
> pt3_fetch_thread()

Hmmm, I would agree with you that the behavior of
schedule_hrtimeout_range() doesn't seem to match the function
comments.

...but I'm not sure I agree with you about what to do here.
Specifically I think that whatever we do we need to try to keep
schedule_hrtimeout_range() and schedule_timeout() parallel.  For
schedule_timeout() we have the same comments but it's my understanding
that you'd expect that wake_up_process() would wake it up.  In any
case, if wake_up_process() doesn't wake it up then it seems like
msleep() and schedule_timeout_uninterruptible() are the same function
with two names, when in fact one is implemented in terms o the other.

NOTE that also it seems as if we need some other return values besides
0 and -EINTR from schedule_hrtimeout_range() (again, to match
schedule_timeout()) since right now we'll return -EINTR if we were
woken up with wake_up_process().  This would be unexpected in the case
where we had TASK_UNINTERRUPTIBLE set.

-Doug