On Wed, Oct 19, 2016 at 4:29 AM, Doug Anderson <dianders at chromium.org> wrote: > > Dan, > > On Tue, Oct 18, 2016 at 6:44 AM, Daniel Kurtz <djkurtz at chromium.org> wrote: > > Hi Doug, > > > > On Tue, Oct 11, 2016 at 5:04 AM, Douglas Anderson <dianders at chromium.org> 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+ at 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. Sounds reasonable. It would be nice to add a note to all of those function comments though to make them sound less absolute - "at least @timeout time is guaranteed to pass before the routine returns unless the current task is explicitly woken up, (e.g. by wake_up_process())" > 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
