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. 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
