On Fri, Jan 18, 2019 at 06:08:01PM +0100, Hugo Lefeuvre wrote:
[...]
> > > +/*
> > > + * like wait_event_hrtimeout() -- except it uses TASK_INTERRUPTIBLE to avoid
> > > + * increasing load and is freezable.
> > > + */
> > > +#define wait_event_freezable_hrtimeout(wq_head, condition, timeout) \
> >
> > You should document the variable names in the header comments.
>
> Agree. This comment was copy/pasted from wait_event_freezable_timeout,
> should I fix it there as well?
>
> > Also, this new API appears to conflict with definition of 'freezable' in
> > wait_event_freezable I think,
> >
> > wait_event_freezable - sleep or freeze until condition is true.
> >
> > wait_event_freezable_hrtimeout - sleep but make sure freezer is not blocked.
> > (your API)
> >
> > It seems to me these are 2 different definitions of 'freezing' (or I could be
> > completely confused). But in the first case it calls try_to_freeze after
> > schedule(). In the second case (your new API), it calls freezable_schedule().
> >
> > So I am wondering why is there this difference in the 'meaning of freezable'.
> > In the very least, any such subtle differences should be documented in the
> > header comments IMO.
>
> It appears that freezable_schedule() and schedule(); try_to_freeze() are
> almost identical:
>
> static inline void freezable_schedule(void)
> {
> freezer_do_not_count();
> schedule();
> freezer_count();
> }
>
> and
>
> static inline void freezer_do_not_count(void)
> {
> current->flags |= PF_FREEZER_SKIP;
> }
>
> static inline void freezer_count(void)
> {
> current->flags &= ~PF_FREEZER_SKIP;
> /*
> * If freezing is in progress, the following paired with smp_mb()
> * in freezer_should_skip() ensures that either we see %true
> * freezing() or freezer_should_skip() sees !PF_FREEZER_SKIP.
> */
> smp_mb();
> try_to_freeze();
> }
>
> as far as I understand this code, freezable_schedule() avoids blocking the
> freezer during the schedule() call, but in the end try_to_freeze() is still
> called so the result is the same, right?
> I wonder why wait_event_freezable is not calling freezable_schedule().
It could be something subtle in my view. freezable_schedule() actually makes
the freezer code not send a wake up to the sleeping task if a freeze happens,
because the PF_FREEZER_SKIP flag is set, as you pointed.
Whereas wait_event_freezable() which uses try_to_freeze() does not seem to have
this behavior and the task will enter the freezer. So I'm a bit skeptical if
your API will behave as expected (or at least consistently with other wait
APIs).
> That being said, I am not sure that the try_to_freeze() call does anything
> in the vsoc case because there is no call to set_freezable() so the thread
> still has PF_NOFREEZE...
I traced this, and PF_NOFREEZE is not set by default for tasks.
thanks,
- Joel
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