Re: Deleting timers

Oleg Nesterov
2009-07-02T16:05:13Z

On 07/02, Alan Stern wrote:
> On Wed, 1 Jul 2009, Andrew Morton wrote:
>
> > On Fri, 26 Jun 2009 15:50:54 -0400 (EDT) Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > > Thomas:
> >
> > I'm not Thomas, but I play one on TV.
> >
> > > The major difference -- in fact, almost the only difference -- between
> > > del_timer() and try_to_del_timer_sync() is that try_to_del_timer_sync
> > > returns a special code (-1) if the timer couldn't be deleted because it
> > > is currently running, whereas del_timer doesn't check this.
> >
> > And del_timer() is heaps faster against a not-pending timer. I have a
> > vague memory that there are some callsites which do this quite a lot.
> >
> > And try_to_del_timer_sync() forgot to do timer_stats_timer_clear_start_info().
> >
> > > Furthermore, the "_sync" in the name suggests that
> > > try_to_del_timer_sync will wait until a running timer has finished,
> > > which it clearly does not do.
> >
> > yup.

Yes, try_to_del_timer_sync() never waits exactly because it fails if the
timer is running.

> > > Despite these facts, the kerneldoc for try_to_del_timer_sync states
> > > that it must not be called in interrupt context. Why not? Isn't that
> > > advice simply wrong?
> >
> > : commit fd450b7318b75343fd76b3d95416853e34e72c95
> > : Author: Oleg Nesterov <oleg@tv-sign.ru>
> > : AuthorDate: Thu Jun 23 00:08:59 2005 -0700
> > : Commit: Linus Torvalds <torvalds@ppc970.osdl.org>
> > : CommitDate: Thu Jun 23 09:45:16 2005 -0700
> > :
> > : [PATCH] timers: introduce try_to_del_timer_sync()
> > :
> > : This patch splits del_timer_sync() into 2 functions. The new one,
> > : try_to_del_timer_sync(), returns -1 when it hits executing timer.
> > :
> > : It can be used in interrupt context, or when the caller hold locks which
> > : can prevent completion of the timer's handler.
> > :
> > : NOTE. Currently it can't be used in interrupt context in UP case, because
> > : ->running_timer is used only with CONFIG_SMP.
> > :
> > : Should the need arise, it is possible to kill #ifdef CONFIG_SMP in
> > : set_running_timer(), it is cheap.
> > :
> >
> > The changelog is somewhat vodka-fogged, but there is a bit of a problem
> > there.

Yeah. try_to_del_timer_sync() should not be used in interrupt context
because in UP case it is equal to del_timer(), this is not what we want.

But with CONFIG_SMP it can work from any context.

> Okay, thanks. That makes sense.
>
> > > With this in mind, would there be any objection if I renamed it to
> > > try_to_del_timer(),

Not sure I understand why try_to_del_timer is better...

try_to_del_timer_sync() means: try to del_timer_sync(), that is why
"_sync" ;)

But I don't really care.

> removed the comment forbidding it to be used in
> > > interrupt context, and made it available even on non-SMP builds?
> >
> > Sounds sane to me, if the set_running_timer() change is also made.

Yes, set_running_timer() should be changed, and

# define try_to_del_timer_sync(t) del_timer(t)

in timer.h should be killed. I think this makes sense.

Oleg.