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    SubjectRe: Deleting timers
    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.
    >
    > > 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.

    Okay, thanks. That makes sense.

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

    It turns out I probably don't need the enhanced functionality after
    all. So never mind for now...

    Alan Stern



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