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SubjectRe: Deleting timers
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.

> 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.

> Alan Stern
>
> P.S.: The only other difference is that del_timer calls
> timer_stats_timer_clear_start_info. Why doesn't try_to_del_timer_sync
> do the same thing?

This could be a day-one bug in

: commit 82f67cd9fca8c8762c15ba7ed0d5747588c1e221
: Author: Ingo Molnar <mingo@elte.hu>
: AuthorDate: Fri Feb 16 01:28:13 2007 -0800
: Commit: Linus Torvalds <torvalds@woody.linux-foundation.org>
: CommitDate: Fri Feb 16 08:13:59 2007 -0800
:
: [PATCH] Add debugging feature /proc/timer_stat

timer-stats omits accumulation for del_timer_sync() also.


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