Messages in this thread | | | Date | Fri, 4 May 2007 21:09:16 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] make cancel_rearming_delayed_work() reliable |
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On 05/03, Andrew Morton wrote: > > On Fri, 4 May 2007 00:42:26 +0400 > Oleg Nesterov <oleg@tv-sign.ru> wrote: > > > Disadvantages: > > > > - this patch adds wmb() to insert_work(). > > > > - slowdowns the fast path (when del_timer() succeeds on entry) of > > cancel_rearming_delayed_work(), because wait_on_work() is called > > unconditionally. In that case, compared to the old version, we are > > doing "unneeded" lock/unlock for each online CPU. > > > > On the other hand, this means we don't need to use cancel_work_sync() > > after cancel_rearming_delayed_work(). > > > > - complicates the code (.text grows by 130 bytes). > > > > hm, this is getting complex.
Yes, and I can't say I like this patch very much.
First, I am not really sure it is terribly useful. Yes, cancel_rearming_delayed_work sucks, but did we have any problem in practice? The most annoying problem is that it cant't cancel @dwork which doesn't re-arm itself unconditionally. But this is not so common, and ata_port_flush_task() shows an example how to do this. However, it also shows that this is not so trivial, and work->func() should participate.
Also, we can solve this problem in more simple way. For example, we can shift "timer->function = delayed_work_timer_fn" from queue_delayed_work() to INIT_DELAYED_WORK(). Then, roughly,
cancel_rearming_delayed_work(dwork) { dwork->timer->function = do_nothing_timer_fn; del_timer_sync(&dwork->timer); wait_on_work(&dwork->work); dwork->timer->function = delayed_work_timer_fn; del_timer(&dwork->timer); work_clear_pending(&dwork->work } But this is so hackish, and doesn't work if work->func() use queue_work() or queue_delayed_work(delay = 0) to re-arm itself. Perhaps we can forbid this, and make a simpler patch.
> > + while (!try_to_grab_pending(work)) > > + ; > > The patch adds a couple of spinloops. Normally we put a cpu_relax() into > such loops. It can make a very large difference under some circumstances.
Ah, yes. I'll send a fix along with a little comments update.
Oleg.
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