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Message-ID: <20151119124349.GP4431@pathway.suse.cz>
Date: Thu, 19 Nov 2015 13:43:49 +0100
From: Petr Mladek <pmladek@...e.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Josh Triplett <josh@...htriplett.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
Vlastimil Babka <vbabka@...e.cz>, linux-api@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 01/22] timer: Allow to check when the timer callback
has not finished yet
On Wed 2015-11-18 23:32:28, Thomas Gleixner wrote:
> On Wed, 18 Nov 2015, Petr Mladek wrote:
> > timer_pending() checks whether the list of callbacks is empty.
> > Each callback is removed from the list before it is called,
> > see call_timer_fn() in __run_timers().
> >
> > Sometimes we need to make sure that the callback has finished.
> > For example, if we want to free some resources that are accessed
> > by the callback.
> >
> > For this purpose, this patch adds timer_active(). It checks both
> > the list of callbacks and the running_timer. It takes the base_lock
> > to see a consistent state.
> >
> > I plan to use it to implement delayed works in kthread worker.
> > But I guess that it will have wider use. In fact, I wonder if
> > timer_pending() is misused in some situations.
>
> Well. That's nice and good. But how will that new function solve
> anything? After you drop the lock the state is not longer valid.
If we prevent anyone from setting up the timer and timer_pending()
returns false, we are sure that the timer will stay as is.
For example, I use it in the function try_to_cancel_kthread_work().
Any manipulation with the timer is protected by worker->lock.
If the timer is not pending but still active, I have to drop
the lock and busy wait for the timer callback. See
http://thread.gmane.org/gmane.linux.kernel.mm/141493/focus=141501
Also I wonder if the following usage in
drivers/infiniband/hw/nes/nes_cm.c is safe:
static int mini_cm_dealloc_core(struct nes_cm_core *cm_core)
{
nes_debug(NES_DBG_CM, "De-Alloc CM Core (%p)\n", cm_core);
if (!cm_core)
return -EINVAL;
barrier();
if (timer_pending(&cm_core->tcp_timer))
del_timer(&cm_core->tcp_timer);
destroy_workqueue(cm_core->event_wq);
destroy_workqueue(cm_core->disconn_wq);
We destroy the workqueue but the timer callback might still
be in progress and queue new work.
There are many more locations where I see the pattern:
if (timer_pending())
del_timer();
clean_up_stuff();
IMHO, we should use:
if (timer_active())
del_timer_sync();
/* really safe to free stuff */
clean_up_stuff();
or just
del_timer_sync();
clean_up_stuff();
I wonder if timer_pending() is used in more racy scenarios. Or maybe,
I just miss something that makes it all safe.
Thanks,
Petr
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