linux-kernel - Re: [PATCH] make cancel_rearming_delayed

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Message-Id: <20070503181513.0b0aa4fb.akpm@linux-foundation.org>
Date:	Thu, 3 May 2007 18:15:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	David Chinner <dgc@....com>, David Howells <dhowells@...hat.com>,
	Gautham Shenoy <ego@...ibm.com>,
	Jarek Poplawski <jarkao2@...pl>, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] make cancel_rearming_delayed_work() reliable

On Fri, 4 May 2007 00:42:26 +0400
Oleg Nesterov <oleg@...sign.ru> wrote:

> Thanks to Jarek Poplawski for the ideas and for spotting the bug in the
> initial draft patch.
> 
> cancel_rearming_delayed_work() currently has many limitations, because it
> requires that dwork always re-arms itself via queue_delayed_work(). So it
> hangs forever if dwork doesn't do this, or cancel_rearming_delayed_work/
> cancel_delayed_work was already called. It uses flush_workqueue() in a loop,
> so it can't be used if workqueue was freezed, and it is potentially live-
> lockable on busy system if delay is small.
> 
> With this patch cancel_rearming_delayed_work() doesn't make any assumptions
> about dwork, it can re-arm itself via queue_delayed_work(), or queue_work(),
> or do nothing.
> 
> As a "side effect", cancel_work_sync() was changed to handle re-arming works
> as well.
> 
> 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.

> +	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.


> +	while (!del_timer(&dwork->timer) &&
> +	       !try_to_grab_pending(&dwork->work))
> +		;

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