linux-kernel - Re: [RFC PATCH 6/6] Convert tasklets to work queues

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Date:	Fri, 22 Jun 2007 19:52:47 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Daniel Walker <dwalker@...sta.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@...radead.org>,
	john stultz <johnstul@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>, kuznet@....inr.ac.ru
Subject: Re: [RFC PATCH 6/6] Convert tasklets to work queues

On 06/22, Steven Rostedt wrote:
>
> > truct tasklet_struct, work);
> > > +
> > > +	if (unlikely(atomic_read(&t->count))) {
> > > +		pr_debug("tasklet disabled %s %p\n", t->n, t);
> > > +		set_bit(TASKLET_STATE_PENDING, &t->state);
> > > +		smp_mb();
> > > +		/* make sure we were not just enabled */
> > > +		if (likely(atomic_read(&t->count)))
> > > +			goto out;
> > > +		clear_bit(TASKLET_STATE_PENDING, &t->state);

Looking closer, I think this is not right, and the smp_mb__before_clear_bit()
can't help.


				/* t->count == 1 */

work_tasklet_exec()					tasklet_enable()

...

set_bit(TASKLET_STATE_PENDING);				atomic_dec_and_test(&t->count);


				/* t->count == 0 */


// False
if (atomic_read(&t->count))
	goto out;

							// True
							if (test_and_clear_bit(_PENDING))
								tasklet_schedule();


clear_bit(TASKLET_STATE_PENDING);

execute t->func();


So, t->func() will be executed twice because tasklet_enable() does
tasklet_schedule().


So I think we need a fix for work_tasklet_exec,

-		clear_bit(TASKLET_STATE_PENDING);
+		if (!test_and_clear_bit(TASKLET_STATE_PENDING))
			goto out;



Steven, a very stupid suggestion, could you move the code for tasklet_enable()
up, closer to tasklet_disable() ?

Oleg.

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