linux-kernel - Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

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Date:	Fri, 29 Jun 2007 21:09:57 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Cc:	Ingo Molnar <mingo@...e.hu>, Jeff Garzik <jeff@...zik.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	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>, matthew@....cx
Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

(the email address of Matthew Wilcox looks wrong, changed to matthew@....cx)

On 06/29, Oleg Nesterov wrote:
>
> Steven, unless you have some objections, could you change tasklet_kill() ?
> 
> > +static inline void tasklet_kill(struct tasklet_struct *t)
> >  {
> > -       return test_bit(TASKLET_STATE_SCHED, &t->state);
> > +       flush_workqueue(ktaskletd_wq);
> >  }
> 
> Just change flush_workqueue(ktaskletd_wq) to cancel_work_sync(t-work).

Ugh, tasklet_disable() should be changed as well.

> @@ -84,35 +50,35 @@ static inline void tasklet_disable_nosyn
>  static inline void tasklet_disable(struct tasklet_struct *t)
>  {
>         tasklet_disable_nosync(t);
> -       tasklet_unlock_wait(t);
> -       smp_mb();
> -}
> -
> -static inline void tasklet_enable(struct tasklet_struct *t)
> -{
> -       smp_mb__before_atomic_dec();
> -       atomic_dec(&t->count);
> +       flush_workqueue(ktaskletd_wq);
> +       /* flush_workqueue should provide us a barrier */
>  }

Suppose we have the tasklets T1 and T2, both are scheduled on the
same CPU. T1 takes some spinlock LOCK.

Currently it is possible to do

	spin_lock(LOCK);
	disable_tasklet(T2);

With this patch, the above code hangs.


The most simple fix is to use wait_on_work(t->work) instead of
flush_workqueue(). Currently it is static, but we can export it.
This change will speedup tasklet_disable), btw.

A better fix imho is to use cancel_work_sync() again, but this
needs some complications to preserve TASKLET_STATE_PENDING.

This in turn means that cancel_work_sync() should return "int", but
not "void". This change makes sense regardless, I'll try to make a
patch on Sunday.

Oleg.

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