linux-kernel - Re: [patch V2 9/9] tasklets: Prevent kill/unlock

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Message-ID: <20201207142727.GU3021@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Dec 2020 15:27:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paul McKenney <paulmck@...nel.org>
Subject: Re: [patch V2 9/9] tasklets: Prevent kill/unlock_wait deadlock on RT

On Mon, Dec 07, 2020 at 03:00:40PM +0100, Sebastian Andrzej Siewior wrote:
> On 2020-12-07 12:47:43 [+0100], Peter Zijlstra wrote:
> > On Fri, Dec 04, 2020 at 06:02:00PM +0100, Thomas Gleixner wrote:
> > > @@ -825,7 +848,20 @@ void tasklet_kill(struct tasklet_struct
> > >  
> > >  	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
> > >  		do {
> > > -			yield();
> > >  		} while (test_bit(TASKLET_STATE_SCHED, &t->state));
> > >  	}
> > >  	tasklet_unlock_wait(t);
> > 
> > 
> > Egads... should we not start by doing something like this?
> 
> So we keep the RT part as-is and replace the non-RT bits with this?

For RT you probably want to wrap the wait_var_event() in that
local_bh_disable()/enable() pear. I just figured those unbounded
spin/yield loops suck and we should get rid of em.