Re: INFO: rcu detected stall in smp_call_function

[Paul E. McKenney]

On Wed, Aug 26, 2020 at 07:07:33AM -0700, Paul E. McKenney wrote:
> On Wed, Aug 26, 2020 at 11:51:44AM +0200, peterz@xxxxxxxxxxxxx wrote:
> > On Tue, Aug 25, 2020 at 08:48:41AM -0700, Paul E. McKenney wrote:
> > 
> > > > Paul, I wanted to use this function, but found it has very weird
> > > > semantics.
> > > > 
> > > > Why do you need it to (remotely) call @func when p is current? The user
> > > > in rcu_print_task_stall() explicitly bails in this case, and the other
> > > > in rcu_wait_for_one_reader() will attempt an IPI.
> > > 
> > > Good question.  Let me look at the invocations:
> > > 
> > > o	trc_wait_for_one_reader() bails on current before
> > > 	invoking try_invoke_on_locked_down_task():
> > > 
> > > 	if (t == current) {
> > > 		t->trc_reader_checked = true;
> > > 		trc_del_holdout(t);
> > > 		WARN_ON_ONCE(t->trc_reader_nesting);
> > > 		return;
> > > 	}
> > > 
> > > o	rcu_print_task_stall() might well invoke on the current task,
> > > 	low though the probability of this happening might be.	(The task
> > > 	has to be preempted within an RCU read-side critical section
> > > 	and resume in time for the scheduling-clock irq that will report
> > > 	the RCU CPU stall to interrupt it.)
> > > 
> > > 	And you are right, no point in an IPI in this case.
> > > 
> > > > Would it be possible to change this function to:
> > > > 
> > > >  - blocked task: call @func with p->pi_lock held
> > > >  - queued, !running task: call @func with rq->lock held
> > > >  - running task: fail.
> > > > 
> > > > ?
> > > 
> > > Why not a direct call in the current-task case, perhaps as follows,
> > > including your change above?  This would allow the RCU CPU stall
> > > case to work naturally and without the IPI.
> > > 
> > > Would that work for your use case?
> > 
> > It would in fact, but at this point I'd almost be inclined to stick the
> > IPI in as well. But small steps I suppose. So yes.
> 
> If interrupts are disabled, won't a self-IPI deadlock?
> 
> But good point that the current-task case could be the only case invoking
> the function with interrupts enabled, which now that you mention it does
> sound like an accident waiting to happen.  So how about the following
> instead?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8471a0f..f8ed04c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2997,7 +2997,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>   * state while invoking @func(@arg).  This function can use ->on_rq and
>   * task_curr() to work out what the state is, if required.  Given that
>   * @func can be invoked with a runqueue lock held, it had better be quite
> - * lightweight.
> + * lightweight.  Note that the current task is implicitly locked down.
>   *
>   * Returns:
>   *	@false if the task slipped out from under the locks.
> @@ -3006,12 +3006,17 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>   */
>  bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg)
>  {
> -	bool ret = false;
>  	struct rq_flags rf;
> +	bool ret = false;
>  	struct rq *rq;
>  
> -	lockdep_assert_irqs_enabled();
> -	raw_spin_lock_irq(&p->pi_lock);
> +	if (p == current) {
> +		local_irq_save(rf.flags);
> +		ret = func(p, arg);
> +		local_irq_restore(rf.flags);
> +		return ret;
> +	}

Is this "if" statement anything more than an optimization, and a dubious
one at that?  It looks to me like the current task would otherwise end
up acquiring its own ->pi_lock, then acquiring the rq lock which cannot
change due to interrupts being disabled.  And then invoking the same
function with interrupts disabled, returning the same result.

Am I missing something subtle here?

						Thanx, Paul

> +	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
>  	if (p->on_rq) {
>  		rq = __task_rq_lock(p, &rf);
>  		if (task_rq(p) == rq)
> @@ -3028,7 +3033,7 @@ bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct t
>  				ret = func(p, arg);
>  		}
>  	}
> -	raw_spin_unlock_irq(&p->pi_lock);
> +	raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
>  	return ret;
>  }
>