linux-kernel - Re: [PATCH rcu 04/32] rcu-tasks: Drive synchronous grace periods from calling task

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Date:   Fri, 2 Sep 2022 05:04:47 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sascha Hauer <sha@...gutronix.de>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Matthew Wilcox <willy@...radead.org>,
        Zhouyi Zhou <zhouzhouyi@...il.com>, kernel@...gutronix.de
Subject: Re: [PATCH rcu 04/32] rcu-tasks: Drive synchronous grace periods
 from calling task

On Fri, Sep 02, 2022 at 01:52:28PM +0200, Sascha Hauer wrote:
> On Thu, Sep 01, 2022 at 10:33:04AM -0700, Paul E. McKenney wrote:
> > On Thu, Sep 01, 2022 at 10:27:25AM -0700, Paul E. McKenney wrote:
> > > On Thu, Sep 01, 2022 at 12:36:25PM +0200, Sascha Hauer wrote:
> > > > Hi Paul,
> > > > 
> > > > On Mon, Jun 20, 2022 at 03:53:43PM -0700, Paul E. McKenney wrote:
> > > > > This commit causes synchronous grace periods to be driven from the task
> > > > > invoking synchronize_rcu_*(), allowing these functions to be invoked from
> > > > > the mid-boot dead zone extending from when the scheduler was initialized
> > > > > to to point that the various RCU tasks grace-period kthreads are spawned.
> > > > > This change will allow the self-tests to run in a consistent manner.
> > > > > 
> > > > > Reported-by: Matthew Wilcox <willy@...radead.org>
> > > > > Reported-by: Zhouyi Zhou <zhouzhouyi@...il.com>
> > > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > > 
> > > > This commit (appeared in mainline as 4a8cc433b8bf) breaks booting my
> > > > ARMv7 based i.MX6ul board when CONFIG_PROVE_RCU is enabled. Reverting
> > > > this patch on v6.0-rc3 makes my board boot again. See below for a boot
> > > > log. The last message is "Running RCU-tasks wait API self tests", after
> > > > that the board hangs. Any idea what goes wrong here?
> > > 
> > > New one on me!
> > > 
> > > Is it possible to get a stack trace of the hang, perhaps via
> > > one form or another of sysrq-T?  Such a stack trace would likely
> > > include synchronize_rcu_tasks(), synchronize_rcu_tasks_rude(), or
> > > synchronize_rcu_tasks_trace() followed by synchronize_rcu_tasks_generic(),
> > > rcu_tasks_one_gp(), and one of rcu_tasks_wait_gp(),
> > > rcu_tasks_rude_wait_gp(), or rcu_tasks_wait_gp().
> > 
> > If there is no chance of sysrq-T, kernel debuggers, kernel crash
> > dumps, or any other source of the stack trace, please decorate the
> > code path with printk() or similar and let me know where it goes.
> > Under normal circumstances, this code path is not sensitive to performance
> > perturbations of the printk() persuasion.
> 
> Some unrelated bug I was searching for made me turn on early console
> output with the "earlycon" parameter. It turned out that when I remove
> this parameter then my board boots fine.
> 
> I then realized that even with earlycon enabled my board boots fine
> when I remove the call to
> 
> 	pr_info("Running RCU-tasks wait API self tests\n");

Ah, there are some printk() fixes in the works.  Maybe this is one area
needed that.  Or maybe not.

> Given that I am not sure how useful it is to add more printk. I did that
> anyway like this:
> 
> > static void rcu_tasks_one_gp(struct rcu_tasks *rtp, bool midboot)
> > {
> > 	int needgpcb;
> > 
> > 	printk("%s: mutex_lock... midboot: %d\n", __func__, midboot);
> > 	mutex_lock(&rtp->tasks_gp_mutex);
> > 	printk("%s: mutex_locked midboot: %d\n", __func__, midboot);
> > 
> > 	// If there were none, wait a bit and start over.
> > 	if (unlikely(midboot)) {
> > 		needgpcb = 0x2;
> > 	} else {
> > 		printk("%s: set_tasks_gp_state(RTGS_WAIT_CBS)...\n", __func__);
> > 		set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
> > 		printk("%s: rcuwait_wait_event...\n", __func__);
> > 		rcuwait_wait_event(&rtp->cbs_wait,
> > 				   (needgpcb = rcu_tasks_need_gpcb(rtp)),
> > 				   TASK_IDLE);
> >		printk("%s: rcuwait_wait_event done\n", __func__);
> > 	}
> >
> 
> What I see then is:
> 
> [    0.156362] synchronize_rcu_tasks_generic: rcu_tasks_one_gp....
> [    0.162087] rcu_tasks_one_gp: mutex_lock... midboot: 1

So one task gets stuck either in mutex_lock() or the printk() above
and some other task below moves ahead?  Or might some printk()s have
been lost?

> [    0.167386] rcu_tasks_one_gp: mutex_lock... midboot: 0
> [    0.172489] rcu_tasks_one_gp: mutex_locked midboot: 0
> [    0.177535] rcu_tasks_one_gp: set_tasks_gp_state(RTGS_WAIT_CBS)...
> [    0.183525] rcu_tasks_one_gp: rcuwait_wait_event...

Given that everything works with printk()s turned off, my current
suspicion is a printk() issue.

> Here the board hangs. After some time I get:
> 
> [  254.493010] random: crng init done

This looks unrelated.

> But that's it.
> 
> > 
> > > At this point in the boot sequence, there is only one online CPU,
> > > correct?
> 
> Yes, it's a single core system.

OK, then we should be able to rule out SMP issues.  ;-)

								Thanx, Paul

> Sascha
> 
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