linux-kernel - Re: There is a Tasks RCU stall warning

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Message-ID: <20170411174953.46adbf1e@gandalf.local.home>
Date:   Tue, 11 Apr 2017 17:49:53 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: There is a Tasks RCU stall warning

On Tue, 11 Apr 2017 14:44:43 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> Works for me!
> 
> Hopefully it will also work for your computer.  :-)
> 
> And whew!  Glad to see that the stall warnings worked!

Ah! but I think I found a bug in synchronize_rcu_tasks()!

Calling schedule isn't good enough. For rcu_tasks to continue, the task
needs to schedule out. With my updated code, I just triggered:

[  196.276868] INFO: rcu_tasks detected stalls on tasks:
[  196.284294] ffff8800c26f8040: .. nvcsw: 2/2 holdout: 1 idle_cpu: -1/1
[  196.293175] event_benchmark R  running task    30536  1127      2 0x10000000
[  196.302746] Call Trace:
[  196.307640]  ? _raw_spin_unlock_irq+0x1f/0x50
[  196.314453]  __schedule+0x222/0x1210
[  196.320476]  ? pci_mmcfg_check_reserved+0xc0/0xc0
[  196.327616]  ? preempt_count_add+0xb7/0xf0
[  196.334174]  ? __asan_store8+0x15/0x70
[  196.340384]  schedule+0x57/0xe0
[  196.345888]  benchmark_event_kthread+0x2e/0x3c0
[  196.352823]  kthread+0x178/0x1d0
[  196.358411]  ? trace_benchmark_reg+0x80/0x80
[  196.365073]  ? kthread_create_on_node+0xa0/0xa0
[  196.371999]  ret_from_fork+0x2e/0x40

And here my benchmark called schedule(), but nothing scheduled it out,
and it still fails on rcu_tasks.

-- Steve