Re: [PATCH printk v2 00/18] add threaded printing + the rest

[John Ogness <john.ogness@linutronix.de>]

Hi Juri,

On 2024-06-04, Juri Lelli <juri.lelli@redhat.com> wrote:
> Our QE reported something like the following while testing the latest
> rt-devel branch (I then could reproduce with this set applied on top of
> linux-next).
>
> ---
> ... kernel: INFO: task khugepaged:351 blocked for more than 1 seconds.
> ... kernel:       Not tainted 6.9.0-thrdprintk+ #3
> ... kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> ... kernel: task:khugepaged      state:D stack:0     pid:351   tgid:351   ppid:2      flags:0x00004000
> ... kernel: Call Trace:
> ... kernel:  <TASK>
> ... kernel:  __schedule+0x2bd/0x7f0
> ... kernel:  ? __lock_release.isra.0+0x5e/0x170
> ... kernel:  schedule+0x3d/0x100
> ... kernel:  schedule_timeout+0x1ca/0x1f0
> ... kernel:  ? mark_held_locks+0x49/0x80
> ... kernel:  ? _raw_spin_unlock_irq+0x24/0x50
> ... kernel:  ? lockdep_hardirqs_on+0x77/0x100
> ... kernel:  __wait_for_common+0xb7/0x220
> ... kernel:  ? __pfx_schedule_timeout+0x10/0x10
> ... kernel:  __flush_work+0x70/0x90
> ... kernel:  ? __pfx_wq_barrier_func+0x10/0x10
> ... kernel:  __lru_add_drain_all+0x179/0x210
> ... kernel:  khugepaged+0x73/0x200
> ... kernel:  ? lockdep_hardirqs_on+0x77/0x100
> ... kernel:  ? _raw_spin_unlock_irqrestore+0x38/0x60
> ... kernel:  ? __pfx_khugepaged+0x10/0x10
> ... kernel:  kthread+0xec/0x120
> ... kernel:  ? __pfx_kthread+0x10/0x10
> ... kernel:  ret_from_fork+0x2d/0x50
> ... kernel:  ? __pfx_kthread+0x10/0x10
> ... kernel:  ret_from_fork_asm+0x1a/0x30
> ... kernel:  </TASK>
> ... kernel:
> ...         Showing all locks held in the system:
> ... kernel: 1 lock held by khungtaskd/345:
> ... kernel:  #0: ffffffff8cbff1c0 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x32/0x1d0
> ... kernel: BUG: using smp_processor_id() in preemptible [00000000] code: khungtaskd/345
> ... kernel: caller is nbcon_get_cpu_emergency_nesting+0x25/0x40
> ... kernel: CPU: 30 PID: 345 Comm: khungtaskd Kdump: loaded Not tainted 6.9.0-thrdprintk+ #3
> ... kernel: Hardware name: Dell Inc. PowerEdge R740/04FC42, BIOS 2.10.2 02/24/2021
> ... kernel: Call Trace:
> ... kernel:  <TASK>
> ... kernel:  dump_stack_lvl+0x7f/0xa0
> ... kernel:  check_preemption_disabled+0xbf/0xe0
> ... kernel:  nbcon_get_cpu_emergency_nesting+0x25/0x40
> ... kernel:  nbcon_cpu_emergency_flush+0xa/0x60
> ... kernel:  debug_show_all_locks+0x9d/0x1d0
> ... kernel:  check_hung_uninterruptible_tasks+0x4f0/0x540
> ... kernel:  ? check_hung_uninterruptible_tasks+0x185/0x540
> ... kernel:  ? __pfx_watchdog+0x10/0x10
> ... kernel:  watchdog+0x99/0xa0
> ... kernel:  kthread+0xec/0x120
> ... kernel:  ? __pfx_kthread+0x10/0x10
> ... kernel:  ret_from_fork+0x2d/0x50
> ... kernel:  ? __pfx_kthread+0x10/0x10
> ... kernel:  ret_from_fork_asm+0x1a/0x30
> ... kernel:  </TASK>
> ---
>
> It requires DEBUG_PREEMPT and LOCKDEP enabled, sched_rt_runtime_us = -1
> and a while(1) loop running at FIFO for some time (I also set sysctl
> kernel.hung_task_timeout_secs=1 to speed up reproduction).
>
> Looks like check_hung_uninterruptible_tasks() requires some care as you
> did already in linux-next for panic, rcu and lockdep ("Make emergency
> sections ...")?

Yes, that probably is a good candidate for emergency mode.

However, your report is also identifying a real issue:
nbcon_cpu_emergency_flush() was implemented to be callable from
non-emergency contexts (in which case it should do nothing). However, in
order to check if it is an emergency context, migration needs to be
disabled.

Perhaps the below change can be made for v2 of this series?

John

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 4b9645e7ed70..eeaf8465f492 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1581,8 +1581,19 @@ void nbcon_cpu_emergency_exit(void)
  */
 void nbcon_cpu_emergency_flush(void)
 {
+	bool is_emergency;
+
+	/*
+	 * If the current context is not an emergency context, preemption
+	 * might be enabled. To be sure, disable preemption when checking
+	 * if this is an emergency context.
+	 */
+	preempt_disable();
+	is_emergency = (*nbcon_get_cpu_emergency_nesting() != 0);
+	preempt_enable();
+
 	/* The explicit flush is needed only in the emergency context. */
-	if (*(nbcon_get_cpu_emergency_nesting()) == 0)
+	if (!is_emergency)
 		return;
 
 	nbcon_atomic_flush_pending();