Re: [RESEND RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic context

[John Ogness]

On 2024-01-16, Leonardo Bras <leobras@xxxxxxxxxx> wrote:
> Well, at least in an PREEMPT_RT=y kernel I have found this same bug 
> reproducing several times, and through the debugging that I went through I 
> saw no mechanism for preventing it.
>
> This is one example of the bug:
> While writing to serial with serial8250_tx_chars in a irq_thread handler
> there is an interruption, and __report_bad_irq() tries to printk 
> stuff to the console, and when printk goes down to 
> serial8250_console_write() and tried to get the port->lock, which causes 
> the "BUG: scheduling while atomic":
>
> [   42.485878] irq 4: nobody cared (try booting with the "irqpoll" option)
> [   42.485886] BUG: scheduling while atomic: irq/4-ttyS0/751/0x00010002
> [   42.485890] Modules linked in:
> [   42.485892] Preemption disabled at:
> [   42.485893] [<ffffffff8118ac80>] irq_enter_rcu+0x10/0x80
> [   42.485919] CPU: 0 PID: 751 Comm: irq/4-ttyS0 Not tainted 6.7.0-rc6+ #6

This is 6.7.0-rc6+. How are you setting PREEMPT_RT?

> [   42.485927] Hardware name: Red Hat KVM/RHEL, BIOS 1.16.3-1.el9 04/01/2014
> [   42.485929] Call Trace:
> [   42.485940]  <IRQ>
> [   42.485944]  dump_stack_lvl+0x33/0x50
> [   42.485976]  __schedule_bug+0x89/0xa0
> [   42.485991]  schedule_debug.constprop.0+0xd1/0x120
> [   42.485996]  __schedule+0x50/0x690
> [   42.486026]  schedule_rtlock+0x1e/0x40
> [   42.486029]  rtlock_slowlock_locked+0xe7/0x2b0
> [   42.486047]  rt_spin_lock+0x41/0x60
> [   42.486051]  serial8250_console_write+0x1be/0x460

On PREEMPT_RT-patched kernel, serial8250_console_write() is not
compiled. So obviously you are not running a PREEMPT_RT-patched kernel.

> [   42.486094]  console_flush_all+0x18d/0x3c0
> [   42.486111]  console_unlock+0x6c/0xd0

Flushing on console_unlock() is the legacy method.

I assume you are using a mainline kernel with forced threading of
irqs. Mainline has many known problems with console printing, including
calling printk when the port->lock is held.

This has been discussed before [0].

> [   42.486117]  vprintk_emit+0x1d6/0x290
> [   42.486122]  _printk+0x58/0x80
> [   42.486139]  __report_bad_irq+0x26/0xc0
> [   42.486147]  note_interrupt+0x2a1/0x2f0
> [   42.486155]  handle_irq_event+0x84/0x90
> [   42.486161]  handle_edge_irq+0x9f/0x260
> [   42.486168]  __common_interrupt+0x68/0x100
> [   42.486178]  common_interrupt+0x9f/0xc0
> [   42.486184]  </IRQ>

If you want to fix any threaded irq problems relating to printk and
console drivers, please use the latest PREEMPT_RT patch series with
CONFIG_PREEMPT_RT enabled. This is the current work that is being
reviewed on LKML for mainline inclusion. Thanks!

John Ogness

[0] https://lore.kernel.org/lkml/87il5o32w9.fsf@xxxxxxxxxxxxxxxxxxxxx