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

Leonardo Bras
2024-01-18T09:37:06Z

On Thu, Jan 18, 2024 at 10:07:45AM +0106, John Ogness wrote:
> On 2024-01-16, Leonardo Bras <leobras@redhat.com> 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?

By setting ARCH_SUPPORTS_RT=y

>
> > [ 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.

Yes, as mentioned to Thomas before, I am on Vanilla torvalds/linux.
I was not aware of any extra patches for PREEMPT_RT, could you please point
the repo, or the patchset for that PREEMPT_RT-patched version?

>
> > [ 42.486094] console_flush_all+0x18d/0x3c0
> > [ 42.486111] console_unlock+0x6c/0xd0
>
> Flushing on console_unlock() is the legacy method.

Great! so this part is solved :)

>
> 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.

I am using mainline (torvalds/linux) kernel, forcing ARCH_SUPPORTS_RT:

diff --git a/arch/Kconfig b/arch/Kconfig
index 5ca66aad0d08..879c34398cb7 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1195,7 +1195,7 @@ config ARCH_NO_PREEMPT
bool

config ARCH_SUPPORTS_RT
- bool
+ def_bool y

Since I was not aware of a PREEMPT_RT-patched tree, I did this so I could
compile a PREEMPT_RT kernel.

>
> 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!
>

Sure, please let me know of where can I find the latest PREEMPT_RT patch
series so I can re-test my bug. By what you comment, it's higly probable
that patch 2/2 will not be necessary.

On the other hand, unless some extra work was done in preventing the
scenario in patch 1/2, I think that can still be discussed.

> John Ogness
>
> [0] https://lore.kernel.org/lkml/87il5o32w9.fsf@jogness.linutronix.de
>

Thanks!
Leo