Re: [PATCH v3] serial: 8250: always disable IRQ during THRE test

[Jiri Slaby <jirislaby@kernel.org>]

On 09. 02. 26, 12:32, Alban Bedel wrote:
> From: Peng Zhang <zhangpeng.00@bytedance.com>
> 
> commit 039d4926379b ("serial: 8250: Toggle IER bits on only after irq
> has been set up") moved IRQ setup before the THRE test, so the interrupt
> handler can run during the test and race with its IIR reads. This can
> produce wrong THRE test results and cause spurious registration of the
> serial8250_backup_timeout timer. Unconditionally disable the IRQ for the
> short duration of the test and re-enable it afterwards to avoid the race.
> 
> Cc: stable@vger.kernel.org
> Fixes: 039d4926379b ("serial: 8250: Toggle IER bits on only after irq has been set up")
> Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> Signed-off-by: Alban Bedel <alban.bedel@lht.dlh.de>
> Tested-by: Maximilian Lueer <maximilian.lueer@lht.dlh.de>
> ---
> Changelog:
> 
> v2: Replaced disable_irq_nosync() with disable_irq() to prevent interrupts
>      that are currently being handled

This made me to check, why/how this is possible. It appears to be, but 
only thanks to:
   205d300aea75 serial: 8250: change lock order in serial8250_do_startup()

And the change should be documented in the commit log. Especially to 
avoid stable backports to trees without 205d300aea75.

thanks,
-- 
js
suse labs