Re: [PATCH 5.15.y] tty: add the option to have a tty reject a new ldisc

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, May 09, 2025 at 05:14:54PM +0800, jianqi.ren.cn@windriver.com wrote:
> From: Linus Torvalds <torvalds@linux-foundation.org>
> 
> [ Upstream commit 6bd23e0c2bb6c65d4f5754d1456bc9a4427fc59b ]
> 
> ... and use it to limit the virtual terminals to just N_TTY.  They are
> kind of special, and in particular, the "con_write()" routine violates
> the "writes cannot sleep" rule that some ldiscs rely on.
> 
> This avoids the
> 
>    BUG: sleeping function called from invalid context at kernel/printk/printk.c:2659
> 
> when N_GSM has been attached to a virtual console, and gsmld_write()
> calls con_write() while holding a spinlock, and con_write() then tries
> to get the console lock.

WHy do you want this in 5.15 and older kernels?  You have already
disabledf n_gsm from your kernels already so this isn't an issue, right?
Unless you have this hardware, and explicitly know what is talking to
it, that is the recommendation for this code.

And how was this and the 5.10.y backport tested?  Did you see the above
"BUG:" line without it?

thanks,

greg k-h