linux-kernel - Re: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst atomic

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Date:   Wed, 4 Oct 2023 08:05:11 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     "Starke, Daniel" <daniel.starke@...mens.com>
Cc:     Lee Jones <lee@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Fedor Pchelkin <pchelkin@...ras.ru>,
        Jiri Slaby <jirislaby@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "syzbot+5f47a8cea6a12b77a876@...kaller.appspotmail.com" 
        <syzbot+5f47a8cea6a12b77a876@...kaller.appspotmail.com>
Subject: Re: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst
 atomic

On Wed, Oct 04, 2023 at 05:59:09AM +0000, Starke, Daniel wrote:
> > Daniel, any thoughts?
> 
> Our application of this protocol is only with specific modems to enable
> circuit switched operation (handling calls, selecting/querying networks,
> etc.) while doing packet switched communication (i.e. IP traffic over PPP).
> The protocol was developed for such use cases.
> 
> Regarding the issue itself:
> There was already an attempt to fix all this by switching from spinlocks to
> mutexes resulting in ~20% performance loss. However, the patch was reverted
> as it did not handle the T1 timer leading into sleep during atomic within
> gsm_dlci_t1() on every mutex lock there.
> There was also a suggestion to fix this in do_con_write() as
> tty_operations::write() appears to be documented as "not allowed to sleep".
> The patch for this was rejected. It did not fix the issue within n_gsm.
> 
> Link: https://lore.kernel.org/all/20221203215518.8150-1-pchelkin@ispras.ru/
> Link: https://lore.kernel.org/all/20221212023530.2498025-1-zengheng4@huawei.com/
> Link: https://lore.kernel.org/all/5a994a13-d1f2-87a8-09e4-a877e65ed166@kernel.org/

Ok, I thought I remembered this, I'll just drop this patch from my
review queue and wait for a better solution if it ever comes up as this
isn't a real issue that people are seeing on actual systems, but just a
syzbot report.

thanks,

greg k-h