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From:   "Starke, Daniel" <daniel.starke@siemens.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Lee Jones <lee@kernel.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Fedor Pchelkin <pchelkin@ispras.ru>,
        Jiri Slaby <jirislaby@kernel.org>,
        "linux-serial@vger.kernel.org" <linux-serial@vger.kernel.org>,
        "syzbot+5f47a8cea6a12b77a876@syzkaller.appspotmail.com" 
        <syzbot+5f47a8cea6a12b77a876@syzkaller.appspotmail.com>
Subject: RE: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst
 atomic
Thread-Topic: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst
 atomic
Thread-Index: AQHZ9hsms1Jp2Ee+x0K6pi4iDpsHIrA4XssAgADExJA=
Date:   Wed, 4 Oct 2023 05:59:09 +0000
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References: <20231003170020.830242-1-lee@kernel.org>
 <2023100320-immorally-outboard-573a@gregkh>
In-Reply-To: <2023100320-immorally-outboard-573a@gregkh>
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> 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@kern=
el.org/

Best regards,
Daniel Starke