RE: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst atomic

Starke, Daniel
2023-10-04T05:59:17Z

> 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/

Best regards,
Daniel Starke