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

[Greg Kroah-Hartman]

On Wed, Oct 04, 2023 at 09:57:20AM +0100, Lee Jones wrote:
> On Wed, 04 Oct 2023, Greg Kroah-Hartman wrote:
> 
> > 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.
> 
> That's correct.  When I initially saw this report, my initial thought
> was to replace the spinlocks with mutexts, but having read the previous
> accepted attempt and it's subsequent reversion I started to think of
> other ways to solve this issue.  This solution, unlike the last, does
> not involve adding sleep inducing locks into atomic contexts, nor
> should it negatively affect performance.
> 
> > > 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@xxxxxxxxx/
> > > Link: https://lore.kernel.org/all/20221212023530.2498025-1-zengheng4@xxxxxxxxxx/
> > > Link: https://lore.kernel.org/all/5a994a13-d1f2-87a8-09e4-a877e65ed166@xxxxxxxxxx/
> > 
> > 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.
> 
> What does the "better solution" look like?

One that actually fixes the root problem here (i.e. does not break the
recursion loop, or cause a performance decrease for normal users, or
prevent this from being bound to the console).

thanks,

greg k-h