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Date:   Thu, 5 Oct 2023 10:03:11 +0100
From:   Lee Jones <lee@kernel.org>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     "Starke, Daniel" <daniel.starke@siemens.com>,
        "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
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On Wed, 04 Oct 2023, Greg Kroah-Hartman wrote:

> On Wed, Oct 04, 2023 at 01:57:04PM +0100, Lee Jones wrote:
> > On Wed, 04 Oct 2023, Greg Kroah-Hartman wrote:
> > 
> > > 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@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.
> > > > 
> > > > 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).
> > 
> > Does this solution break the recursion loop or affect performance?
> 
> This solution broke the recursion by returning an error, right?

This is the part I was least sure about.

If this was considered valid and we were to go forward with a solution
like this, what would a quality improvement look like?  Should we have
stayed in this function and waited for the previous occupant to leave
before continuing through ->write()?

> The performance one was by using mutexes as in previous attempts.

Right.  This solution was designed to avoid that.

-- 
Lee Jones [李琼斯]