Re: [PATCH v3] eventpoll: Fix priority inversion problem

[Xi Ruoyao <xry111@xry111.site>]

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On Thu, 2025-07-10 at 08:21 +0200, Nam Cao wrote:
> On Thu, Jul 10, 2025 at 11:08:18AM +0800, Xi Ruoyao wrote:
> > After upgrading my kernel to the recent mainline I've encountered some
> > stability issue, like:
> > 
> > - When GDM started gnome-shell, the screen often froze and the only
> > thing I could do was to switch into a VT and reboot.
> > - Sometimes gnome-shell started "fine" but then starting an application
> > (like gnome-console) needed to wait for about a minute.
> > - Sometimes the system shutdown process hangs waiting for a service to
> > stop.
> > - Rarely the system boot process hangs for no obvious reason.
> > 
> > Most strangely in all the cases there are nothing alarming in dmesg or
> > system journal.
> > 
> > I'm unsure if this is the culprit but I'm almost sure it's the trigger.
> > Maybe there's some race condition in my userspace that the priority
> > inversion had happened to hide...  but anyway reverting this patch
> > seemed to "fix" the issue.
> > 
> > Any thoughts or pointers to diagnose further?
> 
> I have been running this new epoll on my work machine for weeks by now
> without issue, while you seem to reproduce it reliably. I'm guessing that
> the problem is on some code path which is dead on my system, but executed
> on yours.

I also failed to reproduce it in a VM running the latest Fedora Rawhide
(in 3 attempts).

> I am curious if Gnome is using some epoll options which are unused on my
> system.

> I presume you can still access dmesg despite the freeze. Do you mind
> running the below patch, let me know what's in your dmesg? It may help
> identifying that code path.

Attached the system journal (dmesg was truncated due to too many lines).
I guess the relevant part should be between line 6947 ("New session 2 of
user xry111") and line 8022 ("start operation timed out. Terminating").

-- 
Xi Ruoyao <xry111@xry111.site>

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