Re: [PATCH] livepatch: Avoid hard lockup caused by klp_try_switch_task()

[Petr Mladek <pmladek@suse.com>]

On Fri 2025-01-31 21:22:13, zhang warden wrote:
> 
> 
> > On Jan 22, 2025, at 20:50, Petr Mladek <pmladek@suse.com> wrote:
> > 
> > With this patch, any operation which takes the tasklist_lock might
> > break klp_try_complete_transition(). I am afraid that this might
> > block the transition for a long time on huge systems with some
> > specific loads.
> > 
> > And the problem is caused by a printk() added just for debugging.
> > I wonder if you even use a slow serial port.
> > 
> > You might try to use printk_deferred() instead. Also you might need
> > to disable interrupts around the read_lock()/read_unlock() to
> > make sure that the console handling will be deferred after
> > the tasklist_lock gets released.
> > 
> > Anyway, I am against this patch.
> > 
> > Best Regards,
> > Petr
> 
> Hi, Petr.
> 
> I am unfamiliar with the function `rwlock_is_contended`, but it seems this function will not block and just only check the status of the rw_lock.
> 
> If I understand it right, the problem would raise from the `break` which will stop the process of `for_each_process_thread`, right?

You got it right. I am afraid that it might create a livelock
situation for the livepatch transition. I mean that the check
might almost always break on systems with thousands of processes
and frequently created/exited processes. It always has
to start from the beginning.

Best Regards,
Petr