On Wed 2024-10-23 17:36:04, Marcos Paulo de Souza wrote:
> On Mon, 2024-10-21 at 16:17 +0206, John Ogness wrote:
> > On 2024-10-21, Petr Mladek <pmladek@xxxxxxxx> wrote:
> > > > That will not work because migrate_enable() can only be called
> > > > from
> > > > can_sleep context. Instead, the migrate_disable()/enable() should
> > > > be at
> > > > the few (one?) call sites where
> > > > printk_loud_console_enter()/exit() is
> > > > used from task context.
> > >
> > > Hmm, if I get it correctly, we could not use migrate_disable() in
> > > __handle_sysrq() because it can be called also in atomic context,
> >
> > I am talking about callers of __handle_sysrq() and/or their callers.
> >
> > For example write_sysrq_trigger() could do:
> >
> > migrate_disable();
> > __handle_sysrq(c, false);
> > migrate_enable();
> >
> > Or a new wrapper could be introduced for this purpose:
> >
> > static inline void wrapper handle_sysrq_task(u8 key, bool check_mask)
> > {
> > migrate_disable();
> > __handle_sysrq(key, check_mask);
> > migrate_enable();
> > }
> >
> > A quick grep shows about 25 call sites to check.
> >
> > > I do not see any easy way how to distinguish whether it was called
> > > in
> > > an atomic context or not.
> >
> > There is no clean way to do that. If this information is needed, it
> > must
> > be tracked by the call chain.
> >
> > > So, I see three possibilities:
> > >
> > > 1. Explicitly call preempt_disable() in __handle_sysrq().
> > >
> > > It would be just around the the single line or the help. But
> > > still,
> > > I do not like it much.
> >
> > Not acceptable for PREEMPT_RT since sysrq is exposed to external
> > inputs.
> >
> > > 2. Avoid the per-CPU variable. Force adding the
> > > LOUD_CON/FORCE_CON
> > > flag using a global variable, e.g. printk_force_console.
> > >
> > > The problem is that it might affect also messages printed by
> > > other CPUs. And there might be many.
> > >
> > > Well, console_loglevel is a global variable. The original code
> > > had a similar problem.
> >
> > If disabling migration is not an option for you, this would be my
> > second
> > choice. I assume tagging too many messages is better than not tagging
> > enough. And, as you say, this is effectively what the current code is
> > trying to do.
>
> Thanks for your input John. I talked with Petr and he suggested to
> follow this option. I'll prepare the changes and send them after
> reviewing them with Petr.
Just for record. I propose this way because disabling migration would
require checking all callers (25 as mentioned above).
migrate_disable() is needed and can be called only in the task
context.
I do not think that it is worth the effort. And it would be error
prone (hard to maintain).
Best Regards,
Petr
