Re: [RFC][PATCH v6 1/2] printk: Make printk() completely async
From: Jan Kara
Date: Wed Mar 23 2016 - 09:19:55 EST
On Wed 23-03-16 10:25:41, Petr Mladek wrote:
> On Wed 2016-03-23 10:24:43, Sergey Senozhatsky wrote:
> > On (03/22/16 17:36), Petr Mladek wrote:
> > > > - /* cpu currently holding logbuf_lock in this function */
> > > > - static unsigned int logbuf_cpu = UINT_MAX;
> > > > + bool in_panic = console_loglevel == CONSOLE_LOGLEVEL_MOTORMOUTH;
> > >
> > > I am just looking at the printk in NMI patchset and I will need to
> > > deal with the panic state as well. I am not sure if this detection
> > > is secure.
> > >
> > > This console level is set also by kdb_show_stack()
> > > and kdb_dumpregs(). I am not sure how this kdb stuff works
> > > and if it affects normal kernel but...
> > >
> > > Anyway, it seems that many locations detects the panic situation
> > > via the variable oops_in_progress. It has another advantage
> > > that it can be easily checked and we would not need any extra
> > > variable here.
> > oops_in_progress is not my favorite global. and we can't rely on it
> > in async printk.
> > in panic() we have
> > console_verbose();
> > bust_spinlocks(1); << sets to one
> > pr_emerg("Kernel panic - not syncing: %s\n", buf);
> > smp_send_stop();
> > bust_spinlocks(0); << sets it back to zero
> > console_flush_on_panic();
> > there are several issues here.
> > - first, panic_cpu does not see oops_in_progress right after bust_spinlocks(0).
> > thus all printk issued from panic_cpu can go via async printk.
> I though that it actually could be an advantage. console_verbore() is
> called also by oops_begin() and it does not need to be fatal. But you
> are right that it does not need to be the righ approach.
If we oops, I want printk to be sync regardless whether the machine is able
to live afterwards or not. You never know in advance... That's why I've
chosen the console_verbose() trigger and I still think it is better than
oops_in_progress or special console_panic() trigger.
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR