Re: [PATCH 1/2] printk/panic: Access the main printk log in panic() only when safe

[Sergey Senozhatsky]

On (07/18/19 10:36), Petr Mladek wrote:
> On Wed 2019-07-17 18:56:15, Sergey Senozhatsky wrote:
> > On (07/16/19 09:28), Petr Mladek wrote:
> > > Kernel tries hard to store and show printk messages when panicking. Even
> > > logbuf_lock gets re-initialized when only one CPU is running after
> > > smp_send_stop().
> > > 
> > > Unfortunately, smp_send_stop() might fail on architectures that do not
> > > use NMI as a fallback. Then printk log buffer might stay locked and
> > > a deadlock is almost inevitable.
> > 
> > I'd say that deadlock is still almost inevitable.
> > 
> > panic-CPU syncs with the printing-CPU before it attempts to SMP_STOP.
> > If there is an active printing-CPU, which is looping in console_unlock(),
> > taking logbuf_lock in order to msg_print_text() and stuff, then panic-CPU
> > will spin on console_owner waiting for that printing-CPU to handover
> > printing duties.
> > 
> > 	pr_emerg("Kernel panic - not syncing");
> > 	smp_send_stop();
> 
> Good point. I forgot the handover logic. Well, it is enabled only
> around call_console_drivers(). Therefore it is not under
> lockbuf_lock.
> 
> I had in mind some infinite loop or deadlock in vprintk_store().
> There was at least one long time ago (warning triggered
> by leap second).
> 
> 
> > If printing-CPU goes nuts under logbuf_lock, has corrupted IDT or anything
> > else, then we will not progress with panic(). panic-CPU will deadlock. If
> > not on
> > 	pr_emerg("Kernel panic - not syncing")
> > 
> > then on another pr_emerg(), right before the NMI-fallback.
> 
> Nested printk() should not be problem thanks to printk_safe.

Where do nested printk()-s come from? Which one of the following
scenarios you cover in commit message:

scenario 1

- we have CPUB which holds logbuf_lock
- we have CPUA which panic()-s the system, but can't bring CPUB down,
  so logbuf_lock stays locked on remote CPU

scenario 2

- we have CPUA which holds logbuf_lock
- we have panic() on CPUA, but it cannot bring down some other CPUB
  so logbuf_lock stays locked on local CPU, and it cannot re-init
  logbuf.

	-ss