Re: [PATCH v2 07/17] debugobjects: Move printk out of db lock critical sections

[Sergey Senozhatsky]

On (11/22/18 14:57), Waiman Long wrote:
> > [..]
> >> As a side note, one of the test systems that I used generated a
> >> debugobjects splat in the bootup process and the system hanged
> >> afterward. Applying this patch alone fix the hanging problem and the
> >> system booted up successfully. So it is not really a good idea to call
> >> printk() while holding a raw spinlock.
> > Right, I like this patch.
> > And I think that we, maybe, can go even further.
> >
> > Some serial consoles call mod_timer(). So what we could have with the
> > debug objects enabled was
> >
> > 	mod_timer()
> > 	 lock_timer_base()
> > 	  debug_activate()
> > 	   printk()
> > 	    call_console_drivers()
> > 	     foo_console()
> > 	      mod_timer()
> > 	       lock_timer_base()       << deadlock
> >
> > That's one possible scenario. The other one can involve console's
> > IRQ handler, uart port spinlock, mod_timer, debug objects, printk,
> > and an eventual deadlock on the uart port spinlock. This one can
> > be mitigated with printk_safe. But mod_timer() deadlock will require
> > a different fix.
> >
> > So maybe we need to switch debug objects print-outs to _always_
> > printk_deferred(). Debug objects can be used in code which cannot
> > do direct printk() - timekeeping is just one example.
> 
> Actually, I don't think that was the cause of the hang.

Oh, I didn't suggest that this was the case. Just talked about more
problems with printk in debug objects. Serial consoles call mod_time,
mod_timer calls debug objects, debug objects call printk and end up
in serial console again. Serial consoles are not re-entrant at this
point.

> The debugobjects splat was caused by debug_object_is_on_stack(), below
> was the output:
>
> [    6.890048] ODEBUG: object (____ptrval____) is NOT on stack
> (____ptrval____), but annotated.
> [    6.891000] WARNING: CPU: 28 PID: 1 at lib/debugobjects.c:369
> __debug_object_init.cold.11+0x51/0x2d6
[..]
>    11.270654] systemd[1]: systemd 239 running in system mode. (+PAM
> +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP
> +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN
> +PCRE2 default-hierarchy=legacy)
> [   11.311307] systemd[1]: Detected architecture x86-64.
> [   11.316420] systemd[1]: Running in initial RAM disk.
> 
> Welcome to
> 
> The system is not responsive at this point.
> 
> I am not totally sure what caused this.

Hmm, me neither.

	-ss