Re: [PATCH 2/2] printk: make sure to print log on console.

[Petr Mladek]

On Fri 2018-06-01 10:53:56, Petr Mladek wrote:
> On Fri 2018-06-01 13:40:50, Sergey Senozhatsky wrote:
> > On (05/31/18 14:21), Petr Mladek wrote:
> > > > 
> > > > Upstream printk has no printing kthread. And we also run
> > > > printk()->console_unlock() with disabled preemption.
> > > 
> > > Yes, the comment was wrong
> > 
> > Yes, that was the only thing I meant.
> > I really didn't have any time to look at the patch yesterday, just
> > commented on the most obvious thing.
> 
> Fair enough.
> 
> > > but the problem is real.
> > 
> > Yep, could be. But not exactly the way it is described in the commit
> > messages and the patch does not fully address the problem.
> > 
> > The patch assumes that all those events happen sequentially. While
> > in reality they can happen in parallel on different CPUs.
> > 
> > Example:
> > 
> > 	CPU0					CPU1
> > 
> > 	set console verbose
> > 
> > 	dump_backtrace()
> > 	{
> > 		// for (;;)  print frames
> > 		printk("%pS\n", frame0);
> > 		printk("%pS\n", frame1);
> > 		printk("%pS\n", frame2);
> > 		printk("%pS\n", frame3);
> > 		...				console_loglevel = CONSOLE_LOGLEVEL_SILENT;
> > 		printk("%pS\n", frame12);
> > 		printk("%pS\n", frame13);
> > 	}
> > 
> > Part of backtrace or the entire backtrace will be missed, because
> > we read the global console_loglevel. The problem is still there.
> 
> [...]
> 
> > So I'd say that most likely the following scenarios can suffer:
> > 
> > - NMI comes in, sets loglevel to X, printk-s some data, restores the
> >   loglevel back to Y
> > - IRQ comes in [like sysrq, etc] comes in and does the same thing
> > - software exception comes in and does the same thing [e.g. bust_spinlocks()
> >   at arch/s390/mm/fault.c]

I forgot to say that it was a great point and analyze.

> My view is:
> 
> The race with another printk() (console_lock owner) is much more
> likely than a race between two CPUs manipulating console_loglevel.
> 
> The proposed patch seems to be in the right direction. It is supposed
> to fix the most likely scenario. We could block it and request full
> solution but I wonder if it is worth it.
> 
> I am personally fine with this partial solution for now. We could
> always make it better if people meet the other scenarios.

I am still fine with the partial solution. Well, I will think
more about it before approving any patch.

Best Regards,
Petr