Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

[Petr Mladek]

On Mon 2019-10-07 16:49:37, Michal Hocko wrote:
> [Cc s390 maintainers - the lockdep is http://lkml.kernel.org/r/1570228005-24979-1-git-send-email-cai@xxxxxx
>  Petr has explained it is a false positive
>  http://lkml.kernel.org/r/20191007143002.l37bt2lzqtnqjqxu@xxxxxxxxxxxxxxx]
> On Mon 07-10-19 16:30:02, Petr Mladek wrote:
> [...]
> > I believe that it cannot really happen because:
> > 
> > 	static int __init
> > 	sclp_console_init(void)
> > 	{
> > 	[...]
> > 		rc = sclp_rw_init();
> > 	[...]
> > 		register_console(&sclp_console);
> > 		return 0;
> > 	}
> > 
> > sclp_rw_init() is called before register_console(). And
> > console_unlock() will never call sclp_console_write() before
> > the console is registered.
> > 
> > AFAIK, lockdep only compares existing chain of locks. It does
> > not know about console registration that would make some
> > code paths mutually exclusive.
> > 
> > I believe that it is a false positive. I do not know how to
> > avoid this lockdep report. I hope that it will disappear
> > by deferring all printk() calls rather soon.
> 
> Thanks a lot for looking into this Petr. I have also checked the code
> and I really fail to see why the allocation has to be done under the
> lock in the first place. sclp_read_sccb and sclp_init_sccb are global
> variables but I strongly suspect that they need a synchronization during
> early init, callbacks are registered only later IIUC:

Good idea. It would work when the init function is called only once.
But see below.

> diff --git a/drivers/s390/char/sclp.c b/drivers/s390/char/sclp.c
> index d2ab3f07c008..4b1c033e3255 100644
> --- a/drivers/s390/char/sclp.c
> +++ b/drivers/s390/char/sclp.c
> @@ -1169,13 +1169,13 @@ sclp_init(void)
>  	unsigned long flags;
>  	int rc = 0;
>  
> +	sclp_read_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA);
> +	sclp_init_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA);
>  	spin_lock_irqsave(&sclp_lock, flags);
>  	/* Check for previous or running initialization */
>  	if (sclp_init_state != sclp_init_state_uninitialized)
>  		goto fail_unlock;

It seems that sclp_init() could be called several times in parallel.
I see it called from sclp_register() and sclp_initcall().

I am not sure if it is really needed or if it is just a strange
desing.

It might be still possible to always do the allocation without the lock
and free the memory when it is not really used. But I am not sure
if we want to do this exercise just to avoid lockdep false positive.

Best Regards,
Petr