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

[Michal Hocko <mhocko@kernel.org>]

[Cc s390 maintainers - the lockdep is http://lkml.kernel.org/r/1570228005-24979-1-git-send-email-cai@lca.pw
 Petr has explained it is a false positive
 http://lkml.kernel.org/r/20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz]
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:

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;
 	sclp_init_state = sclp_init_state_initializing;
-	sclp_read_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA);
-	sclp_init_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA);
 	BUG_ON(!sclp_read_sccb || !sclp_init_sccb);
 	/* Set up variables */
 	INIT_LIST_HEAD(&sclp_req_queue);
-- 
Michal Hocko
SUSE Labs