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

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Date:   Mon, 7 Oct 2019 16:49:37 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org,
        sergey.senozhatsky.work@...il.com, rostedt@...dmis.org,
        peterz@...radead.org, linux-mm@...ck.org,
        john.ogness@...utronix.de, david@...hat.com,
        linux-kernel@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

[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