From: Petr Mladek <pmladek@suse.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Qian Cai <cai@lca.pw>,
akpm@linux-foundation.org, sergey.senozhatsky.work@gmail.com,
rostedt@goodmis.org, peterz@infradead.org, linux-mm@kvack.org,
john.ogness@linutronix.de, david@redhat.com,
linux-kernel@vger.kernel.org,
Heiko Carstens <heiko.carstens@de.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>,
Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()
Date: Tue, 8 Oct 2019 09:43:57 +0200 [thread overview]
Message-ID: <20191008074357.f33f6pbs4cw5majk@pathway.suse.cz> (raw)
In-Reply-To: <20191007144937.GO2381@dhcp22.suse.cz>
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@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:
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
next prev parent reply other threads:[~2019-10-08 7:44 UTC|newest]
Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-04 22:26 [PATCH v2] mm/page_isolation: fix a deadlock with printk() Qian Cai
2019-10-07 8:07 ` Michal Hocko
2019-10-07 9:05 ` Petr Mladek
2019-10-07 11:33 ` Michal Hocko
2019-10-07 12:34 ` Qian Cai
2019-10-07 11:04 ` Qian Cai
2019-10-07 11:37 ` Michal Hocko
2019-10-07 12:11 ` Qian Cai
2019-10-07 12:43 ` Michal Hocko
2019-10-07 13:07 ` Qian Cai
2019-10-07 14:10 ` Petr Mladek
2019-10-07 14:30 ` Petr Mladek
2019-10-07 14:49 ` Michal Hocko
2019-10-08 7:43 ` Petr Mladek [this message]
2019-10-08 8:27 ` Michal Hocko
2019-10-08 12:56 ` Christian Borntraeger
2019-10-08 16:08 ` Qian Cai
2019-10-08 18:35 ` Michal Hocko
2019-10-08 19:06 ` Qian Cai
2019-10-08 19:17 ` Michal Hocko
2019-10-08 19:35 ` Qian Cai
2019-10-09 11:49 ` Petr Mladek
2019-10-09 13:06 ` Qian Cai
2019-10-09 13:27 ` Michal Hocko
2019-10-09 13:43 ` Qian Cai
2019-10-09 13:51 ` Michal Hocko
2019-10-09 14:19 ` Qian Cai
2019-10-09 14:34 ` Michal Hocko
2019-10-09 15:08 ` Qian Cai
2019-10-09 16:23 ` Michal Hocko
2019-10-09 16:23 ` Michal Hocko
2019-10-10 9:01 ` Qian Cai
2019-10-10 10:59 ` Michal Hocko
2019-10-10 13:11 ` Qian Cai
2019-10-10 14:18 ` Michal Hocko
2019-10-10 14:47 ` Qian Cai
2019-10-10 17:30 ` Michal Hocko
2019-10-10 17:48 ` Qian Cai
2019-10-10 18:06 ` Michal Hocko
2019-10-10 18:59 ` David Hildenbrand
2019-10-09 14:24 ` Petr Mladek
2019-10-09 14:46 ` Qian Cai
2019-10-10 7:57 ` Petr Mladek
2019-10-09 11:39 ` Petr Mladek
2019-10-09 13:56 ` Peter Oberparleiter
2019-10-09 14:26 ` Michal Hocko
2019-10-10 5:12 ` Sergey Senozhatsky
2019-10-10 7:40 ` Michal Hocko
2019-10-10 8:16 ` Sergey Senozhatsky
2019-10-10 8:37 ` Michal Hocko
2019-10-10 8:21 ` Petr Mladek
2019-10-10 8:39 ` Sergey Senozhatsky
2019-10-10 11:11 ` Petr Mladek
2019-10-09 15:25 ` Qian Cai
2019-10-09 15:25 ` Qian Cai
2019-10-07 14:59 ` Qian Cai
2019-10-07 15:12 ` Michal Hocko
2019-10-07 15:33 ` Qian Cai
2019-10-08 8:15 ` Petr Mladek
2019-10-08 9:32 ` Qian Cai
2019-10-08 13:13 ` Steven Rostedt
2019-10-08 13:23 ` Qian Cai
2019-10-08 13:33 ` Steven Rostedt
2019-10-08 13:42 ` Petr Mladek
2019-10-08 13:48 ` Michal Hocko
2019-10-08 14:03 ` Qian Cai
2019-10-08 14:08 ` Michal Hocko
2019-10-08 8:40 ` Michal Hocko
2019-10-08 10:04 ` Qian Cai
2019-10-08 10:39 ` Michal Hocko
2019-10-08 12:00 ` Qian Cai
2019-10-08 12:39 ` Michal Hocko
2019-10-08 13:06 ` Qian Cai
2019-10-08 13:37 ` Michal Hocko
2019-10-08 13:08 ` Petr Mladek
2019-10-08 13:33 ` Qian Cai
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