From: John Ogness <john.ogness@linutronix.de>
To: Vlastimil Babka <vbabka@suse.cz>,
syzbot <syzbot+23de6daeb71241d36a18@syzkaller.appspotmail.com>,
Liam.Howlett@oracle.com, akpm@linux-foundation.org,
jannh@google.com, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, lorenzo.stoakes@oracle.com, pfalcato@suse.de,
syzkaller-bugs@googlegroups.com
Cc: Suren Baghdasaryan <surenb@google.com>,
Lorenzo Stoakes <lorenzo.stoakes@oracle.com>,
Peter Zijlstra <peterz@infradead.org>,
Petr Mladek <pmladek@suse.com>
Subject: Re: [syzbot] [mm?] possible deadlock in __vma_start_write
Date: Wed, 04 Jun 2025 10:37:43 +0206 [thread overview]
Message-ID: <84qzzzoqkw.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <f03916ab-19fc-4832-b564-58d3c885011d@suse.cz>
On 2025-06-04, Vlastimil Babka <vbabka@suse.cz> wrote:
> After consulting PeterZ on IRC, he said:
>
>> so the thing complains about: vmlock <- mapping.invalidate_lock <- bcachefs_btree <- console_lock <- &helper->lock <- dev->mode_config.idr_mutex <- mm->mmap_lock <- vmlock
>
> so bcachefs is doing printk which wants console_lock, and in
> drm console a possible lock chain exists that can lead to mmap lock
>
> To me it seems all rather theoretical, but not sure where and how we could safely
> break this chain.
> Hopefully one day console_lock goes away? :)
It is a known problem that any caller holding a lock used by a
registered legacy console will result in printk() deadlocking. This is
particularly a problem with the port->lock and power management.
One workaround is to enable CONFIG_PREEMPT_RT. ;-)
A year ago (exactly) there was patch [0] providing a "threadprintk" boot
argument that would also work around this problem.
However, the only real solution is to convert the legacy consoles to
nbcon. This is work in progress.
John
[0] https://lore.kernel.org/lkml/20240603232453.33992-17-john.ogness@linutronix.de
next prev parent reply other threads:[~2025-06-04 8:31 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-05-29 15:40 [syzbot] [mm?] possible deadlock in __vma_start_write syzbot
2025-06-02 13:04 ` Lorenzo Stoakes
2025-06-02 14:16 ` Suren Baghdasaryan
2025-06-02 14:19 ` Lorenzo Stoakes
2025-06-04 0:51 ` Suren Baghdasaryan
2025-06-04 5:17 ` Lorenzo Stoakes
2025-06-04 7:52 ` Vlastimil Babka
2025-06-04 8:31 ` John Ogness [this message]
2025-06-04 14:29 ` Suren Baghdasaryan
2025-06-05 9:40 ` Petr Mladek
2025-06-05 10:48 ` Peter Zijlstra
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