Re: [PATCH v4] mm: Fix possible deadlock in kmemleak

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Aug 18, 2025 at 05:09:44PM +0800, Gu Bowen wrote:
> Our syztester report the lockdep WARNING [1], which was identified in
> stable kernel version 5.10. However, this deadlock path no longer exists
> due to the refactoring of console_lock in v6.2-rc1 [2]. Coincidentally,
> there are two types of deadlocks that we have found here. One is the ABBA
> deadlock, as mentioned above [1], and the other is the AA deadlock was
> reported by Breno [3]. The latter's deadlock issue persists.

It's better to include the lockdep warning here rather than linking to
other threads. Also since we are targeting upstream with this patch,
I don't think we should mention lockdep warnings for 5.10.

> To solve this problem, switch to printk_safe mode before printing warning
> message, this will redirect all printk()-s to a special per-CPU buffer,
> which will be flushed later from a safe context (irq work), and this
> deadlock problem can be avoided. The proper API to use should be
> printk_deferred_enter()/printk_deferred_exit() [4].
> 
> [1]
> https://lore.kernel.org/all/20250730094914.566582-1-gubowen5@huawei.com/
> [2]
> https://lore.kernel.org/all/20221116162152.193147-1-john.ogness@linutronix.de/
> [3]
> https://lore.kernel.org/all/20250731-kmemleak_lock-v1-1-728fd470198f@debian.org/#t
> [4]
> https://lore.kernel.org/all/5ca375cd-4a20-4807-b897-68b289626550@redhat.com/
> ====================
> 
> Signed-off-by: Gu Bowen <gubowen5@huawei.com>
> ---

I suggest you add the 5.10 mention here if you want, text after "---" is
normally stripped (well, not sure with Andrew's scripts).

Otherwise the patch looks fine.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>