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Date: Thu, 29 Feb 2024 15:25:19 +0000
From: Catalin Marinas <catalin.marinas@arm.com>
To: Waiman Long <longman@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Audra Mitchell <aubaker@redhat.com>
Subject: Re: [PATCH] mm/kmemleak: Don't hold kmemleak_lock when calling
 printk()
Message-ID: <ZeCh30o8i-wJVT7N@arm.com>
References: <20240228191444.481048-1-longman@redhat.com>
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In-Reply-To: <20240228191444.481048-1-longman@redhat.com>

On Wed, Feb 28, 2024 at 02:14:44PM -0500, Waiman Long wrote:
> When some error conditions happen (like OOM), some kmemleak functions
> call printk() to dump out some useful debugging information while holding
> the kmemleak_lock. This may cause deadlock as the printk() function
> may need to allocate additional memory leading to a create_object()
> call acquiring kmemleak_lock again.
> 
> Fix this deadlock issue by making sure that printk() is only called
> after releasing the kmemleak_lock.

I can't say I'm familiar with the printk() code but I always thought it
uses some ring buffers as it can be called from all kind of contexts and
allocation is not guaranteed.

If printk() ends up taking kmemleak_lock through the slab allocator, I
wonder whether we have bigger problems. The lock order is always
kmemleak_lock -> object->lock but if printk() triggers a callback into
kmemleak, we can also get object->lock -> kmemleak_lock ordering, so
another potential deadlock.

-- 
Catalin