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Date: Thu, 7 Mar 2024 11:46:30 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Waiman Long <longman@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, Audra Mitchell <aubaker@redhat.com>
Subject: Re: [PATCH v2] mm/kmemleak: Don't hold kmemleak_lock when calling
 printk()
Message-Id: <20240307114630.32702099ac24c182b91da517@linux-foundation.org>
In-Reply-To: <20240307184707.961255-1-longman@redhat.com>
References: <20240307184707.961255-1-longman@redhat.com>
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On Thu,  7 Mar 2024 13:47:07 -0500 Waiman Long <longman@redhat.com> 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.
> 
> An abbreviated lockdep splat is as follows:
>
> ...
> 
> Fix this deadlock issue by making sure that printk() is only called
> after releasing the kmemleak_lock.
> 
> ...
>
> @@ -427,9 +442,19 @@ static struct kmemleak_object *__lookup_object(unsigned long ptr, int alias,
>  		else if (untagged_objp == untagged_ptr || alias)
>  			return object;
>  		else {
> +			if (!get_object(object))
> +				break;
> +			/*
> +			 * Release kmemleak_lock temporarily to avoid deadlock
> +			 * in printk(). dump_object_info() is called without
> +			 * holding object->lock (race unlikely).
> +			 */
> +			raw_spin_unlock(&kmemleak_lock);
>  			kmemleak_warn("Found object by alias at 0x%08lx\n",
>  				      ptr);
>  			dump_object_info(object);
> +			put_object(object);
> +			raw_spin_lock(&kmemleak_lock);
>  			break;

Please include a full description of why this is safe.  Once we've
dropped that lock, the tree is in an unknown state and we shouldn't
touch it again.  This consideration should be added to the relevant
functions' interface documentation and the code should be reviewed to
ensure that we're actually adhering to this.  Or something like that.

To simply drop and reacquire a lock without supporting analysis and
comments does not inspire confidence!