linux-kernel - Re: [PATCH] drivers/base/memory: Fix a reference counting issue in __add_memory

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Date:   Fri, 22 Apr 2022 09:34:57 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Scott Cheloha <cheloha@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] drivers/base/memory: Fix a reference counting issue in
 __add_memory_block()

On Fri 22-04-22 09:15:21, Christophe JAILLET wrote:
> There is no point in doing put_device()/device_unregister() on a
> device that has just been registered a few lines above. This will lead to
> a double reference decrement.

This is a bit confusing. I would rephrase.
"
__add_memory_block calls both put_device and device_unregister when
storing the memory block into the xarray. This is incorrect because
xarray doesn't take an additional reference and device_unregister
already calls put_device.
"

Btw. I do not think this failure path can be triggered, or is there a
way to hit it?
 
> I guess that this put_device()/device_unregister() is a cut'n'paste from
> remove_memory_block() (i.e. unregister_memory() at the time being) which
> does need it.
> 
> Fixes: 4fb6eabf1037 ("drivers/base/memory.c: cache memory blocks in xarray to accelerate lookup")
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>

Other than that looks good to me. With the changelog clarified,
especially the part that evaluates whether this is a real or a
theoretical problem, feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
>  drivers/base/memory.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index 7222ff9b5e05..084d67fd55cc 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -636,10 +636,9 @@ static int __add_memory_block(struct memory_block *memory)
>  	}
>  	ret = xa_err(xa_store(&memory_blocks, memory->dev.id, memory,
>  			      GFP_KERNEL));
> -	if (ret) {
> -		put_device(&memory->dev);
> +	if (ret)
>  		device_unregister(&memory->dev);
> -	}
> +
>  	return ret;
>  }
>  
> -- 
> 2.32.0

-- 
Michal Hocko
SUSE Labs