linux-kernel - Re: [PATCH v2] kernel/resource: Fix locking in request_free_mem

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Date:   Fri, 26 Mar 2021 09:57:51 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>, linux-kernel@...r.kernel.org
Cc:     akpm@...ux-foundation.org, daniel.vetter@...ll.ch,
        dan.j.williams@...el.com, gregkh@...uxfoundation.org,
        jhubbard@...dia.com, jglisse@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH v2] kernel/resource: Fix locking in
 request_free_mem_region

On 26.03.21 02:20, Alistair Popple wrote:
> request_free_mem_region() is used to find an empty range of physical
> addresses for hotplugging ZONE_DEVICE memory. It does this by iterating
> over the range of possible addresses using region_intersects() to see if
> the range is free.

Just a high-level question: how does this iteract with memory 
hot(un)plug? IOW, how defines and manages the "range of possible 
addresses" ?

> 
> region_intersects() obtains a read lock before walking the resource tree
> to protect against concurrent changes. However it drops the lock prior
> to returning. This means by the time request_mem_region() is called in
> request_free_mem_region() another thread may have already reserved the
> requested region resulting in unexpected failures and a message in the
> kernel log from hitting this condition:

I am confused. Why can't we return an error to the caller and let the 
caller continue searching? This feels much simpler than what you propose 
here. What am I missing?



-- 
Thanks,

David / dhildenb