Re: [PATCH v2] kernel/resource: Fix locking in request_free_mem_region

David Hildenbrand
2021-03-26T08:58:02Z

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