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

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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





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