Re: [Xen-devel] [PATCH] gntdev: switch back to rwlocks

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 07/09/2010 08:56 AM, Jan Beulich wrote:
>>> Shouldn't this be solved in a way not depending on an implementation
>>> detail (rw-locks being unfair in that readers can lock out writers
>>> indefinitely)? Is it even certain that all arch-es implement rw-locks
>>> in a manner compatible with this?
>>>       
>> any rwlock implementations that allow multiple readers will do: both
>> mn_invl_range_start and gntdev_mmap only require a read lock.
>>     
> No - if an implementation forces further readers to spin once a
> writer started its attempt to acquire a lock, the code after your
> change still has the potential to deadlock afaict.
>   

Yes, relying on this kind of behaviour from rwlocks doesn't pass the
smell test.  rwlocks are just a performance optimisation for particular
locking patterns; it should always be safe to implement them as plain
spinlocks (or convert them into spinlocks).

I think removing the notifier calls from apply_to_page_range fixes the
root of the problem.

    J

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