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xen-devel
Re: [Xen-devel] [PATCH] gntdev: switch back to rwlocks
>>> On 09.07.10 at 16:55, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>>> wrote:
> On Fri, 9 Jul 2010, Jan Beulich wrote:
>> >>> On 09.07.10 at 16:32, Stefano Stabellini
>> >>> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> > Hi Jeremy,
>> > this patch switches back from spinlocks to rwlocks in gntdev, solving
>> > the locking issue that was preventing stubdoms from working.
>> > In particular the problem is that gntdev_mmap calls apply_to_page_range
>> > after acquiring the spinlock. apply_to_page_range causes the mmu
>> > notifiers to be called, so mn_invl_range_start will be called that will
>> > try to acquire again the same spinlock.
>>
>> 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.
Jan
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