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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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.


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