Re: [RFC 01/20] iommu/iommufd: Add /dev/iommu core

Jacob Pan
2021-10-19T17:07:47Z

Hi Jason,

On Tue, 19 Oct 2021 13:57:47 -0300, Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Tue, Oct 19, 2021 at 09:57:34AM -0700, Jacob Pan wrote:
> > Hi Jason,
> >
> > On Fri, 15 Oct 2021 08:18:07 -0300, Jason Gunthorpe <jgg@nvidia.com>
> > wrote:
> > > On Fri, Oct 15, 2021 at 09:18:06AM +0000, Liu, Yi L wrote:
> > >
> > > > > Acquire from the xarray is
> > > > > rcu_lock()
> > > > > ioas = xa_load()
> > > > > if (ioas)
> > > > > if (down_read_trylock(&ioas->destroying_lock))
> > > >
> > > > all good suggestions, will refine accordingly. Here destroying_lock
> > > > is a rw_semaphore. right? Since down_read_trylock() accepts a
> > > > rwsem.
> > >
> > > Yes, you probably need a sleeping lock
> > >
> > I am not following why we want a sleeping lock inside RCU protected
> > section?
>
> trylock is not sleeping
Of course, thanks for clarifying.

> > For ioas, do we really care about the stale data to choose rw_lock vs
> > RCU? Destroying can be kfree_rcu?
>
> It needs a hard fence so things don't continue to use the IOS once it
> is destroyed.
I guess RCU can do that also perhaps can scale better?

> Jason

Thanks,

Jacob