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

Jason Gunthorpe
2021-10-19T16:57:53Z

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

> 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.

Jason