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

[Jason Gunthorpe]

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@xxxxxxxxxx> 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