Re: [RFC PATCH v2 16/19] RDMA/uverbs: Add back pointer to system file object

[Jason Gunthorpe]

On Mon, Aug 12, 2019 at 02:15:37PM -0700, Ira Weiny wrote:
> On Mon, Aug 12, 2019 at 02:56:15PM -0300, Jason Gunthorpe wrote:
> > On Mon, Aug 12, 2019 at 10:28:27AM -0700, Ira Weiny wrote:
> > > On Mon, Aug 12, 2019 at 10:00:40AM -0300, Jason Gunthorpe wrote:
> > > > On Fri, Aug 09, 2019 at 03:58:30PM -0700, ira.weiny@xxxxxxxxx wrote:
> > > > > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > > > 
> > > > > In order for MRs to be tracked against the open verbs context the ufile
> > > > > needs to have a pointer to hand to the GUP code.
> > > > > 
> > > > > No references need to be taken as this should be valid for the lifetime
> > > > > of the context.
> > > > > 
> > > > > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > > >  drivers/infiniband/core/uverbs.h      | 1 +
> > > > >  drivers/infiniband/core/uverbs_main.c | 1 +
> > > > >  2 files changed, 2 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/infiniband/core/uverbs.h b/drivers/infiniband/core/uverbs.h
> > > > > index 1e5aeb39f774..e802ba8c67d6 100644
> > > > > +++ b/drivers/infiniband/core/uverbs.h
> > > > > @@ -163,6 +163,7 @@ struct ib_uverbs_file {
> > > > >  	struct page *disassociate_page;
> > > > >  
> > > > >  	struct xarray		idr;
> > > > > +	struct file             *sys_file; /* backpointer to system file object */
> > > > >  };
> > > > 
> > > > The 'struct file' has a lifetime strictly shorter than the
> > > > ib_uverbs_file, which is kref'd on its own lifetime. Having a back
> > > > pointer like this is confouding as it will be invalid for some of the
> > > > lifetime of the struct.
> > > 
> > > Ah...  ok.  I really thought it was the other way around.
> > > 
> > > __fput() should not call ib_uverbs_close() until the last reference on struct
> > > file is released...  What holds references to struct ib_uverbs_file past that?
> > 
> > Child fds hold onto the internal ib_uverbs_file until they are closed
> 
> The FDs hold the struct file, don't they?

Only dups, there are other 'child' FDs we can create

> > Now this has unlocked updates to that data.. you'd need some lock and
> > get not zero pattern
> 
> You can't call "get" here because I'm 99% sure we only get here when struct
> file has no references left...

Nope, like I said the other FDs hold the uverbs_file independent of
the struct file it is related too. 

This is why having a back pointer like this is so ugly, it creates a
reference counting cycle

Jason