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

[Ira Weiny]

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?

> 
> > Perhaps I need to add this (untested)?
> > 
> > diff --git a/drivers/infiniband/core/uverbs_main.c
> > b/drivers/infiniband/core/uverbs_main.c
> > index f628f9e4c09f..654e774d9cf2 100644
> > +++ b/drivers/infiniband/core/uverbs_main.c
> > @@ -1125,6 +1125,8 @@ static int ib_uverbs_close(struct inode *inode, struct file *filp)
> >         list_del_init(&file->list);
> >         mutex_unlock(&file->device->lists_mutex);
> >  
> > +       file->sys_file = NULL;
> 
> 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...  I could be wrong.  It took me a while to work
through the reference counting so I could have missed something.

Ira