Re: [PATCH v2 15/24] proc/fd: In proc_readfd_common use task_lookup_next_fd_rcu

[Al Viro]

On Tue, Dec 08, 2020 at 10:24:57PM -0600, Eric W. Biederman wrote:
> Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes:
> 
> > On Tue, Dec 08, 2020 at 04:38:09PM -0600, Eric W. Biederman wrote:
> >
> >> Is there any reason we don't simply rcu free the files_struct?
> >> That would remove the need for the task_lock entirely.
> >
> > Umm...  Potentially interesting part here is the interaction with
> > close_files(); currently that can't overlap with any of those
> > 3rd-party accesses to descriptor table, but with your changes
> > here it's very much possible.
> 
> Good point.
> 
> I was worried there might have been a concern about the overhead
> introduced by always rcu freeing files table.
> 
> > OTOH, it's not like close_files() did much beyond the effects of already
> > possible close(2) done by one of the threads sharing that sucker.
> > It's _probably_ safe (at least for proc_readfd_common()), but I'll need
> > to look at the other places doing that kind of access.  Especially the
> > BPF foulness...

Still digging, unfortunately ;-/

> > Oh, and in any case, the trick with repurposing ->rcu of embedded
> > fdtable deserves a comment.  It's not hard to explain, so...
> 
> Agreed.  Something like fdtable.rcu isn't used so use it so by reusing
> it we keep from wasting memory in files_struct to have a dedicated
> rcu_head.

I'd probably go for something along the lines of "we can avoid adding
a separate rcu_head into files_struct, since rcu_head in struct fdtable
is only used for separately allocated instances, allowing us to repurpose
files_struct->fdtab.rcu for RCU-delayed freeing of files_struct"...