Re: [PATCH] VFS: use synchronize_rcu_expedited() in namespace_unlock()

[Paul E. McKenney]

On Mon, Nov 27, 2017 at 12:27:04PM +0100, Florian Weimer wrote:
> On 10/26/2017 02:27 PM, Paul E. McKenney wrote:
> >But just for completeness, one way to make this work across the board
> >might be to instead use call_rcu(), with the callback function kicking
> >off a workqueue handler to do the rest of the unmount.  Of course,
> >in saying that, I am ignoring any mutexes that you might be holding
> >across this whole thing, and also ignoring any problems that might arise
> >when returning to userspace with some portion of the unmount operation
> >still pending.  (For example, someone unmounting a filesystem and then
> >immediately remounting that same filesystem.)
> 
> You really need to complete all side effects of deallocating a
> resource before returning to user space.  Otherwise, it will never
> be possible to allocate and deallocate resources in a tight loop
> because you either get spurious failures because too many
> unaccounted deallocations are stuck somewhere in the system (and the
> user can't tell that this is due to a race), or you get an OOM
> because the user manages to queue up too much state.
> 
> We already have this problem with RLIMIT_NPROC, where waitpid etc.
> return before the process is completely gone.  On some
> kernels/configurations, the resulting race is so wide that parallel
> make no longer works reliable because it runs into fork failures.

Or alternatively, use rcu_barrier() occasionally to wait for all
preceding deferred deallocations.  And there are quite a few other
ways to take on this problem.

							Thanx, Paul