Re: [PATCH RFC fs/namespace] Make kern_unmount() use synchronize_rcu_expedited()

[Chris Mason]

> On Feb 14, 2022, at 2:05 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> 
> Experimental.  Not for inclusion.  Yet, anyway.
> 
> Freeing large numbers of namespaces in quick succession can result in
> a bottleneck on the synchronize_rcu() invoked from kern_unmount().
> This patch applies the synchronize_rcu_expedited() hammer to allow
> further testing and fault isolation.
> 
> Hey, at least there was no need to change the comment!  ;-)
> 

I don’t think this will be fast enough.  I think the problem is that commit e1eb26fa62d04ec0955432be1aa8722a97cb52e7 is putting all of the ipc namespace frees onto a list, and every free includes one call to synchronize_rcu()

The end result is that we can create new namespaces much much faster than we can free them, and eventually we run out.  I found this while debugging clone() returning ENOSPC because create_ipc_ns() was returning ENOSPC.

-chris