Re: [PATCH 1/2] vfs: free vfsmount through rcu work from kern_unmount

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Feb 18, 2022 at 02:33:31PM -0500, Rik van Riel wrote:
> On Fri, 2022-02-18 at 19:26 +0000, Al Viro wrote:
> > On Fri, Feb 18, 2022 at 01:31:13PM -0500, Rik van Riel wrote:
> > > After kern_unmount returns, callers can no longer access the
> > > vfsmount structure. However, the vfsmount structure does need
> > > to be kept around until the end of the RCU grace period, to
> > > make sure other accesses have all gone away too.
> > > 
> > > This can be accomplished by either gating each kern_unmount
> > > on synchronize_rcu (the comment in the code says it all), or
> > > by deferring the freeing until the next grace period, where
> > > it needs to be handled in a workqueue due to the locking in
> > > mntput_no_expire().
> > 
> > NAK.  There's code that relies upon kern_unmount() being
> > synchronous.  That's precisely the reason why MNT_INTERNAL
> > is treated that way in mntput_no_expire().
> 
> Fair enough. Should I make a kern_unmount_rcu() version
> that gets called just from mq_put_mnt()?

Umm...  I'm not sure you can afford having struct ipc_namespace
freed and reused before the mqueue superblock gets at least to
deactivate_locked_super().