Re: [PATCH v4 4/6] kernfs: Don't re-lock kernfs_root::kernfs_rwsem in kernfs_fop_readdir().

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-01-24 13:15:32 [-1000], Tejun Heo wrote:
> Hello,
> 
> On Fri, Jan 24, 2025 at 06:46:12PM +0100, Sebastian Andrzej Siewior wrote:
> ...
> > to avoid holding a global lock during a page fault. The lock drop is
> > wrong since the support of renames and not a big burden since the lock
> > is no longer global.
> 
> It's still a pretty big lock. Hopefully, at least name accesses can be
> converted to rcu protected ones later?

Not sure what you mean by "rcu protected ones". The name is already RCU
protected which is part of the problem :P
Assuming an upper length of name to be around 255 and
kernfs_fop_readdir() to be very early on the chain then we could copy
the name on stack within the RCU section and do this without
::kernfs_rwsem.

> > Don't re-acquire kernfs_root::kernfs_rwsem while copying the name to the
> > userpace buffer.
> > 
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> 
> Thanks.

Sebastian