Re: [PATCH] netfs: Fix lockdep warning from taking sb_writers whilst holding mmap_lock

[David Howells]

Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> On Tue, Dec 07, 2021 at 11:19:35AM +0000, David Howells wrote:
> > Taking sb_writers whilst holding mmap_lock isn't allowed and will result in
> > a lockdep warning like that below.  The problem comes from cachefiles
> > needing to take the sb_writers lock in order to do a write to the cache,
> > but being asked to do this by netfslib called from readpage, readahead or
> > write_begin[1].
> 
> Isn't it taking sb_writers _on a different filesystem_?  So there's not
> a real deadlock here, just a need to tell lockdep that this is a
> different subclass of lock?

Jann thinks it can be turned into a real deadlock.  See the link I put in the
patch description:

> > Link: https://lore.kernel.org/r/20210922110420.GA21576@xxxxxxxxxxxxxx/ [1]

David