linux-kernel - Re: [PATCH] netfs: Fix lockdep warning from taking sb_writers whilst holding mmap

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Message-ID: <1602527.1638883933@warthog.procyon.org.uk>
Date:   Tue, 07 Dec 2021 13:32:13 +0000
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.com, jack@...e.cz, jlayton@...nel.org,
        linux-cachefs@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netfs: Fix lockdep warning from taking sb_writers whilst holding mmap_lock

Matthew Wilcox <willy@...radead.org> 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@quack2.suse.cz/ [1]

David