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Date:   Tue, 7 Dec 2021 13:13:50 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     jack@suse.cz, jlayton@kernel.org, linux-cachefs@redhat.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] netfs: Fix lockdep warning from taking sb_writers whilst
 holding mmap_lock
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References: <163887597541.1596626.2668163316598972956.stgit@warthog.procyon.org.uk>
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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?