linux-ext4 - Re: [PATCH RESEND v2 19/25] vfs: Handle case-exact lookup in d_add

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Date:   Sun, 7 Oct 2018 14:09:53 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Gabriel Krisman Bertazi <krisman@...labora.co.uk>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH RESEND v2 19/25] vfs: Handle case-exact lookup in d_add_ci

On Mon, Sep 24, 2018 at 05:56:49PM -0400, Gabriel Krisman Bertazi wrote:
> This prevents a soft hang if called d_add_ci is called from the FS
> layer, when doing a CI search but the result dentry is the exact match.

This isn't the right way to fix this problem.  Take a look at how xfs
handles this in fs/xfs/xfs_iops.c:xfs_vn_ci_lookup().  This logic
should be in the file system, not in d_add_ci().  Also, we don't want
to use d_same_name(), since that is *not* guaranteed to do an exact
match.  It happens to do so for ext4 since we don't provide d_compare,
but it's better just check for an exact match and call
d_splice_alias() instead of d_add_ci() in ext4_lookup().

Also note that d_same_name() is *not* guaranteeed to do an exact
match, in particular if the file system provides d_compare (which
granted, ext4 doesn't right now).  It's simpler to just do a direct
strcmp in ext4_lookup.

					- Ted

P.S.  Apologies for not having a chance to look at this series in
detail until now.  It's been a crazy month...