linux-ext4 - Re: [PATCH 1/2] fs: dcache: Handle case-exact lookup in d_alloc

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Date:   Sun, 3 Oct 2021 13:38:40 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Shreeya Patel <shreeya.patel@...labora.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca, krisman@...labora.com,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH 1/2] fs: dcache: Handle case-exact lookup in
 d_alloc_parallel

On Wed, Sep 29, 2021 at 04:23:38PM +0530, Shreeya Patel wrote:
> There is a soft hang caused by a deadlock in d_alloc_parallel which
> waits up on lookups to finish for the dentries in the parent directory's
> hash_table.
> In case when d_add_ci is called from the fs layer's lookup functions,
> the dentry being looked up is already in the hash table (created before
> the fs lookup function gets called). We should not be processing the
> same dentry that is being looked up, hence, in case of case-insensitive
> filesystems we are making it a case-exact match to prevent this from
> happening.

NAK.  What you are doing would lead to parallel calls of ->lookup() in the
same directory for names that would compare as equal.  Which violates
all kinds of assumptions in the analysis of dentry tree locking.

d_add_ci() is used to force the "exact" spelling of the name on lookup -
that's the whole point of that thing.  What are you trying to achieve,
and what's the point of mixing that with non-trivial ->d_compare()?

If it's "force to exact spelling on lookup, avoid calling ->lookup() on
aliases", d_add_ci() is simply not a good match.