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

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Message-ID: <589db4cf-5cab-2d1f-10ce-3a5009685948@collabora.com>
Date:   Tue, 5 Oct 2021 18:39:59 +0530
From:   Shreeya Patel <shreeya.patel@...labora.com>
To:     Al Viro <viro@...iv.linux.org.uk>
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 03/10/21 7:08 pm, Al Viro wrote:
> 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()?
>
Sending again as plain text...

Hi Al Viro,

This patch was added to resolve some of the issues faced in patch 02/02 
of the series.

Originally, the 'native', per-directory case-insensitive implementation
merged in ext4/f2fs stores the case of the first lookup on the dcache,
regardless of the disk exact file name case. This gets reflected in symlink
returned by /proc/self/cwd.

To solve this we are calling d_add_ci from the fs lookup function to 
store the
disk exact name in the dcache even if an inexact-match string is used on 
the FIRST lookup.
But this caused a soft hang since there was a deadlock in d_wait_lookup 
called from d_alloc_parallel.

The reason for the hang is that d_same_name uses d_compare which does a
case-insensitive match and is able to find the dentry name in the 
secondary hash table
leading it to d_wait_lookup which would wait for the lookup to finish on 
that dentry
causing a deadlock.

To avoid the hang, we are doing a case-sensitive match using dentry_cmp 
here.

Thanks

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