linux-ext4 - Re: [PATCH v2] ext2: Silence lockdep warning about reclaim under xattr

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Message-ID: <30602.1586151377@jrobl>
Date:   Mon, 06 Apr 2020 14:36:17 +0900
From:   "J. R. Okajima" <hooanon05g@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2] ext2: Silence lockdep warning about reclaim under xattr_sem

Jan Kara:
> Lockdep complains about a chain:
>   sb_internal#2 --> &ei->xattr_sem#2 --> fs_reclaim
>
> and shrink_dentry_list -> ext2_evict_inode -> ext2_xattr_delete_inode ->
> down_write(ei->xattr_sem) creating a locking cycle in the reclaim path.
> This is however a false positive because when we are in
> ext2_evict_inode() we are the only holder of the inode reference and
> nobody else should touch xattr_sem of that inode. So we cannot ever
> block on acquiring the xattr_sem in the reclaim path.
>
> Silence the lockdep warning by using down_write_trylock() in
> ext2_xattr_delete_inode() to not create false locking dependency.

v5.6 is released.
But I cannot see this patch applied.  Sad.

Anyway I am wondering whether acquiring xattr_sem in
ext2_xattr_delete_inode() is really necessary or not.
It is necessary because this function refers and clears i_file_acl,
right?

But this function handles the removed (nlink==0) and unused inodes only.
If nobody else touches xattr_sem as you wrote, then it is same to
i_file_acl, isn't it?  Can we replace xattr_sem (only here) by memory
barrier, or remove xattr_sem from ext2_xattr_delete_inode()?

J. R. Okajima