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Subject: Re: [PATCH v2] ext2: Silence lockdep warning about reclaim under
 xattr_sem
To:     Jan Kara <jack@suse.cz>, linux-ext4@vger.kernel.org
Cc:     "J. R. Okajima" <hooanon05g@gmail.com>
References: <20200225120803.7901-1-jack@suse.cz>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Wed, 26 Feb 2020 17:02:18 +0530
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On 2/25/20 5:38 PM, Jan Kara wrote:
> 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.
> 
> Reported-by: "J. R. Okajima" <hooanon05g@gmail.com>
> Signed-off-by: Jan Kara <jack@suse.cz>

Agreed with evict() will only be called when it's the last reference 
going down and so we won't be blocked on xattr_sem.
Thanks for clearly explaining the problem in the cover letter.

Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>


> ---
>   fs/ext2/xattr.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> Changes since v1:
> - changed WARN_ON to WARN_ON_ONCE
> 
> diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c
> index 0456bc990b5e..9ad07c7ef0b3 100644
> --- a/fs/ext2/xattr.c
> +++ b/fs/ext2/xattr.c
> @@ -790,7 +790,15 @@ ext2_xattr_delete_inode(struct inode *inode)
>   	struct buffer_head *bh = NULL;
>   	struct ext2_sb_info *sbi = EXT2_SB(inode->i_sb);
> 
> -	down_write(&EXT2_I(inode)->xattr_sem);
> +	/*
> +	 * We are the only ones holding inode reference. The xattr_sem should
> +	 * better be unlocked! We could as well just not acquire xattr_sem at
> +	 * all but this makes the code more futureproof. OTOH we need trylock
> +	 * here to avoid false-positive warning from lockdep about reclaim
> +	 * circular dependency.
> +	 */
> +	if (WARN_ON_ONCE(!down_write_trylock(&EXT2_I(inode)->xattr_sem)))
> +		return;
>   	if (!EXT2_I(inode)->i_file_acl)
>   		goto cleanup;
>