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Date:   Tue, 5 May 2020 07:25:40 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Jia-Ju Bai <baijiaju1990@gmail.com>
Cc:     darrick.wong@oracle.com, linux-xfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: xfs: fix a possible data race in
 xfs_inode_set_reclaim_tag()
Message-ID: <20200504212540.GK2040@dread.disaster.area>
References: <20200504161530.14059-1-baijiaju1990@gmail.com>
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On Tue, May 05, 2020 at 12:15:30AM +0800, Jia-Ju Bai wrote:
> We find that xfs_inode_set_reclaim_tag() and xfs_reclaim_inode() are
> concurrently executed at runtime in the following call contexts:
> 
> Thread1:
>   xfs_fs_put_super()
>     xfs_unmountfs()
>       xfs_rtunmount_inodes()
>         xfs_irele()
>           xfs_fs_destroy_inode()
>             xfs_inode_set_reclaim_tag()
> 
> Thread2:
>   xfs_reclaim_worker()
>     xfs_reclaim_inodes()
>       xfs_reclaim_inodes_ag()
>         xfs_reclaim_inode()
> 
> In xfs_inode_set_reclaim_tag():
>   pag = xfs_perag_get(mp, XFS_INO_TO_AGNO(mp, ip->i_ino));
>   ...
>   spin_lock(&ip->i_flags_lock);
> 
> In xfs_reclaim_inode():
>   spin_lock(&ip->i_flags_lock);
>   ...
>   ip->i_ino = 0;
>   spin_unlock(&ip->i_flags_lock);
> 
> Thus, a data race can occur for ip->i_ino.
> 
> To fix this data race, the spinlock ip->i_flags_lock is used to protect
> the access to ip->i_ino in xfs_inode_set_reclaim_tag().
> 
> This data race is found by our concurrency fuzzer.

This data race cannot happen.

xfs_reclaim_inode() will not be called on this inode until -after-
the XFS_ICI_RECLAIM_TAG is set in the radix tree for this inode, and
setting that is protected by the i_flags_lock.

So while the xfs_perag_get() call doesn't lock the ip->i_ino access,
there is are -multiple_ iflags_lock lock/unlock cycles before
ip->i_ino is cleared in the reclaim worker. Hence there is a full
unlock->lock memory barrier for the ip->i_ino reset inside the
critical section vs xfs_inode_set_reclaim_tag().

Hence even if the reclaim worker could access the inode before the
XFS_ICI_RECLAIM_TAG is set, no data race exists here.

> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
> ---
>  fs/xfs/xfs_icache.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index 8bf1d15be3f6..a2de08222ff5 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -229,9 +229,9 @@ xfs_inode_set_reclaim_tag(
>  	struct xfs_mount	*mp = ip->i_mount;
>  	struct xfs_perag	*pag;
>  
> +	spin_lock(&ip->i_flags_lock);
>  	pag = xfs_perag_get(mp, XFS_INO_TO_AGNO(mp, ip->i_ino));
>  	spin_lock(&pag->pag_ici_lock);
> -	spin_lock(&ip->i_flags_lock);

Also, this creates a lock inversion deadlock here with
xfs_iget_cache_hit() clearing the XFS_IRECLAIMABLE flag.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com