Re: [PATCH 1/4] xfs: fix locking in xfs_iget_cache_hit

[Eric Sandeen <sandeen@sandeen.net>]

Christoph Hellwig wrote:

> New version below:

> The locking in xfs_iget_cache_hit currently has numerous problems:
> 
>  - we clear the reclaim tag without i_flags_lock which protects modifications
>    to it
>  - we call inode_init_always which can sleep with pag_ici_lock held
>    (this is oss.sgi.com BZ #819)
>  - we acquire and drop i_flags_lock a lot and thus provide no consistency
>    between the various flags we set/clear under it
> 
> This patch fixes all that with a major revamp of the locking in the function.
> The new version acquires i_flags_lock early and only drops it once we need to
> call into inode_init_always or before calling xfs_ilock.
> 
> This patch fixes a bug seen in the wild where we race modifying the reclaim tag.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

This seems ok to me but I have to be honest, I'm having a hard time
getting my head around back into the inode lifecycle.

one comment, I wonder if it's worth capturing the actual error from
inode_init_always() vs. turning every error into ENOMEM?  True, today
it's the only error we can get but why re-set it?

-Eric

> Index: linux-2.6/fs/xfs/xfs_iget.c
> ===================================================================
> --- linux-2.6.orig/fs/xfs/xfs_iget.c	2009-08-10 13:10:19.141974933 -0300
> +++ linux-2.6/fs/xfs/xfs_iget.c	2009-08-10 13:19:06.913056731 -0300
> @@ -191,80 +191,83 @@ xfs_iget_cache_hit(
>  	int			flags,
>  	int			lock_flags) __releases(pag->pag_ici_lock)
>  {
> +	struct inode		*inode = VFS_I(ip);
>  	struct xfs_mount	*mp = ip->i_mount;
> -	int			error = EAGAIN;
> +	int			error;
> +
> +	spin_lock(&ip->i_flags_lock);
>  
>  	/*
> -	 * If INEW is set this inode is being set up
> -	 * If IRECLAIM is set this inode is being torn down
> -	 * Pause and try again.
> +	 * If we are racing with another cache hit that is currently
> +	 * instantiating this inode or currently recycling it out of
> +	 * reclaimabe state, wait for the initialisation to complete
> +	 * before continuing.
> +	 *
> +	 * XXX(hch): eventually we should do something equivalent to
> +	 *	     wait_on_inode to wait for these flags to be cleared
> +	 *	     instead of polling for it.
>  	 */
> -	if (xfs_iflags_test(ip, (XFS_INEW|XFS_IRECLAIM))) {
> +	if (ip->i_flags & (XFS_INEW|XFS_IRECLAIM)) {
>  		XFS_STATS_INC(xs_ig_frecycle);
> +		error = EAGAIN;
>  		goto out_error;
>  	}
>  
> -	/* If IRECLAIMABLE is set, we've torn down the vfs inode part */
> -	if (xfs_iflags_test(ip, XFS_IRECLAIMABLE)) {
> -
> -		/*
> -		 * If lookup is racing with unlink, then we should return an
> -		 * error immediately so we don't remove it from the reclaim
> -		 * list and potentially leak the inode.
> -		 */
> -		if ((ip->i_d.di_mode == 0) && !(flags & XFS_IGET_CREATE)) {
> -			error = ENOENT;
> -			goto out_error;
> -		}
> +	/*
> +	 * If lookup is racing with unlink return an error immediately.
> +	 */
> +	if (ip->i_d.di_mode == 0 && !(flags & XFS_IGET_CREATE)) {
> +		error = ENOENT;
> +		goto out_error;
> +	}
>  
> +	/*
> +	 * If IRECLAIMABLE is set, we've torn down the VFS inode already.
> +	 * Need to carefully get it back into useable state.
> +	 */
> +	if (ip->i_flags & XFS_IRECLAIMABLE) {
>  		xfs_itrace_exit_tag(ip, "xfs_iget.alloc");
>  
>  		/*
> -		 * We need to re-initialise the VFS inode as it has been
> -		 * 'freed' by the VFS. Do this here so we can deal with
> -		 * errors cleanly, then tag it so it can be set up correctly
> -		 * later.
> +		 * We need to set XFS_INEW atomically with clearing the
> +		 * reclaimable tag so that we do have an indicator of the
> +		 * inode still being initialized.
>  		 */
> -		if (inode_init_always(mp->m_super, VFS_I(ip))) {
> +		ip->i_flags |= XFS_INEW;
> +		ip->i_flags &= ~XFS_IRECLAIMABLE;
> +		__xfs_inode_clear_reclaim_tag(mp, pag, ip);
> +
> +		spin_unlock(&ip->i_flags_lock);
> +		read_unlock(&pag->pag_ici_lock);
> +
> +		if (unlikely(inode_init_always(mp->m_super, inode))) {
> +			/*
> +			 * Re-initializing the inode failed, and we are in deep
> +			 * trouble.  Try to re-add it to the reclaim list.
> +			 */
> +			read_lock(&pag->pag_ici_lock);
> +			spin_lock(&ip->i_flags_lock);
> +
> +			ip->i_flags &= ~XFS_INEW;
> +			ip->i_flags |= XFS_IRECLAIMABLE;
> +			__xfs_inode_set_reclaim_tag(pag, ip);
> +
>  			error = ENOMEM;
>  			goto out_error;
>  		}
> -
> -		/*
> -		 * We must set the XFS_INEW flag before clearing the
> -		 * XFS_IRECLAIMABLE flag so that if a racing lookup does
> -		 * not find the XFS_IRECLAIMABLE above but has the igrab()
> -		 * below succeed we can safely check XFS_INEW to detect
> -		 * that this inode is still being initialised.
> -		 */
> -		xfs_iflags_set(ip, XFS_INEW);
> -		xfs_iflags_clear(ip, XFS_IRECLAIMABLE);
> -
> -		/* clear the radix tree reclaim flag as well. */
> -		__xfs_inode_clear_reclaim_tag(mp, pag, ip);
> -	} else if (!igrab(VFS_I(ip))) {
> +		inode->i_state = I_LOCK|I_NEW;
> +	} else {
>  		/* If the VFS inode is being torn down, pause and try again. */
> -		XFS_STATS_INC(xs_ig_frecycle);
> -		goto out_error;
> -	} else if (xfs_iflags_test(ip, XFS_INEW)) {
> -		/*
> -		 * We are racing with another cache hit that is
> -		 * currently recycling this inode out of the XFS_IRECLAIMABLE
> -		 * state. Wait for the initialisation to complete before
> -		 * continuing.
> -		 */
> -		wait_on_inode(VFS_I(ip));
> -	}
> +		if (!igrab(inode)) {
> +			error = EAGAIN;
> +			goto out_error;
> +		}
>  
> -	if (ip->i_d.di_mode == 0 && !(flags & XFS_IGET_CREATE)) {
> -		error = ENOENT;
> -		iput(VFS_I(ip));
> -		goto out_error;
> +		/* We've got a live one. */
> +		spin_unlock(&ip->i_flags_lock);
> +		read_unlock(&pag->pag_ici_lock);
>  	}
>  
> -	/* We've got a live one. */
> -	read_unlock(&pag->pag_ici_lock);
> -
>  	if (lock_flags != 0)
>  		xfs_ilock(ip, lock_flags);
>  
> @@ -274,6 +277,7 @@ xfs_iget_cache_hit(
>  	return 0;
>  
>  out_error:
> +	spin_unlock(&ip->i_flags_lock);
>  	read_unlock(&pag->pag_ici_lock);
>  	return error;
>  }
> Index: linux-2.6/fs/xfs/linux-2.6/xfs_sync.c
> ===================================================================
> --- linux-2.6.orig/fs/xfs/linux-2.6/xfs_sync.c	2009-08-10 13:10:19.146974522 -0300
> +++ linux-2.6/fs/xfs/linux-2.6/xfs_sync.c	2009-08-10 13:10:59.958993938 -0300
> @@ -708,6 +708,16 @@ xfs_reclaim_inode(
>  	return 0;
>  }
>  
> +void
> +__xfs_inode_set_reclaim_tag(
> +	struct xfs_perag	*pag,
> +	struct xfs_inode	*ip)
> +{
> +	radix_tree_tag_set(&pag->pag_ici_root,
> +			   XFS_INO_TO_AGINO(ip->i_mount, ip->i_ino),
> +			   XFS_ICI_RECLAIM_TAG);
> +}
> +
>  /*
>   * We set the inode flag atomically with the radix tree tag.
>   * Once we get tag lookups on the radix tree, this inode flag
> @@ -722,8 +732,7 @@ xfs_inode_set_reclaim_tag(
>  
>  	read_lock(&pag->pag_ici_lock);
>  	spin_lock(&ip->i_flags_lock);
> -	radix_tree_tag_set(&pag->pag_ici_root,
> -			XFS_INO_TO_AGINO(mp, ip->i_ino), XFS_ICI_RECLAIM_TAG);
> +	__xfs_inode_set_reclaim_tag(pag, ip);
>  	__xfs_iflags_set(ip, XFS_IRECLAIMABLE);
>  	spin_unlock(&ip->i_flags_lock);
>  	read_unlock(&pag->pag_ici_lock);
> Index: linux-2.6/fs/xfs/linux-2.6/xfs_sync.h
> ===================================================================
> --- linux-2.6.orig/fs/xfs/linux-2.6/xfs_sync.h	2009-08-10 13:10:19.153974227 -0300
> +++ linux-2.6/fs/xfs/linux-2.6/xfs_sync.h	2009-08-10 13:10:59.962994168 -0300
> @@ -48,6 +48,7 @@ int xfs_reclaim_inode(struct xfs_inode *
>  int xfs_reclaim_inodes(struct xfs_mount *mp, int mode);
>  
>  void xfs_inode_set_reclaim_tag(struct xfs_inode *ip);
> +void __xfs_inode_set_reclaim_tag(struct xfs_perag *pag, struct xfs_inode *ip);
>  void xfs_inode_clear_reclaim_tag(struct xfs_inode *ip);
>  void __xfs_inode_clear_reclaim_tag(struct xfs_mount *mp, struct xfs_perag *pag,
>  				struct xfs_inode *ip);