[PATCH 1/4] xfs: fix locking in xfs_iget_cache_hit
Felix Blyakher
felixb at sgi.com
Fri Aug 7 12:25:47 CDT 2009
On Aug 4, 2009, at 9:15 AM, Christoph Hellwig wrote:
> 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
> - 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.
Some comments below as well as couple of questions based on the
difference with previous code, not necessarily pointing to a
problem. Just trying to figure it out for myself.
>
>
> Signed-off-by: Christoph Hellwig <hch at lst.de>
>
> Index: linux-2.6/fs/xfs/xfs_iget.c
> ===================================================================
> --- linux-2.6.orig/fs/xfs/xfs_iget.c 2009-08-01 23:20:54.775080254
> +0200
> +++ linux-2.6/fs/xfs/xfs_iget.c 2009-08-01 23:20:54.807080483 +0200
> @@ -133,80 +133,90 @@ 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.
> + * This inode is being torn down, pause and try again.
> */
> - if (xfs_iflags_test(ip, (XFS_INEW|XFS_IRECLAIM))) {
> + if (ip->i_flags & 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 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.
> + */
> + if (ip->i_flags & XFS_INEW) {
Another case when we find XFS_INEW set is the race with the
cache miss, which just set up a new inode. Would the proposed
code be still sensible in that case? If yes, at least comments
should be updated.
>
> + spin_unlock(&ip->i_flags_lock);
> + read_unlock(&pag->pag_ici_lock);
>
> - /*
> - * 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;
> - }
> + XFS_STATS_INC(xs_ig_frecycle);
> + wait_on_inode(inode);
It's possible to have XFS_INEW set, but no I_LOCK|I_NEW yet.
Then the wait_on_inode() would return quickly even before the
linux inode is reinitialized. Though, that was the case with
the old code as well.
>
> + return EAGAIN;
This return seems inconsistent with usual goto end of function
convention. I understand that in this case goto out_error would
be incorrect. Should we create new label out_error_unlocked?
>
> + }
>
> + /*
> + * 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)) {
Previously the conclusion of the race with unlink was based on
XFS_IRECLAIMABLE i_flag set in addition to the test above.
Is is no longer a case, or not really necessary?
>
> + 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;
> + __xfs_inode_clear_reclaim_tag(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;
> + __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);
>
> @@ -216,6 +226,7 @@ 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 2009-08-01
> 23:20:31.441330970 +0200
> +++ linux-2.6/fs/xfs/linux-2.6/xfs_sync.c 2009-08-01
> 23:20:54.807080483 +0200
> @@ -708,6 +708,17 @@ 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);
> + ip->i_flags |= XFS_IRECLAIMABLE;
> +}
> +
> /*
> * We set the inode flag atomically with the radix tree tag.
> * Once we get tag lookups on the radix tree, this inode flag
> @@ -722,9 +733,7 @@ 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_iflags_set(ip, XFS_IRECLAIMABLE);
> + __xfs_inode_set_reclaim_tag(pag, ip);
> spin_unlock(&ip->i_flags_lock);
> read_unlock(&pag->pag_ici_lock);
> xfs_put_perag(mp, pag);
> @@ -732,27 +741,13 @@ xfs_inode_set_reclaim_tag(
>
> void
> __xfs_inode_clear_reclaim_tag(
> - xfs_mount_t *mp,
> - xfs_perag_t *pag,
> - xfs_inode_t *ip)
> + struct xfs_perag *pag,
> + struct xfs_inode *ip)
> {
> + ip->i_flags &= ~XFS_IRECLAIMABLE;
> radix_tree_tag_clear(&pag->pag_ici_root,
> - XFS_INO_TO_AGINO(mp, ip->i_ino), XFS_ICI_RECLAIM_TAG);
> -}
> -
> -void
> -xfs_inode_clear_reclaim_tag(
> - xfs_inode_t *ip)
> -{
> - xfs_mount_t *mp = ip->i_mount;
> - xfs_perag_t *pag = xfs_get_perag(mp, ip->i_ino);
> -
> - read_lock(&pag->pag_ici_lock);
> - spin_lock(&ip->i_flags_lock);
> - __xfs_inode_clear_reclaim_tag(mp, pag, ip);
> - spin_unlock(&ip->i_flags_lock);
> - read_unlock(&pag->pag_ici_lock);
> - xfs_put_perag(mp, pag);
> + XFS_INO_TO_AGINO(ip->i_mount, ip->i_ino),
> + XFS_ICI_RECLAIM_TAG);
> }
>
> STATIC int
> Index: linux-2.6/fs/xfs/linux-2.6/xfs_sync.h
> ===================================================================
> --- linux-2.6.orig/fs/xfs/linux-2.6/xfs_sync.h 2009-08-01
> 23:20:31.449329683 +0200
> +++ linux-2.6/fs/xfs/linux-2.6/xfs_sync.h 2009-08-01
> 23:20:54.808079772 +0200
> @@ -48,9 +48,8 @@ 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_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);
> +void __xfs_inode_set_reclaim_tag(struct xfs_perag *pag, struct
> xfs_inode *ip);
> +void __xfs_inode_clear_reclaim_tag(struct xfs_perag *pag, struct
> xfs_inode *ip);
>
> int xfs_sync_inode_valid(struct xfs_inode *ip, struct xfs_perag *pag);
> int xfs_inode_ag_iterator(struct xfs_mount *mp,
>
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