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On Fri, May 02, 2008 at 12:58:03PM +0200, Christoph Hellwig wrote:
> When multiple inodes are locked in XFS it happens in order of the inode
> number, with the everything but the first inode trylocked if any of
> the previous inodes is in the AIL.
>
> Except for the sorting of the inodes this logic is implemented in
> xfs_lock_inodes, but also partially duplicated in xfs_lock_dir_and_entry
> in a particularly stupid way adds a lock roundtrip if the inode ordering
> is not optimal.
>
> This patch adds a new helper xfs_lock_two_inodes that takes two inodes
> and locks them in the most optimal way according to the above locking
> protocol and uses it for all places that want to lock two inodes.
>
> The only caller of xfs_lock_inodes is xfs_rename which might lock up to
> four inodes.
>
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
>
> Index: linux-2.6-xfs/fs/xfs/xfs_vnodeops.c
> ===================================================================
> --- linux-2.6-xfs.orig/fs/xfs/xfs_vnodeops.c 2008-05-02 08:30:24.000000000
> +0200
> +++ linux-2.6-xfs/fs/xfs/xfs_vnodeops.c 2008-05-02 08:30:30.000000000
> +0200
> @@ -1897,111 +1897,6 @@ std_return:
> }
>
> #ifdef DEBUG
> -/*
> - * Some counters to see if (and how often) we are hitting some deadlock
> - * prevention code paths.
> - */
> -
> -int xfs_rm_locks;
> -int xfs_rm_lock_delays;
> -int xfs_rm_attempts;
> -#endif
> -
> -/*
> - * The following routine will lock the inodes associated with the
> - * directory and the named entry in the directory. The locks are
> - * acquired in increasing inode number.
> - *
> - * If the entry is "..", then only the directory is locked. The
> - * vnode ref count will still include that from the .. entry in
> - * this case.
> - *
> - * There is a deadlock we need to worry about. If the locked directory is
> - * in the AIL, it might be blocking up the log. The next inode we lock
> - * could be already locked by another thread waiting for log space (e.g
> - * a permanent log reservation with a long running transaction (see
> - * xfs_itruncate_finish)). To solve this, we must check if the directory
> - * is in the ail and use lock_nowait. If we can't lock, we need to
> - * drop the inode lock on the directory and try again. xfs_iunlock will
> - * potentially push the tail if we were holding up the log.
> - */
> -STATIC int
> -xfs_lock_dir_and_entry(
> - xfs_inode_t *dp,
> - xfs_inode_t *ip) /* inode of entry 'name' */
> -{
> - int attempts;
> - xfs_ino_t e_inum;
> - xfs_inode_t *ips[2];
> - xfs_log_item_t *lp;
> -
> -#ifdef DEBUG
> - xfs_rm_locks++;
> -#endif
> - attempts = 0;
> -
> -again:
> - xfs_ilock(dp, XFS_ILOCK_EXCL | XFS_ILOCK_PARENT);
> -
> - e_inum = ip->i_ino;
> -
> - xfs_itrace_ref(ip);
> -
> - /*
> - * We want to lock in increasing inum. Since we've already
> - * acquired the lock on the directory, we may need to release
> - * if if the inum of the entry turns out to be less.
> - */
> - if (e_inum > dp->i_ino) {
> - /*
> - * We are already in the right order, so just
> - * lock on the inode of the entry.
> - * We need to use nowait if dp is in the AIL.
> - */
> -
> - lp = (xfs_log_item_t *)dp->i_itemp;
> - if (lp && (lp->li_flags & XFS_LI_IN_AIL)) {
> - if (!xfs_ilock_nowait(ip, XFS_ILOCK_EXCL)) {
> - attempts++;
> -#ifdef DEBUG
> - xfs_rm_attempts++;
> -#endif
> -
> - /*
> - * Unlock dp and try again.
> - * xfs_iunlock will try to push the tail
> - * if the inode is in the AIL.
> - */
> -
> - xfs_iunlock(dp, XFS_ILOCK_EXCL);
> -
> - if ((attempts % 5) == 0) {
> - delay(1); /* Don't just spin the CPU */
> -#ifdef DEBUG
> - xfs_rm_lock_delays++;
> -#endif
> - }
> - goto again;
> - }
> - } else {
> - xfs_ilock(ip, XFS_ILOCK_EXCL);
> - }
> - } else if (e_inum < dp->i_ino) {
> - xfs_iunlock(dp, XFS_ILOCK_EXCL);
> -
> - ips[0] = ip;
> - ips[1] = dp;
> - xfs_lock_inodes(ips, 2, XFS_ILOCK_EXCL);
> - }
> - /* else e_inum == dp->i_ino */
> - /* This can happen if we're asked to lock /x/..
> - * the entry is "..", which is also the parent directory.
> - */
> -
> - return 0;
> -}
> -
> -#ifdef DEBUG
> int xfs_locked_n;
> int xfs_small_retries;
> int xfs_middle_retries;
> @@ -2135,6 +2030,45 @@ again:
> #endif
> }
>
> +void
> +xfs_lock_two_inodes(
> + xfs_inode_t *ip0,
> + xfs_inode_t *ip1,
> + uint lock_mode)
> +{
> + xfs_inode_t *temp;
> + int attempts = 0;
> + xfs_log_item_t *lp;
> +
> + ASSERT(ip0->i_ino != ip1->i_ino);
> +
> + if (ip0->i_ino > ip1->i_ino) {
> + temp = ip0;
> + ip0 = ip1;
> + ip1 = temp;
> + }
> +
> + again:
> + xfs_ilock(ip0, xfs_lock_inumorder(lock_mode, 0));
> +
> + /*
> + * If the first lock we have locked is in the AIL, we must TRY to get
> + * the second lock. If we can't get it, we must release the first one
> + * and try again.
> + */
> + lp = (xfs_log_item_t *)ip0->i_itemp;
> + if (lp && (lp->li_flags & XFS_LI_IN_AIL)) {
> + if (!xfs_ilock_nowait(ip1, xfs_lock_inumorder(lock_mode, 1))) {
> + xfs_iunlock(ip0, lock_mode);
> + if ((++attempts % 5) == 0)
> + delay(1); /* Don't just spin the CPU */
> + goto again;
> + }
> + } else {
> + xfs_ilock(ip1, xfs_lock_inumorder(lock_mode, 1));
> + }
> +}
> +
> int
> xfs_remove(
> xfs_inode_t *dp,
> @@ -2210,9 +2144,7 @@ xfs_remove(
> goto out_trans_cancel;
> }
>
> - error = xfs_lock_dir_and_entry(dp, ip);
> - if (error)
> - goto out_trans_cancel;
> + xfs_lock_two_inodes(dp, ip, XFS_ILOCK_EXCL);
>
> /*
> * At this point, we've gotten both the directory and the entry
> @@ -2239,9 +2171,6 @@ xfs_remove(
> }
> }
>
> - /*
> - * Entry must exist since we did a lookup in xfs_lock_dir_and_entry.
> - */
> XFS_BMAP_INIT(&free_list, &first_block);
> error = xfs_dir_removename(tp, dp, name, ip->i_ino,
> &first_block, &free_list, resblks);
> @@ -2347,7 +2276,6 @@ xfs_link(
> {
> xfs_mount_t *mp = tdp->i_mount;
> xfs_trans_t *tp;
> - xfs_inode_t *ips[2];
> int error;
> xfs_bmap_free_t free_list;
> xfs_fsblock_t first_block;
> @@ -2395,15 +2323,7 @@ xfs_link(
> goto error_return;
> }
>
> - if (sip->i_ino < tdp->i_ino) {
> - ips[0] = sip;
> - ips[1] = tdp;
> - } else {
> - ips[0] = tdp;
> - ips[1] = sip;
> - }
> -
> - xfs_lock_inodes(ips, 2, XFS_ILOCK_EXCL);
> + xfs_lock_two_inodes(sip, tdp, XFS_ILOCK_EXCL);
>
> /*
> * Increment vnode ref counts since xfs_trans_commit &
> Index: linux-2.6-xfs/fs/xfs/xfs_dfrag.c
> ===================================================================
> --- linux-2.6-xfs.orig/fs/xfs/xfs_dfrag.c 2008-04-26 17:43:14.000000000
> +0200
> +++ linux-2.6-xfs/fs/xfs/xfs_dfrag.c 2008-05-02 08:30:30.000000000 +0200
> @@ -128,7 +128,6 @@ xfs_swap_extents(
> xfs_swapext_t *sxp)
> {
> xfs_mount_t *mp;
> - xfs_inode_t *ips[2];
> xfs_trans_t *tp;
> xfs_bstat_t *sbp = &sxp->sx_stat;
> bhv_vnode_t *vp, *tvp;
> @@ -153,16 +152,7 @@ xfs_swap_extents(
> vp = XFS_ITOV(ip);
> tvp = XFS_ITOV(tip);
>
> - /* Lock in i_ino order */
> - if (ip->i_ino < tip->i_ino) {
> - ips[0] = ip;
> - ips[1] = tip;
> - } else {
> - ips[0] = tip;
> - ips[1] = ip;
> - }
> -
> - xfs_lock_inodes(ips, 2, lock_flags);
> + xfs_lock_two_inodes(ip, tip, lock_flags);
> locked = 1;
>
> /* Verify that both files have the same format */
> @@ -265,7 +255,7 @@ xfs_swap_extents(
> locked = 0;
> goto error0;
> }
> - xfs_lock_inodes(ips, 2, XFS_ILOCK_EXCL);
> + xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);
>
> /*
> * Count the number of extended attribute blocks
> Index: linux-2.6-xfs/fs/xfs/xfs_inode.h
> ===================================================================
> --- linux-2.6-xfs.orig/fs/xfs/xfs_inode.h 2008-05-01 22:56:57.000000000
> +0200
> +++ linux-2.6-xfs/fs/xfs/xfs_inode.h 2008-05-02 08:30:30.000000000 +0200
> @@ -522,6 +522,7 @@ void xfs_iflush_all(struct xfs_mount *)
> void xfs_ichgtime(xfs_inode_t *, int);
> xfs_fsize_t xfs_file_last_byte(xfs_inode_t *);
> void xfs_lock_inodes(xfs_inode_t **, int, uint);
> +void xfs_lock_two_inodes(xfs_inode_t *, xfs_inode_t *, uint);
>
> void xfs_synchronize_atime(xfs_inode_t *);
> void xfs_mark_inode_dirty_sync(xfs_inode_t *);
---end quoted text---
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