xfs: copy li_lsn before dropping AIL lock

Access to log items on the AIL is generally protected by m_ail_lock;
this is particularly needed when we're getting or setting the 64-bit
li_lsn on a 32-bit platform.  This patch fixes a couple places where we
were accessing the log item after dropping the AIL lock on 32-bit
machines.

This can result in a partially-zeroed log->l_tail_lsn if
xfs_trans_ail_delete is racing with xfs_trans_ail_update, and in at
least some cases, this can leave the l_tail_lsn with a zero cycle
number, which means xlog_space_left will think the log is full (unless
CONFIG_XFS_DEBUG is set, in which case we'll trip an ASSERT), leading to
processes stuck forever in xlog_grant_log_space.

Thanks to Adrian VanderSpek for first spotting the race potential and to
Dave Chinner for debug assistance.

Signed-off-by: Nathaniel W. Turner <nate@houseofnate.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

1 files changed, 20 insertions, 3 deletions

diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
index f31271c30de9..2ffc570679be 100644
--- a/fs/xfs/xfs_trans_ail.c
+++ b/fs/xfs/xfs_trans_ail.c
@@ -467,6 +467,7 @@ xfs_trans_ail_update(
 {
         xfs_log_item_t          *dlip = NULL;
         xfs_log_item_t          *mlip;  /* ptr to minimum lip */
+        xfs_lsn_t               tail_lsn;
 
         mlip = xfs_ail_min(ailp);
 
@@ -483,8 +484,16 @@ xfs_trans_ail_update(
 
         if (mlip == dlip) {
                 mlip = xfs_ail_min(ailp);
+                /*
+                 * It is not safe to access mlip after the AIL lock is
+                 * dropped, so we must get a copy of li_lsn before we do
+                 * so.  This is especially important on 32-bit platforms
+                 * where accessing and updating 64-bit values like li_lsn
+                 * is not atomic.
+                 */
+                tail_lsn = mlip->li_lsn;
                 spin_unlock(&ailp->xa_lock);
-                xfs_log_move_tail(ailp->xa_mount, mlip->li_lsn);
+                xfs_log_move_tail(ailp->xa_mount, tail_lsn);
         } else {
                 spin_unlock(&ailp->xa_lock);
         }
@@ -514,6 +523,7 @@ xfs_trans_ail_delete(
 {
         xfs_log_item_t          *dlip;
         xfs_log_item_t          *mlip;
+        xfs_lsn_t               tail_lsn;
 
         if (lip->li_flags & XFS_LI_IN_AIL) {
                 mlip = xfs_ail_min(ailp);
@@ -527,9 +537,16 @@ xfs_trans_ail_delete(
 
                 if (mlip == dlip) {
                         mlip = xfs_ail_min(ailp);
+                        /*
+                         * It is not safe to access mlip after the AIL lock
+                         * is dropped, so we must get a copy of li_lsn
+                         * before we do so.  This is especially important
+                         * on 32-bit platforms where accessing and updating
+                         * 64-bit values like li_lsn is not atomic.
+                         */
+                        tail_lsn = mlip ? mlip->li_lsn : 0;
                         spin_unlock(&ailp->xa_lock);
-                        xfs_log_move_tail(ailp->xa_mount,
-                                                (mlip ? mlip->li_lsn : 0));
+                        xfs_log_move_tail(ailp->xa_mount, tail_lsn);
                 } else {
                         spin_unlock(&ailp->xa_lock);
                 }