xfs: fix filesystsem freeze race in xfs_trans_alloc

As pointed out by Jan xfs_trans_alloc can race with a concurrent filesystem
freeze when it sleeps during the memory allocation.  Fix this by moving the
wait_for_freeze call after the memory allocation.  This means moving the
freeze into the low-level _xfs_trans_alloc helper, which thus grows a new
argument.  Also fix up some comments in that area while at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <david@fromorbit.com>

5 files changed, 27 insertions, 29 deletions

diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
index 9153d2c77ca..b7c492d293b 100644
--- a/fs/xfs/xfs_fsops.c
+++ b/fs/xfs/xfs_fsops.c
@@ -626,7 +626,7 @@ xfs_fs_log_dummy(
         xfs_trans_t     *tp;
         int             error;
 
-        tp = _xfs_trans_alloc(mp, XFS_TRANS_DUMMY1, KM_SLEEP);
+        tp = _xfs_trans_alloc(mp, XFS_TRANS_DUMMY1, KM_SLEEP, false);
         error = xfs_trans_reserve(tp, 0, mp->m_sb.sb_sectsize + 128, 0, 0,
                                         XFS_DEFAULT_LOG_COUNT);
         if (error) {
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 091d82b94c4..982b71108f3 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -688,8 +688,7 @@ xfs_iomap_write_unwritten(
                  * the same inode that we complete here and might deadlock
                  * on the iolock.
                  */
-                xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
-                tp = _xfs_trans_alloc(mp, XFS_TRANS_STRAT_WRITE, KM_NOFS);
+                tp = _xfs_trans_alloc(mp, XFS_TRANS_STRAT_WRITE, KM_NOFS, true);
                 tp->t_flags |= XFS_TRANS_RESERVE;
                 error = xfs_trans_reserve(tp, resblks,
                                 XFS_WRITE_LOG_RES(mp), 0,
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index b49b82363d2..63659ee316b 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1566,15 +1566,9 @@ xfs_fs_writable(xfs_mount_t *mp)
 }
 
 /*
- * xfs_log_sbcount
- *
  * Called either periodically to keep the on disk superblock values
  * roughly up to date or from unmount to make sure the values are
  * correct on a clean unmount.
- *
- * Note this code can be called during the process of freezing, so
- * we may need to use the transaction allocator which does not not
- * block when the transaction subsystem is in its frozen state.
  */
 int
 xfs_log_sbcount(
@@ -1596,7 +1590,13 @@ xfs_log_sbcount(
         if (!xfs_sb_version_haslazysbcount(&mp->m_sb))
                 return 0;
 
-        tp = _xfs_trans_alloc(mp, XFS_TRANS_SB_COUNT, KM_SLEEP);
+        /*
+         * We can be called during the process of freezing, so make sure
+         * we go ahead even if the frozen for new transactions.  We will
+         * always use a sync transaction in the freeze path to make sure
+         * the transaction has completed by the time we return.
+         */
+        tp = _xfs_trans_alloc(mp, XFS_TRANS_SB_COUNT, KM_SLEEP, false);
         error = xfs_trans_reserve(tp, 0, mp->m_sb.sb_sectsize + 128, 0, 0,
                                         XFS_DEFAULT_LOG_COUNT);
         if (error) {
diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
index c83f63b33aa..2837220ea5c 100644
--- a/fs/xfs/xfs_trans.c
+++ b/fs/xfs/xfs_trans.c
@@ -566,31 +566,24 @@ xfs_trans_init(
 
 /*
  * This routine is called to allocate a transaction structure.
+ *
  * The type parameter indicates the type of the transaction.  These
  * are enumerated in xfs_trans.h.
- *
- * Dynamically allocate the transaction structure from the transaction
- * zone, initialize it, and return it to the caller.
  */
-xfs_trans_t *
-xfs_trans_alloc(
-        xfs_mount_t     *mp,
-        uint            type)
-{
-        xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
-        return _xfs_trans_alloc(mp, type, KM_SLEEP);
-}
-
-xfs_trans_t *
+struct xfs_trans *
 _xfs_trans_alloc(
-        xfs_mount_t     *mp,
-        uint            type,
-        uint            memflags)
+        struct xfs_mount        *mp,
+        uint                    type,
+        uint                    memflags,
+        bool                    wait_for_freeze)
 {
-        xfs_trans_t     *tp;
+        struct xfs_trans        *tp;
 
         atomic_inc(&mp->m_active_trans);
 
+        if (wait_for_freeze)
+                xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
+
         tp = kmem_zone_zalloc(xfs_trans_zone, memflags);
         tp->t_magic = XFS_TRANS_MAGIC;
         tp->t_type = type;
diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
index 06a9759b635..4e78432ce14 100644
--- a/fs/xfs/xfs_trans.h
+++ b/fs/xfs/xfs_trans.h
@@ -447,8 +447,14 @@ typedef struct xfs_trans {
 /*
  * XFS transaction mechanism exported interfaces.
  */
-xfs_trans_t     *xfs_trans_alloc(struct xfs_mount *, uint);
-xfs_trans_t     *_xfs_trans_alloc(struct xfs_mount *, uint, uint);
+xfs_trans_t     *_xfs_trans_alloc(struct xfs_mount *, uint, uint, bool);
+
+static inline struct xfs_trans *
+xfs_trans_alloc(struct xfs_mount *mp, uint type)
+{
+        return _xfs_trans_alloc(mp, type, KM_SLEEP, true);
+}
+
 xfs_trans_t     *xfs_trans_dup(xfs_trans_t *);
 int             xfs_trans_reserve(xfs_trans_t *, uint, uint, uint,
                                   uint, uint);