xfs: use ->b_state to fix buffer I/O accounting release race

We've had user reports of unmount hangs in xfs_wait_buftarg() that
analysis shows is due to btp->bt_io_count == -1. bt_io_count
represents the count of in-flight asynchronous buffers and thus
should always be >= 0. xfs_wait_buftarg() waits for this value to
stabilize to zero in order to ensure that all untracked (with
respect to the lru) buffers have completed I/O processing before
unmount proceeds to tear down in-core data structures.

The value of -1 implies an I/O accounting decrement race. Indeed,
the fact that xfs_buf_ioacct_dec() is called from xfs_buf_rele()
(where the buffer lock is no longer held) means that bp->b_flags can
be updated from an unsafe context. While a user-level reproducer is
currently not available, some intrusive hacks to run racing buffer
lookups/ioacct/releases from multiple threads was used to
successfully manufacture this problem.

Existing callers do not expect to acquire the buffer lock from
xfs_buf_rele(). Therefore, we can not safely update ->b_flags from
this context. It turns out that we already have separate buffer
state bits and associated serialization for dealing with buffer LRU
state in the form of ->b_state and ->b_lock. Therefore, replace the
_XBF_IN_FLIGHT flag with a ->b_state variant, update the I/O
accounting wrappers appropriately and make sure they are used with
the correct locking. This ensures that buffer in-flight state can be
modified at buffer release time without racing with modifications
from a buffer lock holder.

Fixes: 9c7504aa72b6 ("xfs: track and serialize in-flight async buffers against unmount")
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Libor Pechacek <lpechacek@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 26 insertions, 12 deletions

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 62fa39276a24..07b77b73b024 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -97,12 +97,16 @@ static inline void
 xfs_buf_ioacct_inc(
         struct xfs_buf  *bp)
 {
-        if (bp->b_flags & (XBF_NO_IOACCT|_XBF_IN_FLIGHT))
+        if (bp->b_flags & XBF_NO_IOACCT)
                 return;
 
         ASSERT(bp->b_flags & XBF_ASYNC);
-        bp->b_flags |= _XBF_IN_FLIGHT;
-        percpu_counter_inc(&bp->b_target->bt_io_count);
+        spin_lock(&bp->b_lock);
+        if (!(bp->b_state & XFS_BSTATE_IN_FLIGHT)) {
+                bp->b_state |= XFS_BSTATE_IN_FLIGHT;
+                percpu_counter_inc(&bp->b_target->bt_io_count);
+        }
+        spin_unlock(&bp->b_lock);
 }
 
 /*
@@ -110,14 +114,24 @@ xfs_buf_ioacct_inc(
  * freed and unaccount from the buftarg.
  */
 static inline void
-xfs_buf_ioacct_dec(
+__xfs_buf_ioacct_dec(
         struct xfs_buf  *bp)
 {
-        if (!(bp->b_flags & _XBF_IN_FLIGHT))
-                return;
+        ASSERT(spin_is_locked(&bp->b_lock));
 
-        bp->b_flags &= ~_XBF_IN_FLIGHT;
-        percpu_counter_dec(&bp->b_target->bt_io_count);
+        if (bp->b_state & XFS_BSTATE_IN_FLIGHT) {
+                bp->b_state &= ~XFS_BSTATE_IN_FLIGHT;
+                percpu_counter_dec(&bp->b_target->bt_io_count);
+        }
+}
+
+static inline void
+xfs_buf_ioacct_dec(
+        struct xfs_buf  *bp)
+{
+        spin_lock(&bp->b_lock);
+        __xfs_buf_ioacct_dec(bp);
+        spin_unlock(&bp->b_lock);
 }
 
 /*
@@ -149,9 +163,9 @@ xfs_buf_stale(
          * unaccounted (released to LRU) before that occurs. Drop in-flight
          * status now to preserve accounting consistency.
          */
-        xfs_buf_ioacct_dec(bp);
-
         spin_lock(&bp->b_lock);
+        __xfs_buf_ioacct_dec(bp);
+
         atomic_set(&bp->b_lru_ref, 0);
         if (!(bp->b_state & XFS_BSTATE_DISPOSE) &&
             (list_lru_del(&bp->b_target->bt_lru, &bp->b_lru)))
@@ -979,12 +993,12 @@ xfs_buf_rele(
                  * ensures the decrement occurs only once per-buf.
                  */
                 if ((atomic_read(&bp->b_hold) == 1) && !list_empty(&bp->b_lru))
-                        xfs_buf_ioacct_dec(bp);
+                        __xfs_buf_ioacct_dec(bp);
                 goto out_unlock;
         }
 
         /* the last reference has been dropped ... */
-        xfs_buf_ioacct_dec(bp);
+        __xfs_buf_ioacct_dec(bp);
         if (!(bp->b_flags & XBF_STALE) && atomic_read(&bp->b_lru_ref)) {
                 /*
                  * If the buffer is added to the LRU take a new reference to the