xfs: fix a buffer lookup against removal race

When a buffer is freed either by LRU eviction or because it is unset,
the lockref is marked as dead instantly, which prevents the buffer from
being used after finding it in the buffer hash in xfs_buf_lookup and
xfs_buf_find_insert.  But the latter will then not add the new buffer to
the hash because it already found an existing buffer.

Fix this using in two places:  Remove the buffer from the hash before
marking the lockref dead so that that no buffer with a dead lockref can
be found in the hash, but if we find one in xfs_buf_find_insert due to
store reordering, handle this case correctly instead of returning an
unhashed buffer.

Fixes: 67fe430 ("xfs: don't keep a reference for buffers on the LRU")
Reported-by: Andrey Albershteyn <aalbersh@redhat.com>
Reported-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: Christoph Hellwig

fs/xfs/xfs_buf.c

@@ -472,6 +472,7 @@ xfs_buf_find_insert(
 	/* The new buffer keeps the perag reference until it is freed. */
 	new_bp->b_pag = pag;
 
+retry:
 	rcu_read_lock();
 	bp = rhashtable_lookup_get_insert_fast(&btp->bt_hash,
 			&new_bp->b_rhash_head, xfs_buf_hash_params);
@@ -480,8 +481,16 @@ xfs_buf_find_insert(
 		error = PTR_ERR(bp);
 		goto out_free_buf;
 	}
-	if (bp && lockref_get_not_dead(&bp->b_lockref)) {
-		/* found an existing buffer */
+	if (bp) {
+		/*
+		 * If there is an existing buffer with a dead lockref, retry
+		 * until the new buffer is added, or a usable buffer is found.
+		 */
+		if (!lockref_get_not_dead(&bp->b_lockref)) {
+			rcu_read_unlock();
+			cpu_relax();
+			goto retry;
+		}
 		rcu_read_unlock();
 		error = xfs_buf_find_lock(bp, flags);
 		if (error)
@@ -820,15 +829,20 @@ xfs_buf_destroy(
 	ASSERT(__lockref_is_dead(&bp->b_lockref));
 	ASSERT(!(bp->b_flags & _XBF_DELWRI_Q));
 
+	if (bp->b_pag)
+		xfs_perag_put(bp->b_pag);
+	xfs_buf_free(bp);
+}
+
+static inline void
+xfs_buf_kill(
+	struct xfs_buf		*bp)
+{
+	lockref_mark_dead(&bp->b_lockref);
 	if (!xfs_buf_is_uncached(bp)) {
 		rhashtable_remove_fast(&bp->b_target->bt_hash,
 				&bp->b_rhash_head, xfs_buf_hash_params);
-
-		if (bp->b_pag)
-			xfs_perag_put(bp->b_pag);
 	}
-
-	xfs_buf_free(bp);
 }
 
 /*
@@ -851,7 +865,7 @@ xfs_buf_rele(
 	return;
 
 kill:
-	lockref_mark_dead(&bp->b_lockref);
+	xfs_buf_kill(bp);
 	list_lru_del_obj(&bp->b_target->bt_lru, &bp->b_lru);
 	spin_unlock(&bp->b_lockref.lock);
 
@@ -1433,7 +1447,7 @@ xfs_buftarg_drain_rele(
 		return LRU_SKIP;
 	}
 
-	lockref_mark_dead(&bp->b_lockref);
+	xfs_buf_kill(bp);
 	list_lru_isolate_move(lru, item, dispose);
 	spin_unlock(&bp->b_lockref.lock);
 	return LRU_REMOVED;
@@ -1545,7 +1559,7 @@ xfs_buftarg_isolate(
 		return LRU_ROTATE;
 	}
 
-	lockref_mark_dead(&bp->b_lockref);
+	xfs_buf_kill(bp);
 	list_lru_isolate_move(lru, item, dispose);
 	spin_unlock(&bp->b_lockref.lock);
 	return LRU_REMOVED;