From 7f4078ff3ae0baf0a6df52f14c0a0680366931f8 Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka@redhat.com>
Date: Mon, 21 Jul 2008 12:00:34 +0100
Subject: [PATCH] --- yaml --- r: 104294 b: refs/heads/master c:
 a8d41b59f3f5a7ac19452ef442a7fc1b5fa17366 h: refs/heads/master v: v3

---
 [refs]                     |  2 +-
 trunk/drivers/md/dm-snap.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/[refs] b/[refs]
index c61f1428b95a..b2301be2244a 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: cd45daffd1f7b53aac0835b23e97f814ec3f10dc
+refs/heads/master: a8d41b59f3f5a7ac19452ef442a7fc1b5fa17366
diff --git a/trunk/drivers/md/dm-snap.c b/trunk/drivers/md/dm-snap.c
index de302702ab3e..f4fd0cee9c3d 100644
--- a/trunk/drivers/md/dm-snap.c
+++ b/trunk/drivers/md/dm-snap.c
@@ -134,6 +134,27 @@ static void stop_tracking_chunk(struct dm_snapshot *s,
 	mempool_free(c, s->tracked_chunk_pool);
 }
 
+static int __chunk_is_tracked(struct dm_snapshot *s, chunk_t chunk)
+{
+	struct dm_snap_tracked_chunk *c;
+	struct hlist_node *hn;
+	int found = 0;
+
+	spin_lock_irq(&s->tracked_chunk_lock);
+
+	hlist_for_each_entry(c, hn,
+	    &s->tracked_chunk_hash[DM_TRACKED_CHUNK_HASH(chunk)], node) {
+		if (c->chunk == chunk) {
+			found = 1;
+			break;
+		}
+	}
+
+	spin_unlock_irq(&s->tracked_chunk_lock);
+
+	return found;
+}
+
 /*
  * One of these per registered origin, held in the snapshot_origins hash
  */
@@ -839,6 +860,13 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
 		goto out;
 	}
 
+	/*
+	 * Check for conflicting reads. This is extremely improbable,
+	 * so yield() is sufficient and there is no need for a wait queue.
+	 */
+	while (__chunk_is_tracked(s, pe->e.old_chunk))
+		yield();
+
 	/*
 	 * Add a proper exception, and remove the
 	 * in-flight exception from the list.