svn commit: r187474 - head/sys/ufs/ufs
jhb at FreeBSD.org
Tue Jan 20 08:35:35 PST 2009
Date: Tue Jan 20 16:35:34 2009
New Revision: 187474
Add a comment explaining why the "bufwait" / "dirhash" LOR reported by
WITNESS will not actually result in a deadlock.
Discussed with: kib
MFC after: 1 week
--- head/sys/ufs/ufs/ufs_dirhash.c Tue Jan 20 14:09:12 2009 (r187473)
+++ head/sys/ufs/ufs/ufs_dirhash.c Tue Jan 20 16:35:34 2009 (r187474)
@@ -126,6 +126,18 @@ static struct mtx ufsdirhash_mtx;
* free a dirhash structure that was recycled by ufsdirhash_recycle().
* The dirhash lock may be held across io operations.
+ * WITNESS reports a lock order reversal between the "bufwait" lock
+ * and the "dirhash" lock. However, this specific reversal will not
+ * cause a deadlock. To get a deadlock, one would have to lock a
+ * buffer followed by the dirhash while a second thread locked a
+ * buffer while holding the dirhash lock. The second order can happen
+ * under a shared or exclusive vnode lock for the associated directory
+ * in lookup(). The first order, however, can only happen under an
+ * exclusive vnode lock (e.g. unlink(), rename(), etc.). Thus, for
+ * a thread to be doing a "bufwait" -> "dirhash" order, it has to hold
+ * an exclusive vnode lock. That exclusive vnode lock will prevent
+ * any other threads from doing a "dirhash" -> "bufwait" order.
More information about the svn-src-head