From 19686e364d4eaa10fb7ddc1226b367ef47a22480 Mon Sep 17 00:00:00 2001 From: jhb Date: Tue, 20 Jan 2009 16:35:34 +0000 Subject: 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 --- sys/ufs/ufs/ufs_dirhash.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'sys/ufs') diff --git a/sys/ufs/ufs/ufs_dirhash.c b/sys/ufs/ufs/ufs_dirhash.c index 0eacb58..3b6d5e1 100644 --- a/sys/ufs/ufs/ufs_dirhash.c +++ b/sys/ufs/ufs/ufs_dirhash.c @@ -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. */ static void -- cgit v1.1