Make uma_mtx MTX_RECURSE. Here's why:

The general UMA lock is a recursion-allowed lock because
there is a code path where, while we're still configured
to use startup_alloc() for backend page allocations, we
may end up in uma_reclaim() which calls zone_foreach(zone_drain),
which grabs uma_mtx, only to later call into startup_alloc()
because while freeing we needed to allocate a bucket.  Since
startup_alloc() also takes uma_mtx, we need to be able to
recurse on it.

This exact explanation also added as comment above mtx_init().

Trace showing recursion reported by: Peter Holm <peter-at-holm.cc>

1 files changed, 11 insertions, 1 deletions

diff --git a/sys/vm/uma_core.c b/sys/vm/uma_core.c
index 6a4c457..d49deea 100644
--- a/sys/vm/uma_core.c
+++ b/sys/vm/uma_core.c
@@ -1393,7 +1393,17 @@ uma_startup(void *bootmem)
 #ifdef UMA_DEBUG
 	printf("Creating uma keg headers zone and keg.\n");
 #endif
-	mtx_init(&uma_mtx, "UMA lock", NULL, MTX_DEF);
+	/*
+	 * The general UMA lock is a recursion-allowed lock because
+	 * there is a code path where, while we're still configured
+	 * to use startup_alloc() for backend page allocations, we
+	 * may end up in uma_reclaim() which calls zone_foreach(zone_drain),
+	 * which grabs uma_mtx, only to later call into startup_alloc()
+	 * because while freeing we needed to allocate a bucket.  Since
+	 * startup_alloc() also takes uma_mtx, we need to be able to
+	 * recurse on it.
+	 */
+	mtx_init(&uma_mtx, "UMA lock", NULL, MTX_DEF | MTX_RECURSE);
 
 	/* "manually" create the initial zone */
 	args.name = "UMA Kegs";