diff --git a/src/prof.c b/src/prof.c
index 28d30f29..5aeefb28 100644
--- a/src/prof.c
+++ b/src/prof.c
@@ -708,7 +708,7 @@ prof_lookup_global(tsd_t *tsd, prof_bt_t *bt, prof_tdata_t *tdata,
 	union {
 		prof_gctx_t	*p;
 		void		*v;
-	} gctx;
+	} gctx, tgctx;
 	union {
 		prof_bt_t	*p;
 		void		*v;
@@ -718,21 +718,32 @@ prof_lookup_global(tsd_t *tsd, prof_bt_t *bt, prof_tdata_t *tdata,
 	prof_enter(tsd, tdata);
 	if (ckh_search(&bt2gctx, bt, &btkey.v, &gctx.v)) {
 		/* bt has never been seen before.  Insert it. */
-		gctx.p = prof_gctx_create(tsd_tsdn(tsd), bt);
-		if (gctx.v == NULL) {
-			prof_leave(tsd, tdata);
+		prof_leave(tsd, tdata);
+		tgctx.p = prof_gctx_create(tsd_tsdn(tsd), bt);
+		if (tgctx.v == NULL) {
 			return true;
 		}
-		btkey.p = &gctx.p->bt;
-		if (ckh_insert(tsd, &bt2gctx, btkey.v, gctx.v)) {
-			/* OOM. */
-			prof_leave(tsd, tdata);
-			idalloctm(tsd_tsdn(tsd), iealloc(tsd_tsdn(tsd), gctx.v),
-			    gctx.v, NULL, true, true);
-			return true;
+		prof_enter(tsd, tdata);
+		if (ckh_search(&bt2gctx, bt, &btkey.v, &gctx.v)) {
+			gctx.p = tgctx.p;
+			btkey.p = &gctx.p->bt;
+			if (ckh_insert(tsd, &bt2gctx, btkey.v, gctx.v)) {
+				/* OOM. */
+				prof_leave(tsd, tdata);
+				idalloctm(tsd_tsdn(tsd), iealloc(tsd_tsdn(tsd),
+				    gctx.v), gctx.v, NULL, true, true);
+				return true;
+			}
+			new_gctx = true;
+		} else {
+			new_gctx = false;
 		}
-		new_gctx = true;
 	} else {
+		tgctx.v = NULL;
+		new_gctx = false;
+	}
+
+	if (!new_gctx) {
 		/*
 		 * Increment nlimbo, in order to avoid a race condition with
 		 * prof_tctx_destroy()/prof_gctx_try_destroy().
@@ -741,6 +752,12 @@ prof_lookup_global(tsd_t *tsd, prof_bt_t *bt, prof_tdata_t *tdata,
 		gctx.p->nlimbo++;
 		malloc_mutex_unlock(tsd_tsdn(tsd), gctx.p->lock);
 		new_gctx = false;
+
+		if (tgctx.v != NULL) {
+			/* Lost race to insert. */
+			idalloctm(tsd_tsdn(tsd), iealloc(tsd_tsdn(tsd),
+			    tgctx.v), tgctx.v, NULL, true, true);
+		}
 	}
 	prof_leave(tsd, tdata);