FAILED: patch "[PATCH] KVM: arm/arm64: vgic: Fix potential deadlock when ap_list is" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d4a8061a7c5f7c27a2dc002ee4cb89b3e6637e44 Mon Sep 17 00:00:00 2001
From: Heyi Guo <[email protected]>
Date: Tue, 27 Aug 2019 12:26:50 +0100
Subject: [PATCH] KVM: arm/arm64: vgic: Fix potential deadlock when ap_list is
long

If the ap_list is longer than 256 entries, merge_final() in list_sort() will call the comparison callback with the same element twice, causing a deadlock in vgic_irq_cmp().

Fix it by returning early when irqa == irqb.

index 04786c8ec77e..ca5e6c6866a4 100644
@@ -254,6 +254,13 @@ static int vgic_irq_cmp(void *priv, struct list_head *a, struct list_head *b)
 	bool penda, pendb;
 	int ret;
 
+	/*
+	 * list_sort may call this function with the same element when
+	 * the list is fairly long.
+	 */
+	if (unlikely(irqa == irqb))
+		return 0;
+
 	raw_spin_lock(&irqa->irq_lock);
 	raw_spin_lock_nested(&irqb->irq_lock, SINGLE_DEPTH_NESTING);