linux-kernel - [PATCH 2/3] KVM: Use atomic_long_cmpxchg() instead of an open-coded variant

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Date:   Fri, 26 Nov 2021 01:31:08 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Igor Mammedov <imammedo@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/3] KVM: Use atomic_long_cmpxchg() instead of an open-coded variant

From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>

Open-coding a cmpxchg()-like operation is significantly less readable than
a direct call.

Also, the open-coded version compiles to multiple instructions with
a branch on x86, instead of just a single instruction.

Since technically the open-coded variant didn't guarantee actual atomicity
add a comment there, too, that this property isn't strictly required in
this case.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
---
 virt/kvm/kvm_main.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d4399db06d49..367c1cba26d2 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1378,8 +1378,12 @@ static void kvm_replace_memslot(struct kvm *kvm,
 		hash_del(&old->id_node[idx]);
 		interval_tree_remove(&old->hva_node[idx], &slots->hva_tree);
 
-		if ((long)old == atomic_long_read(&slots->last_used_slot))
-			atomic_long_set(&slots->last_used_slot, (long)new);
+		/*
+		 * The atomicity isn't strictly required here since we are
+		 * operating on an inactive memslots set anyway.
+		 */
+		atomic_long_cmpxchg(&slots->last_used_slot,
+				    (unsigned long)old, (unsigned long)new);
 
 		if (!new) {
 			kvm_erase_gfn_node(slots, old);