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

Commit Message

Maciej S. Szmigiero Nov. 26, 2021, 12:31 a.m. UTC

From: "Maciej S. Szmigiero" <[e-mail]>

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 <[e-mail]>
---
 virt/kvm/kvm_main.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

Comments

Paolo Bonzini Nov. 26, 2021, 10:34 a.m. UTC | #1

On 11/26/21 01:31, Maciej S. Szmigiero wrote:
> -		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);

I think using read/set is more readable than a comment saying that 
atomicity is not required.

It's a fairly common pattern, and while I agree that it's a PITA to 
write atomic_long_read and atomic_long_set, the person that reads the 
code is also helped by read/set, because they know they have to think 
about ownership invariants rather than concurrency invariants.

Paolo

Patch

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);