[PATCH] netdev: Use try_cmpxchg in napi_if_scheduled_mark_missed

[PATCH] netdev: Use try_cmpxchg in napi_if_scheduled_mark_missed

[Uros Bizjak]

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
napi_if_scheduled_mark_missed. x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails, enabling further code simplifications.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
 include/linux/netdevice.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 1a3cb93c3dcc..51c916894661 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -546,8 +546,8 @@ static inline bool napi_if_scheduled_mark_missed(struct napi_struct *n)
 {
 	unsigned long val, new;
 
+	val = READ_ONCE(n->state);
 	do {
-		val = READ_ONCE(n->state);
 		if (val & NAPIF_STATE_DISABLE)
 			return true;
 
@@ -555,7 +555,7 @@ static inline bool napi_if_scheduled_mark_missed(struct napi_struct *n)
 			return false;
 
 		new = val | NAPIF_STATE_MISSED;
-	} while (cmpxchg(&n->state, val, new) != val);
+	} while (!try_cmpxchg(&n->state, &val, new));
 
 	return true;
 }
-- 
2.37.1

Re: [PATCH] netdev: Use try_cmpxchg in napi_if_scheduled_mark_missed

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Mon, 22 Aug 2022 16:32:43 +0200 you wrote:
> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
> napi_if_scheduled_mark_missed. x86 CMPXCHG instruction returns
> success in ZF flag, so this change saves a compare after cmpxchg
> (and related move instruction in front of cmpxchg).
> 
> Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
> fails, enabling further code simplifications.
> 
> [...]

Here is the summary with links:
  - netdev: Use try_cmpxchg in napi_if_scheduled_mark_missed
    https://git.kernel.org/netdev/net-next/c/b9030780971b

You are awesome, thank you!
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