linux-kernel - [PATCH] netdev: Use try_cmpxchg in napi_if_scheduled_mark

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Date:   Mon, 22 Aug 2022 16:32:43 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: [PATCH] netdev: Use try_cmpxchg in napi_if_scheduled_mark_missed

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@...emloft.net>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>
Cc: Paolo Abeni <pabeni@...hat.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.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