linux-kernel - [PATCH] net: optimize cmpxchg in ip_idents

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Date:   Wed, 15 Jan 2020 11:23:40 +0800
From:   Shaokun Zhang <zhangshaokun@...ilicon.com>
To:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     Yuqi Jin <jinyuqi@...wei.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Yang Guo <guoyang2@...wei.com>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>
Subject: [PATCH] net: optimize cmpxchg in ip_idents_reserve

From: Yuqi Jin <jinyuqi@...wei.com>

atomic_try_cmpxchg is called instead of atomic_cmpxchg that can reduce
the access number of the global variable @p_id in the loop. Let's
optimize it for performance.

Cc: "David S. Miller" <davem@...emloft.net>
Cc: Alexey Kuznetsov <kuznet@....inr.ac.ru>
Cc: Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Yang Guo <guoyang2@...wei.com>
Signed-off-by: Yuqi Jin <jinyuqi@...wei.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@...ilicon.com>
---
 net/ipv4/route.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 87e979f2b74a..7e28c7121c20 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -496,10 +496,10 @@ u32 ip_idents_reserve(u32 hash, int segs)
 		delta = prandom_u32_max(now - old);
 
 	/* Do not use atomic_add_return() as it makes UBSAN unhappy */
+	old = (u32)atomic_read(p_id);
 	do {
-		old = (u32)atomic_read(p_id);
 		new = old + delta + segs;
-	} while (atomic_cmpxchg(p_id, old, new) != old);
+	} while (!atomic_try_cmpxchg(p_id, &old, new));
 
 	return new - segs;
 }
-- 
2.7.4