From: Eric Dumazet <edumazet@google.com>
Date: Wed, 9 Oct 2019 19:55:53 +0000 (-0700)
Subject: net: avoid possible false sharing in sk_leave_memory_pressure()
X-Git-Tag: v5.4-rc4~6^2~61
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=503978aca46124cd714703e180b9c8292ba50ba7;p=users%2Fjedix%2Flinux-maple.git

net: avoid possible false sharing in sk_leave_memory_pressure()

As mentioned in https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#it-may-improve-performance
a C compiler can legally transform :

if (memory_pressure && *memory_pressure)
        *memory_pressure = 0;

to :

if (memory_pressure)
        *memory_pressure = 0;

Fixes: 0604475119de ("tcp: add TCPMemoryPressuresChrono counter")
Fixes: 180d8cd942ce ("foundations of per-cgroup memory pressure controlling.")
Fixes: 3ab224be6d69 ("[NET] CORE: Introducing new memory accounting interface.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
---

diff --git a/net/core/sock.c b/net/core/sock.c
index fac2b4d80de5..50647a10fdb7 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2334,8 +2334,8 @@ static void sk_leave_memory_pressure(struct sock *sk)
 	} else {
 		unsigned long *memory_pressure = sk->sk_prot->memory_pressure;
 
-		if (memory_pressure && *memory_pressure)
-			*memory_pressure = 0;
+		if (memory_pressure && READ_ONCE(*memory_pressure))
+			WRITE_ONCE(*memory_pressure, 0);
 	}
 }