[PATCH net] net: avoid possible false sharing in sk_leave_memory_pressure()

[Eric Dumazet]

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 <eduma...@google.com>
---
 net/core/sock.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 
fac2b4d80de5e54c77628073d7930bddf8a10cb3..50647a10fdb7f050e963e2734f0d3555fa4bd7aa
 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);
        }
 }
 
-- 
2.23.0.581.g78d2f28ef7-goog