net: fix sk_memory_allocated_{add|sub} vs softirqs

Jonathan Heathcote reported a regression caused by blamed commit
on aarch64 architecture.

x86 happens to have irq-safe __this_cpu_add_return()
and __this_cpu_sub(), but this is not generic.

I think my confusion came from "struct sock" argument,
because these helpers are called with a locked socket.
But the memory accounting is per-proto (and per-cpu after
the blamed commit). We might cleanup these helpers later
to directly accept a "struct proto *proto" argument.

Switch to this_cpu_add_return() and this_cpu_xchg()
operations, and get rid of preempt_disable()/preempt_enable() pairs.

Fast path becomes a bit faster as a result :)

Many thanks to Jonathan Heathcote for his awesome report and
investigations.

Fixes: 3cd3399 ("net: implement per-cpu reserves for memory_allocated")
Reported-by: Jonathan Heathcote <jonathan.heathcote@bbc.co.uk>
Closes: https://lore.kernel.org/netdev/VI1PR01MB42407D7947B2EA448F1E04EFD10D2@VI1PR01MB4240.eurprd01.prod.exchangelabs.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Link: https://lore.kernel.org/r/20240421175248.1692552-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/sock.h

@@ -1410,32 +1410,34 @@ sk_memory_allocated(const struct sock *sk)
 #define SK_MEMORY_PCPU_RESERVE (1 << (20 - PAGE_SHIFT))
 extern int sysctl_mem_pcpu_rsv;
 
+static inline void proto_memory_pcpu_drain(struct proto *proto)
+{
+	int val = this_cpu_xchg(*proto->per_cpu_fw_alloc, 0);
+
+	if (val)
+		atomic_long_add(val, proto->memory_allocated);
+}
+
 static inline void
-sk_memory_allocated_add(struct sock *sk, int amt)
+sk_memory_allocated_add(const struct sock *sk, int val)
 {
-	int local_reserve;
+	struct proto *proto = sk->sk_prot;
 
-	preempt_disable();
-	local_reserve = __this_cpu_add_return(*sk->sk_prot->per_cpu_fw_alloc, amt);
-	if (local_reserve >= READ_ONCE(sysctl_mem_pcpu_rsv)) {
-		__this_cpu_sub(*sk->sk_prot->per_cpu_fw_alloc, local_reserve);
-		atomic_long_add(local_reserve, sk->sk_prot->memory_allocated);
-	}
-	preempt_enable();
+	val = this_cpu_add_return(*proto->per_cpu_fw_alloc, val);
+
+	if (unlikely(val >= READ_ONCE(sysctl_mem_pcpu_rsv)))
+		proto_memory_pcpu_drain(proto);
 }
 
 static inline void
-sk_memory_allocated_sub(struct sock *sk, int amt)
+sk_memory_allocated_sub(const struct sock *sk, int val)
 {
-	int local_reserve;
+	struct proto *proto = sk->sk_prot;
 
-	preempt_disable();
-	local_reserve = __this_cpu_sub_return(*sk->sk_prot->per_cpu_fw_alloc, amt);
-	if (local_reserve <= -READ_ONCE(sysctl_mem_pcpu_rsv)) {
-		__this_cpu_sub(*sk->sk_prot->per_cpu_fw_alloc, local_reserve);
-		atomic_long_add(local_reserve, sk->sk_prot->memory_allocated);
-	}
-	preempt_enable();
+	val = this_cpu_sub_return(*proto->per_cpu_fw_alloc, val);
+
+	if (unlikely(val <= -READ_ONCE(sysctl_mem_pcpu_rsv)))
+		proto_memory_pcpu_drain(proto);
 }
 
 #define SK_ALLOC_PERCPU_COUNTER_BATCH 16