tcp/dccp: add SLAB_DESTROY_BY_RCU flag for request sockets

Before letting request sockets being put in TCP/DCCP regular
ehash table, we need to add either :

- SLAB_DESTROY_BY_RCU flag to their kmem_cache
- add RCU grace period before freeing them.

Since we carefully respected the SLAB_DESTROY_BY_RCU protocol
like ESTABLISH and TIMEWAIT sockets, use it here.

req_prot_init() being only used by TCP and DCCP, I did not add
a new slab_flags into their rsk_prot, but reuse prot->slab_flags

Since all reqsk_alloc() users are correctly dealing with a failure,
add the __GFP_NOWARN flag to avoid traces under pressure.

Fixes: 079096f1 ("tcp/dccp: install syn_recv requests into ehash table")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/request_sock.h

@@ -82,7 +82,9 @@ static inline struct sock *req_to_sk(struct request_sock *req)
 static inline struct request_sock *
 reqsk_alloc(const struct request_sock_ops *ops, struct sock *sk_listener)
 {
-	struct request_sock *req = kmem_cache_alloc(ops->slab, GFP_ATOMIC);
+	struct request_sock *req;
+
+	req = kmem_cache_alloc(ops->slab, GFP_ATOMIC | __GFP_NOWARN);
 
 	if (req) {
 		req->rsk_ops = ops;

net/core/sock.c

@@ -2758,7 +2758,7 @@ static int req_prot_init(const struct proto *prot)
 
 	rsk_prot->slab = kmem_cache_create(rsk_prot->slab_name,
 					   rsk_prot->obj_size, 0,
-					   0, NULL);
+					   prot->slab_flags, NULL);
 
 	if (!rsk_prot->slab) {
 		pr_crit("%s: Can't create request sock SLAB cache!\n",