[v2 PATCH] netlink: Do not schedule work from sk_destruct

[Herbert Xu]

On Mon, Dec 05, 2016 at 03:19:46PM +0800, Herbert Xu wrote:
>
> Thanks for the patch.  It'll obviously work but I wanted avoid that
> because it penalises the common path for the rare case.
> 
> Andrey, please try this patch and let me know if it's any better.
> 
> ---8<---
> Subject: netlink: Do not schedule work from sk_destruct

Crap, I screwed it up again.  Here is a v2 which moves the atomic
call into the RCU callback as otherwise the socket can be freed from
another path while we await the RCU callback.

---8<---
It is wrong to schedule a work from sk_destruct using the socket
as the memory reserve because the socket will be freed immediately
after the return from sk_destruct.

Instead we should do the deferral prior to sk_free.

This patch does just that.

Fixes: 707693c8a498 ("netlink: Call cb->done from a worker thread")
Signed-off-by: Herbert Xu <herb...@gondor.apana.org.au>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 602e5eb..463f5cf 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -322,11 +322,13 @@ static void netlink_skb_set_owner_r(struct sk_buff *skb, 
struct sock *sk)
        sk_mem_charge(sk, skb->truesize);
 }
 
-static void __netlink_sock_destruct(struct sock *sk)
+static void netlink_sock_destruct(struct sock *sk)
 {
        struct netlink_sock *nlk = nlk_sk(sk);
 
        if (nlk->cb_running) {
+               if (nlk->cb.done)
+                       nlk->cb.done(&nlk->cb);
                module_put(nlk->cb.module);
                kfree_skb(nlk->cb.skb);
        }
@@ -348,21 +350,7 @@ static void netlink_sock_destruct_work(struct work_struct 
*work)
        struct netlink_sock *nlk = container_of(work, struct netlink_sock,
                                                work);
 
-       nlk->cb.done(&nlk->cb);
-       __netlink_sock_destruct(&nlk->sk);
-}
-
-static void netlink_sock_destruct(struct sock *sk)
-{
-       struct netlink_sock *nlk = nlk_sk(sk);
-
-       if (nlk->cb_running && nlk->cb.done) {
-               INIT_WORK(&nlk->work, netlink_sock_destruct_work);
-               schedule_work(&nlk->work);
-               return;
-       }
-
-       __netlink_sock_destruct(sk);
+       sk_free(&nlk->sk);
 }
 
 /* This lock without WQ_FLAG_EXCLUSIVE is good on UP and it is _very_ bad on
@@ -664,11 +652,21 @@ static int netlink_create(struct net *net, struct socket 
*sock, int protocol,
        goto out;
 }
 
-static void deferred_put_nlk_sk(struct rcu_head *head)
+static void deferred_free_nlk_sk(struct rcu_head *head)
 {
        struct netlink_sock *nlk = container_of(head, struct netlink_sock, rcu);
+       struct sock *sk = &nlk->sk;
+
+       if (!atomic_dec_and_test(&sk->sk_refcnt))
+               return;
+
+       if (nlk->cb_running && nlk->cb.done) {
+               INIT_WORK(&nlk->work, netlink_sock_destruct_work);
+               schedule_work(&nlk->work);
+               return;
+       }
 
-       sock_put(&nlk->sk);
+       sk_free(sk);
 }
 
 static int netlink_release(struct socket *sock)
@@ -743,7 +741,7 @@ static int netlink_release(struct socket *sock)
        local_bh_disable();
        sock_prot_inuse_add(sock_net(sk), &netlink_proto, -1);
        local_bh_enable();
-       call_rcu(&nlk->rcu, deferred_put_nlk_sk);
+       call_rcu(&nlk->rcu, deferred_free_nlk_sk);
        return 0;
 }
 
-- 
Email: Herbert Xu <herb...@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt