From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH] net: Fix sock_wfree() race Date: Wed, 23 Sep 2009 15:44:15 +0200 Message-ID: <4ABA262F.9040407@gmail.com> References: <4AA64A11.7090804@gmail.com> <4AA6DF7B.7060105@gmail.com> <20090911.114337.150207703.davem@davemloft.net> <20090911.125242.244008840.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: albcamus@gmail.com, parag.lkml@gmail.com, linux-kernel@vger.kernel.org, netdev@vger.kernel.org To: David Miller Return-path: Received: from gw1.cosmosbay.com ([212.99.114.194]:42689 "EHLO gw1.cosmosbay.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751311AbZIWNoP (ORCPT ); Wed, 23 Sep 2009 09:44:15 -0400 In-Reply-To: <20090911.125242.244008840.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-ID: David Miller a =E9crit : > From: David Miller > Date: Fri, 11 Sep 2009 11:43:37 -0700 (PDT) >=20 >> From: Eric Dumazet >> Date: Wed, 09 Sep 2009 00:49:31 +0200 >> >>> [PATCH] net: Fix sock_wfree() race >>> >>> Commit 2b85a34e911bf483c27cfdd124aeb1605145dc80 >>> (net: No more expensive sock_hold()/sock_put() on each tx) >>> opens a window in sock_wfree() where another cpu >>> might free the socket we are working on. >>> >>> Fix is to call sk->sk_write_space(sk) only >>> while still holding a reference on sk. >>> >>> Since doing this call is done before the=20 >>> atomic_sub(truesize, &sk->sk_wmem_alloc), we should pass truesize a= s=20 >>> a bias for possible sk_wmem_alloc evaluations. >>> >>> Reported-by: Jike Song >>> Signed-off-by: Eric Dumazet >> Applied to net-next-2.6, thanks. I'll queue up your simpler >> version for -stable. >=20 > Eric, I have to revert, as you didn't update the callbacks > of several protocols such as SCTP and RDS in this change. >=20 > Let me know when you have a fixed version of this patch :-) Sorry for the delay David. But this is complex. I am not sure we can do a clean and safe thing, not counting the added bloat. If we do : void sock_wfree(struct sk_buff *skb) { struct sock *sk =3D skb->sk; int res; if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE)) sk->sk_write_space(sk, skb->truesize); res =3D atomic_sub_return(skb->truesize, &sk->sk_wmem_alloc); /* * if sk_wmem_alloc reached 0, we are last user and should * free this sock, as sk_free() call could not do it. */ if (res =3D=3D 0) __sk_free(sk); } There is still a possibility multiple cpus call sock_wfree() for the same socket, and that they all call sk_write_space() with their bias, yet the protocol still has a possible too big estimation of sk_wmem_alloc We could miss to wakeup a blocked writer in case low sk->sk_sndbuf values are setup. (One could argue that with small sk_sndbuf values we should not have many packets in flight : Keep in mind sk_sndbuf can be lowered by the user) With second patch we instead have : void sock_wfree(struct sk_buff *skb) { struct sock *sk =3D skb->sk; unsigned int len =3D skb->truesize; if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE)) { /* * Keep a reference on sk_wmem_alloc, this will be released * after sk_write_space() call */ atomic_sub(len - 1, &sk->sk_wmem_alloc); sk->sk_write_space(sk); len =3D 1; } /* * if sk_wmem_alloc reaches 0, we must finish what sk_free() * could not do because of in-flight packets */ if (atomic_sub_return(len, &sk->sk_wmem_alloc) =3D=3D 0) __sk_free(sk); } The accumulated transient error on sk_wmem_alloc is then < num_online_c= pus(), that should be OK even for very small sk_sndbuf values. Of course TCP doesnt have to pay the price of sk_write_space() and the = second atomic operation re-added by this fix. Here is the patch for reference : [PATCH] net: Fix sock_wfree() race Commit 2b85a34e911bf483c27cfdd124aeb1605145dc80 (net: No more expensive sock_hold()/sock_put() on each tx) opens a window in sock_wfree() where another cpu might free the socket we are working on. A fix is to call sk->sk_write_space(sk) while still holding a reference on sk. Reported-by: Jike Song Signed-off-by: Eric Dumazet --- net/core/sock.c | 19 ++++++++++++------- 1 files changed, 12 insertions(+), 7 deletions(-) diff --git a/net/core/sock.c b/net/core/sock.c index 30d5446..e1f034e 100644 --- a/net/core/sock.c +++ b/net/core/sock.c @@ -1228,17 +1228,22 @@ void __init sk_init(void) void sock_wfree(struct sk_buff *skb) { struct sock *sk =3D skb->sk; - int res; + unsigned int len =3D skb->truesize; =20 - /* In case it might be waiting for more memory. */ - res =3D atomic_sub_return(skb->truesize, &sk->sk_wmem_alloc); - if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE)) + if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE)) { + /* + * Keep a reference on sk_wmem_alloc, this will be released + * after sk_write_space() call + */ + atomic_sub(len - 1, &sk->sk_wmem_alloc); sk->sk_write_space(sk); + len =3D 1; + } /* - * if sk_wmem_alloc reached 0, we are last user and should - * free this sock, as sk_free() call could not do it. + * if sk_wmem_alloc reaches 0, we must finish what sk_free() + * could not do because of in-flight packets */ - if (res =3D=3D 0) + if (atomic_sub_return(len, &sk->sk_wmem_alloc) =3D=3D 0) __sk_free(sk); } EXPORT_SYMBOL(sock_wfree);