Em 19-01-2016 16:37, Vlad Yasevich escreveu:
On 01/19/2016 10:59 AM, Marcelo Ricardo Leitner wrote:
Em 19-01-2016 12:19, Vlad Yasevich escreveu:
On 01/15/2016 04:40 PM, Marcelo Ricardo Leitner wrote:
On Fri, Jan 15, 2016 at 08:11:03PM +0100, Dmitry Vyukov wrote:
On Fri, Jan 15, 2016 at 7:46 PM, Marcelo Ricardo Leitner
<marcelo.leitner@xxxxxxxxx> wrote:
On Wed, Dec 30, 2015 at 09:42:27PM +0100, Dmitry Vyukov wrote:
Hello,
The following program leads to a leak of two sock objects:
...
On commit 8513342170278468bac126640a5d2d12ffbff106 (Dec 28).
I'm afraid I cannot reproduce this one?
I enabled dynprintk at sctp_destroy_sock and it does print twice when I
run this test app.
Also added debugs to check association lifetime, and then it was
destroyed. Same for endpoint.
Checking with trace-cmd, both calls to sctp_close() resulted in
sctp_destroy_sock() being called.
As for sock_hold/put, they are matched too.
Ideas? Log is below for double checking
Hummm... I can reproduce it pretty reliably.
[ 197.459024] kmemleak: 11 new suspected memory leaks (see
/sys/kernel/debug/kmemleak)
[ 307.494874] kmemleak: 409 new suspected memory leaks (see
/sys/kernel/debug/kmemleak)
[ 549.784022] kmemleak: 125 new suspected memory leaks (see
/sys/kernel/debug/kmemleak)
I double checked via /proc/slabinfo:
SCTPv6 4373 4420 2368 13 8 : tunables 0 0
0 : slabdata 340 340 0
SCTPv6 starts with almost 0, but grows infinitely while I run the
program in a loop.
Here is my SCTP related configs:
CONFIG_IP_SCTP=y
CONFIG_NET_SCTPPROBE=y
CONFIG_SCTP_DBG_OBJCNT=y
# CONFIG_SCTP_DEFAULT_COOKIE_HMAC_MD5 is not set
# CONFIG_SCTP_DEFAULT_COOKIE_HMAC_SHA1 is not set
CONFIG_SCTP_DEFAULT_COOKIE_HMAC_NONE=y
# CONFIG_SCTP_COOKIE_HMAC_MD5 is not set
# CONFIG_SCTP_COOKIE_HMAC_SHA1 is not set
I am on commit 67990608c8b95d2b8ccc29932376ae73d5818727 and I don't
seem to have any sctp-related changes on top.
Ok, now I can. Enabled slub debugs, now I cannot see calls to
sctp_destroy_sock. I see to sctp_close, but not to sctp_destroy_sock.
And SCTPv6 grew by 2 sockets after the execution.
Further checking, it's a race within SCTP asoc migration:
thread 0 thread 1
- app creates a sock
- sends a packet to itself
- sctp will create an asoc and do implicit
handshake
- send the packet
- listen()
- accept() is called and
that asoc is migrated
- packet is delivered
- skb->destructor is called, BUT:
(note that if accept() is called after packet is delivered and skb is freed, it
doesn't happen)
static void sctp_wfree(struct sk_buff *skb)
{
struct sctp_chunk *chunk = skb_shinfo(skb)->destructor_arg;
struct sctp_association *asoc = chunk->asoc;
struct sock *sk = asoc->base.sk;
...
atomic_sub(sizeof(struct sctp_chunk), &sk->sk_wmem_alloc);
and it's pointing to the new socket already. So one socket gets a leak
on sk_wmem_alloc and another gets a negative value:
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -1537,12 +1537,14 @@ static void sctp_close(struct sock *sk, long timeout)
/* Hold the sock, since sk_common_release() will put sock_put()
* and we have just a little more cleanup.
*/
+ printk("%s sock_hold %p\n", __func__, sk);
sock_hold(sk);
sk_common_release(sk);
bh_unlock_sock(sk);
spin_unlock_bh(&net->sctp.addr_wq_lock);
+ printk("%s sock_put %p %d %d\n", __func__, sk, atomic_read(&sk->sk_refcnt),
atomic_read(&sk->sk_wmem_alloc));
sock_put(sk);
SCTP_DBG_OBJCNT_DEC(sock);
gave me:
[ 99.456944] sctp_close sock_hold ffff880137df8940
...
[ 99.457337] sctp_close sock_put ffff880137df8940 1 -247
[ 99.458313] sctp_close sock_hold ffff880137dfef00
...
[ 99.458383] sctp_close sock_put ffff880137dfef00 1 249
That's why the socket is not freed..
Interesting... sctp_sock_migrate() accounts for this race in the
receive buffer, but not the send buffer.
On the one hand I am not crazy about the connect-to-self scenario.
On the other, I think to support it correctly, we should support
skb migrations for the send case just like we do the receive case.
Yes, not thrilled here either about connect-to-self.
But there is a big difference on how both works. For rx we can just look for wanted skbs
in rx queue, as they aren't going anywhere, but for tx I don't think we can easily block
sctp_wfree() call because that may be happening on another CPU (or am I mistaken here?
sctp still doesn't have RFS but even irqbalance could affect this AFAICT) and more than
one skb may be in transit at a time.
The way it's done now, we wouldn't have to block sctp_wfree. Chunks are released under
lock when they are acked, so we are OK here. The tx completions will just put 1 byte back
to the socket associated with the tx'ed skb, and that should still be ok as
sctp_packet_release_owner will call sk_free().
Please let me rephrase it. I'm actually worried about the asoc->base.sk
part of the story and how it's fetched in sctp_wfree(). I think we can
update that sk pointer after sock_wfree() has fetched it but not used it
yet, possibly leading to accounting it twice, one during migration and
one on sock_wfree.
In sock_wfree() it will update some sk stats like sk->sk_wmem_alloc,
among others.
That is, I don't see anything that would avoid that.
The lockings for this on sctp_chunk would be pretty nasty, I think, and normal usage lets
say wouldn't be benefit from it. Considering the possible migration, as we can't trust
chunk->asoc right away in sctp_wfree, the lock would reside in sctp_chunk and we would
have to go on taking locks one by one on tx queue for the migration. Ugh ;)
No, the chunks manipulation is done under the socket locket so I don't think we have to
worry about a per chunk lock. We should be able to trust chunk->asoc pointer always
because each chunk holds a ref on the association. The only somewhat ugly thing
about moving tx chunks is that you have to potentially walk a lot of lists to move
things around. There are all the lists in the sctp_outqueue struct, plus the
per-transport retransmit list...
Agreed, no per-chunk lock needed, maybe just one to protect
sctp_ep_common.sk ?
Even though the above seems to be a PITA, my main reason for recommending this is
that can happen in normal situations too. Consider a very busy association that is
transferring a lot of a data on a 1-to-many socket. The app decides to move do a
peel-off, and we could now be stuck not being able to peel-off for a quite a while
if there is a hick-up in the network and we have to rtx multiple times.
Fair point.
Marcelo
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