RE: [PATCH net] hv_sock: Fix hang when a connection is closed

[Sunil Muthuswamy <sunilmut@microsoft.com>]

> -----Original Message-----
> From: Dexuan Cui <decui@microsoft.com>
> Sent: Sunday, July 28, 2019 11:32 AM
> To: Sunil Muthuswamy <sunilmut@microsoft.com>; David Miller <davem@davemloft.net>; netdev@vger.kernel.org
> Cc: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>; Stephen Hemminger
> <sthemmin@microsoft.com>; sashal@kernel.org; Michael Kelley <mikelley@microsoft.com>; linux-hyperv@vger.kernel.org; linux-
> kernel@vger.kernel.org; olaf@aepfle.de; apw@canonical.com; jasowang@redhat.com; vkuznets <vkuznets@redhat.com>;
> marcelo.cerri@canonical.com
> Subject: [PATCH net] hv_sock: Fix hang when a connection is closed
> 
> 
> hvs_do_close_lock_held() may decrease the reference count to 0 and free the
> sk struct completely, and then the following release_sock(sk) may hang.
> 
> Fixes: a9eeb998c28d ("hv_sock: Add support for delayed close")
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> Cc: stable@vger.kernel.org
> 
> ---
> With the proper kernel debugging options enabled, first a warning can
> appear:
> 
> kworker/1:0/4467 is freeing memory ..., with a lock still held there!
> stack backtrace:
> Workqueue: events vmbus_onmessage_work [hv_vmbus]
> Call Trace:
>  dump_stack+0x67/0x90
>  debug_check_no_locks_freed.cold.52+0x78/0x7d
>  slab_free_freelist_hook+0x85/0x140
>  kmem_cache_free+0xa5/0x380
>  __sk_destruct+0x150/0x260
>  hvs_close_connection+0x24/0x30 [hv_sock]
>  vmbus_onmessage_work+0x1d/0x30 [hv_vmbus]
>  process_one_work+0x241/0x600
>  worker_thread+0x3c/0x390
>  kthread+0x11b/0x140
>  ret_from_fork+0x24/0x30
> 
> and then the following release_sock(sk) can hang:
> 
> watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [kworker/1:0:4467]
> ...
> irq event stamp: 62890
> CPU: 1 PID: 4467 Comm: kworker/1:0 Tainted: G        W         5.2.0+ #39
> Workqueue: events vmbus_onmessage_work [hv_vmbus]
> RIP: 0010:queued_spin_lock_slowpath+0x2b/0x1e0
> ...
> Call Trace:
>  do_raw_spin_lock+0xab/0xb0
>  release_sock+0x19/0xb0
>  vmbus_onmessage_work+0x1d/0x30 [hv_vmbus]
>  process_one_work+0x241/0x600
>  worker_thread+0x3c/0x390
>  kthread+0x11b/0x140
>  ret_from_fork+0x24/0x30
> 
>  net/vmw_vsock/hyperv_transport.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/net/vmw_vsock/hyperv_transport.c b/net/vmw_vsock/hyperv_transport.c
> index f2084e3f7aa4..efbda8ef1eff 100644
> --- a/net/vmw_vsock/hyperv_transport.c
> +++ b/net/vmw_vsock/hyperv_transport.c
> @@ -309,9 +309,16 @@ static void hvs_close_connection(struct vmbus_channel *chan)
>  {
>  	struct sock *sk = get_per_channel_state(chan);
> 
> +	/* Grab an extra reference since hvs_do_close_lock_held() may decrease
> +	 * the reference count to 0 by calling sock_put(sk).
> +	 */
> +	sock_hold(sk);
> +

To me, it seems like when 'hvs_close_connection' is called, there should always be
an outstanding reference to the socket. The reference that is dropped by
' hvs_do_close_lock_held' is a legitimate reference that was taken by 'hvs_close_lock_held'.
Or, in other words, I think the right solution is to always maintain a reference to socket
until this routine is called and drop that here. That can be done by taking the reference to
the socket prior to ' vmbus_set_chn_rescind_callback(chan, hvs_close_connection)' and
dropping that reference at the end of 'hvs_close_connection'.

>  	lock_sock(sk);
>  	hvs_do_close_lock_held(vsock_sk(sk), true);
>  	release_sock(sk);
> +
> +	sock_put(sk);
>  }
> 
>  static void hvs_open_connection(struct vmbus_channel *chan)
> --
> 2.19.1

Thanks for taking a look at this. We should queue this fix and the other hvsocket fixes
for the stable branch.