[PATCH net-next] net/smc: reduce rtnl pressure in smc_pnet_create_pnetids_list()

[PATCH net-next] net/smc: reduce rtnl pressure in smc_pnet_create_pnetids_list()

[Eric Dumazet]

Many syzbot reports show extreme rtnl pressure, and many of them hint
that smc acquires rtnl in netns creation for no good reason [1]

This patch returns early from smc_pnet_net_init()
if there is no netdevice yet.

I am not even sure why smc_pnet_create_pnetids_list() even exists,
because smc_pnet_netdev_event() is also calling
smc_pnet_add_base_pnetid() when handling NETDEV_UP event.

[1] extract of typical syzbot reports

2 locks held by syz-executor.3/12252:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878
2 locks held by syz-executor.4/12253:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878
2 locks held by syz-executor.1/12257:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878
2 locks held by syz-executor.2/12261:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878
2 locks held by syz-executor.0/12265:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878
2 locks held by syz-executor.3/12268:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878
2 locks held by syz-executor.4/12271:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878
2 locks held by syz-executor.1/12274:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878
2 locks held by syz-executor.2/12280:
  #0: ffffffff8f369610 (pernet_ops_rwsem){++++}-{3:3}, at: copy_net_ns+0x4c7/0x7b0 net/core/net_namespace.c:491
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_create_pnetids_list net/smc/smc_pnet.c:809 [inline]
  #1: ffffffff8f375b88 (rtnl_mutex){+.+.}-{3:3}, at: smc_pnet_net_init+0x10a/0x1e0 net/smc/smc_pnet.c:878

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Wenjia Zhang <wenjia@linux.ibm.com>
Cc: Jan Karcher <jaka@linux.ibm.com>
Cc: "D. Wythe" <alibuda@linux.alibaba.com>
Cc: Tony Lu <tonylu@linux.alibaba.com>
Cc: Wen Gu <guwen@linux.alibaba.com>
---
 net/smc/smc_pnet.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/smc/smc_pnet.c b/net/smc/smc_pnet.c
index 9f2c58c5a86b7d78cdd07a996e6f7c3766d6886c..2adb92b8c4699c592eddeefb4222f70713d13895 100644
--- a/net/smc/smc_pnet.c
+++ b/net/smc/smc_pnet.c
@@ -806,6 +806,16 @@ static void smc_pnet_create_pnetids_list(struct net *net)
 	u8 ndev_pnetid[SMC_MAX_PNETID_LEN];
 	struct net_device *dev;
 
+	/* Newly created netns do not have devices.
+	 * Do not even acquire rtnl.
+	 */
+	if (list_empty(&net->dev_base_head))
+		return;
+
+	/* Note: This might not be needed, because smc_pnet_netdev_event()
+	 * is also calling smc_pnet_add_base_pnetid() when handling
+	 * NETDEV_UP event.
+	 */
 	rtnl_lock();
 	for_each_netdev(net, dev)
 		smc_pnet_add_base_pnetid(net, dev, ndev_pnetid);
-- 
2.44.0.278.ge034bb2e1d-goog

Re: [PATCH net-next] net/smc: reduce rtnl pressure in smc_pnet_create_pnetids_list()

[Wenjia Zhang]

On 02.03.24 11:07, Eric Dumazet wrote:
> Many syzbot reports show extreme rtnl pressure, and many of them hint
> that smc acquires rtnl in netns creation for no good reason [1]
> 
> This patch returns early from smc_pnet_net_init()
> if there is no netdevice yet.
> 
> I am not even sure why smc_pnet_create_pnetids_list() even exists,
> because smc_pnet_netdev_event() is also calling
> smc_pnet_add_base_pnetid() when handling NETDEV_UP event.
> 
Hi Eric,

I think that was because of double check. But I do not see the necessary 
of the function. I'll check it later if there is any scenario which 
would need the function. Before we decide if we need some clean up on 
it, the patch looks good to me! Thank you for fixing it!

Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>

Thanks,
Wenjia

Re: [PATCH net-next] net/smc: reduce rtnl pressure in smc_pnet_create_pnetids_list()

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Sat,  2 Mar 2024 10:07:44 +0000 you wrote:
> Many syzbot reports show extreme rtnl pressure, and many of them hint
> that smc acquires rtnl in netns creation for no good reason [1]
> 
> This patch returns early from smc_pnet_net_init()
> if there is no netdevice yet.
> 
> I am not even sure why smc_pnet_create_pnetids_list() even exists,
> because smc_pnet_netdev_event() is also calling
> smc_pnet_add_base_pnetid() when handling NETDEV_UP event.
> 
> [...]

Here is the summary with links:
  - [net-next] net/smc: reduce rtnl pressure in smc_pnet_create_pnetids_list()
    https://git.kernel.org/netdev/net-next/c/00af2aa93b76

You are awesome, thank you!
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