Patch "net/smc: reduce rtnl pressure in smc_pnet_create_pnetids_list()" has been added to the 6.6-stable tree

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This is a note to let you know that I've just added the patch titled

    net/smc: reduce rtnl pressure in smc_pnet_create_pnetids_list()

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     net-smc-reduce-rtnl-pressure-in-smc_pnet_create_pnet.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit d6d526468cd5bf2e7b180fdbb4b446ae02716fc7
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Sat Mar 2 10:07:44 2024 +0000

    net/smc: reduce rtnl pressure in smc_pnet_create_pnetids_list()
    
    [ Upstream commit 00af2aa93b76b1bade471ad0d0525d4d29ca5cc0 ]
    
    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@xxxxxxxxxx>
    Cc: Wenjia Zhang <wenjia@xxxxxxxxxxxxx>
    Cc: Jan Karcher <jaka@xxxxxxxxxxxxx>
    Cc: "D. Wythe" <alibuda@xxxxxxxxxxxxxxxxx>
    Cc: Tony Lu <tonylu@xxxxxxxxxxxxxxxxx>
    Cc: Wen Gu <guwen@xxxxxxxxxxxxxxxxx>
    Reviewed-by: Wenjia Zhang <wenjia@xxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20240302100744.3868021-1-edumazet@xxxxxxxxxx
    Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/smc/smc_pnet.c b/net/smc/smc_pnet.c
index 11775401df689..306b536fa89e9 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);




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