Re: [PATCH net] team: postpone features update to avoid deadlock

On Thu, 21 Jan 2021 18:34:52 -0800
Jakub Kicinski [off-list ref] wrote:

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On Thu, 21 Jan 2021 11:29:37 +0100 Ivan Vecera wrote:  

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On Wed, 20 Jan 2021 15:18:20 -0800
Cong Wang [off-list ref] wrote:    

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On Wed, Jan 20, 2021 at 4:56 AM Ivan Vecera [off-list ref] wrote:      

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Team driver protects port list traversal by its team->lock mutex
in functions like team_change_mtu(), team_set_rx_mode(),    

The set_rx_mode part can't be true, set_rx_mode can't sleep and
team->lock is a mutex.
  

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To fix the problem __team_compute_features() needs to be postponed
for these cases.        

Is there any user-visible effect after deferring this feature change?    

An user should not notice this change.    

I think Cong is right, can you expand a little on your assertion?
User should be able to assume that the moment syscall returns the
features had settled.

What does team->mutex actually protect in team_compute_features()?
All callers seem to hold RTNL at a quick glance. This is a bit of 
a long shot but isn't it just tryin to protect the iteration over 
ports which could be under RCU?  

In fact the mutex could be removed at all because all port-list
writers are running under rtnl_lock, some readers like team_change_mtu()
or team_device_event() [notifier] as well and hot path readers are
protected by RCU.
I have discussed this with Jiri but he don't want to introduce any dependency
on RTNL to team as it was designed as RTNL-independent from beginning.

Anyway your idea to run team_compute_features under RCU could be fine
as subsequent __team_compute_features() cannot sleep...

Do you mean something like this?

diff --git a/drivers/net/team/team.c b/drivers/net/team/team.c
index c19dac21c468..dd7917cab2b1 100644
--- a/drivers/net/team/team.c
+++ b/drivers/net/team/team.c

@@ -992,7 +992,8 @@ static void __team_compute_features(struct team *team)
        unsigned int dst_release_flag = IFF_XMIT_DST_RELEASE |
                                        IFF_XMIT_DST_RELEASE_PERM;
 
-       list_for_each_entry(port, &team->port_list, list) {
+       rcu_read_lock();
+       list_for_each_entry_rcu(port, &team->port_list, list) {
                vlan_features = netdev_increment_features(vlan_features,
                                        port->dev->vlan_features,
                                        TEAM_VLAN_FEATURES);

@@ -1006,6 +1007,7 @@ static void __team_compute_features(struct team *team)
                if (port->dev->hard_header_len > max_hard_header_len)
                        max_hard_header_len = port->dev->hard_header_len;
        }
+       rcu_read_unlock();
 
        team->dev->vlan_features = vlan_features;
        team->dev->hw_enc_features = enc_features | NETIF_F_GSO_ENCAP_ALL |

@@ -1020,9 +1022,7 @@ static void __team_compute_features(struct team *team)
 
 static void team_compute_features(struct team *team)
 {
-       mutex_lock(&team->lock);
        __team_compute_features(team);
-       mutex_unlock(&team->lock);
        netdev_change_features(team->dev);
 }