netdev - Re: DSA: Suspicious RCU usage (via rtnl_bridge

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Date:   Tue, 20 Sep 2016 16:46:05 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Vivien Didelot <vivien.didelot@...oirfairelinux.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Jiri Pirko <jiri@...lanox.com>, netdev@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: DSA: Suspicious RCU usage (via rtnl_bridge_getlink)

Tue, Sep 20, 2016 at 04:32:53PM CEST, vivien.didelot@...oirfairelinux.com wrote:
>Hi Andrew, Russell,
>
>Andrew Lunn <andrew@...n.ch> writes:
>
>> On Tue, Sep 20, 2016 at 11:26:12AM +0100, Russell King - ARM Linux wrote:
>>> Issuing "bridge vlan show" on clearfog provokes a "suspicious RCU usage"
>>> warning from the kernel (see below).
>>> 
>>> As it's illegal to schedule while holding the RCU read lock, there's the
>>> possibility for this happening much earlier in the call sequence -
>>> mv88e6xxx_port_vlan_dump() takes a mutex, and if that mutex were already
>>> held, we'd schedule at that point.  The RCU read lock was taken by
>>> rtnl_bridge_getlink().
>>> 
>>> It looks horrible to fix - mvmdio.c as well as DSA locking are involved.
>>
>> I would say this needs fixing higher up, in the bridge code. DSA has
>> to be able to sleep, since the switch can be on any arbitrary bus,
>> MDIO, SPI, etc. This will affect pure switchdev devices as well, since
>> they often need to send a request to the switch and wait for a reply.
>
>It looks similar to when a switchdev object/attribute is added/deleted
>without the SWITCHDEV_F_DEFER flag, used in the bridge code to defer
>switchdev operations until switchdev_deferred_process() is called.
>
>This is usually used to process switchdev ops outside the bridge lock.
>
>Jiri, can switchdev_port_vlan_fill not using SWITCHDEV_F_DEFER be the
>reason for this suspicious RCU usage when issuing "bridge vlan show"?

If it is called from atomic context, it should be deferred.


>
>Thanks,
>
>        Vivien