Re: [PATCH v2 net-next 0/5] Make SWITCHDEV_FDB_{ADD,DEL}_TO_DEVICE blocking

On Mon, Aug 23, 2021 at 03:16:48PM +0300, Ido Schimmel wrote:

I was thinking about the following case:

t0 - <MAC1,VID1,P1> is added in syscall context under 'hash_lock'
t1 - br_fdb_delete_by_port() flushes entries under 'hash_lock' in
     response to STP state. Notifications are added to 'deferred' list
t2 - switchdev_deferred_process() is called in syscall context
t3 - <MAC1,VID1,P1> is notified as blocking

Updates to the SW FDB are protected by 'hash_lock', but updates to the
HW FDB are not. In this case, <MAC1,VID1,P1> does not exist in SW, but
it will exist in HW.

Another case assuming switchdev_deferred_process() is called first:

t0 - switchdev_deferred_process() is called in syscall context
t1 - <MAC1,VID,P1> is learned under 'hash_lock'. Notification is added
     to 'deferred' list
t2 - <MAC1,VID1,P1> is modified in syscall context under 'hash_lock' to
     <MAC1,VID1,P2>
t3 - <MAC1,VID1,P2> is notified as blocking
t4 - <MAC1,VID1,P1> is notified as blocking (next time the 'deferred'
     list is processed)

In this case, the HW will have <MAC1,VID1,P1>, but SW will have
<MAC1,VID1,P2>

Ok, so if the hardware FDB entry needs to be updated under the same
hash_lock as the software FDB entry, then it seems that the goal of
updating the hardware FDB synchronously and in a sleepable manner is if
the data path defers the learning to sleepable context too. That in turn
means that there will be 'dead time' between the reception of a packet
from a given {MAC SA, VID} flow and the learning of that address. So I
don't think that is really desirable. So I don't know if it is actually
realistic to do this.

Can we drop it from the requirements of this change, or do you feel like
it's not worth it to make my change if this problem is not solved?

There is of course the option of going half-way too, just like for
SWITCHDEV_PORT_ATTR_SET. You notify it once, synchronously, on the
atomic chain, the switchdev throws as many errors as it can reasonably
can, then you defer the actual installation which means a hardware access.