Re: [PATCH v2 3/4] net/failsafe: replace local sub-device with shared data

[Thomas Monjalon <thomas@monjalon.net>]

06/03/2019 11:46, Gaëtan Rivet:
> On Tue, Mar 05, 2019 at 06:58:04PM +0100, Thomas Monjalon wrote:
> > 05/03/2019 18:38, Gaëtan Rivet:
> > > What happens when a primary process closes a device before a secondary?
> > > Is the secondary unable to stop / close its own then? Isn't there some
> > > missing uninit?
> > 
> > Is the secondary process supposed to do any closing?
> > The device management should be done only by the primary process.
> > 
> > Note: anyway all this hotplug related code should be dropped
> > from failsafe to be replaced by EAL hotplug management.
> > 
> 
> I don't know, I've never used secondary process.
> However, cursory reading the code of rte_eth_dev_close(), I don't see
> a guard against calling it from a secondary process?

Yes indeed, there is no guard.
That's something not clear in DPDK, previously we were
attaching some vdevs in secondary only.

> Reading code like
> 
>    rte_free(dev->data->rx_queues);
>    dev->data->rx_queues = NULL;
> 
> within makes me think the issue has been seen at least once, where
> shared data is freed multiple times, so I guessed some secondary
> processes were calling it. Maybe they are not meant to, but what
> prevents them from being badly written?
> 
> Also, given rte_dev_remove IPC call to transfer the order to the
> primary, it seems that at least secondary processes are expected to call
> rte_dev_remove() at some point? So are they only authorized to call
> rte_dev_remove() (to manage hotplug), but not rte_eth_dev_close()? Is
> there a specific documentation detailing the design of secondary process
> and the related responsibilities in the lifetime of a device? How are
> they synching their rte_eth_devices list if they are not calling
> rte_eth_dev_close(), ever?

All these calls should be done in primary.
The IPC mechanism calls the attach/detach in secondary at EAL level.
The PMDs does the bridge between EAL device and ethdev port status.
But you are right, there can be a sync issue if closing an ethdev port
and not removing the EAL device.
This is a generic question about deciding whether we want all ethdev ports
to be synced in multi-process or not.

In failsafe context, we are closing the EAL device and change
the state of the sub-device accordingly. So I think there is no issue.

> > > This seems dangerous to me. Why not instead allocating a per-process
> > > slab of memory that would hold the relevant references and outlive the
> > > shared data (a per-process rte_eth_dev private data...).
> > 
> > Which data do you think should be allocated per process?
> > 
> > 
> 
> [-------- SHARED SPACE --------------] [-- PER-PROCESS --------]
> +--------------------------------------------------------------+
> | +------------------+                +- rte_eth_devices[n] -+ |
> | |rte_eth_dev_data  |<---------------+ data                 | | PRIMARY
> | |                  |   +dev_priv-+  |                      | |
> | |      dev_private +-->|         |  |                      | |
> | |              ... |   |         |  |                      | |
> | |          port_id |   |         |  |                      | |
> | |                  |   |         |  |                      | |
> | |                  |   |         |  |                      | |
> | |                  |   |         |  +----------------------+ |
> | |                  |   |         |  +- rte_eth_devices[n] -+ |
> | |                  |   |         |  |                      | | SECONDARY
> | |                  |   |         |  |                      | |
> | |                  |   |         |  |                      | |
> | |                  |   |         |  |                      | |
> | |                  |   +---------+  |                      | |
> | |                  |<---------------+ data                 | |
> | +------------------+                +----------------------+ |
> +--------------------------------------------------------------+
> 
> Here port_id is used within fail-safe to get back to rte_eth_devices[n].
> This disappears once a device is closed, as all shared space is zeroed.
> 
> This means that sometimes ETH(sdev) and PORT_ID(sdev) is correct,
> and at some point it is not anymore, once a sub-device has been
> closed. This seems dangerous.

The state of the sub-device is changed.
I don't see any issue.

> I was thinking initially that allocating a place where each sdev would
> store their rte_eth_devices / port_id back-reference could alleviate the
> issue, meaning that the fail-safe would not zero it on sdev_close(), and
> it would remain valid for the lifetime of a sub-device, so even when a
> sub-device is in DEV_PROBED state.
> 
> But now that I think about it, it could probably be simpler: instead of
> using (ETH(sdev)->data->port_id) for the port_id of an sdev (meaning
> that it is dependent on the lifetime of the sdev, instead of the
> lifetime of the failsafe), the port-id itself should be stored in the
> sub_device structure. This structure will be available for the lifetime
> of the failsafe, and the port_id is correct accross all processes.
> 
> So PORT_ID(sdev) would be defined to something like (sdev->port_id), and
> ETH(sdev) would be (&rte_eth_devices[PORT_ID(sdev)]). It would remain
> correct even once the primary has closed the sub-device.
> 
> What do you think? Do you agree that the current state is dangerous, and
> do you think the solution would alleviate the issue? Maybe the concern
> is unfounded and the only issue is within fs_dev_remove().

Yes it is only seen in fs_dev_remove().
I discussed about your proposal with Raslan, and we agree we
could change from sub_device.data to sub_device.port_id,
it may be more future-proof.

I have only one doubt: look at the macro in this patch:

#define ETH(sdev) \
	((sdev)->data == NULL ? NULL : &rte_eth_devices[(sdev)->data->port_id])

The NULL check cannot be done with a port id.
I think it was needed to manage one case. Raslan?