Date:	Thu, 1 Mar 2012 09:51:58 +0200
From:	Jack Morgenstein <jackm@....mellanox.co.il>
To:	cascardo@...ux.vnet.ibm.com
Cc:	David Miller <davem@...emloft.net>, yevgenyp@...lanox.co.il,
	roland@...estorage.com, netdev@...r.kernel.org,
	linux-rdma@...r.kernel.org
Subject: Re: [PATCH] mlx4: prevent the device from being removed concurrently

Mr Cascardo,

If this is at a customer site, there is a workaround available for you.

In file /etc/modprobe.d/mlx4_core.conf, enter a line:
options mlx4_core internal_err_reset=0

(do "modinfo mlx4_core" to see a description of the module parameter).

Setting this parameter to 0 in the modprobe conf file will cause the driver to simply report the
internal error failure (in /var/log/messages), but the device will not be restarted.

This will avoid your Oops -- the EEH flow will then be the only flow restarting the device.

(Note that even if you implement my suggestion, you will still have a situation where the device
will be reset TWICE).

Again, this is a workaround for current users.  The issue needs to be fixed so that even when the
internal error does cause a reset, the kernel will not crash.

Unfortunately, I do not have the time now to generate a patch.

-Jack

On Wednesday 29 February 2012 17:19, Jack Morgenstein wrote:
> On Wednesday 29 February 2012 16:47, Jack Morgenstein wrote:
> > Some comments.
> > 
> > 1. Mr Cascardo's solution is only partial, and does not cover all the problem cases. He
> >    simply uncovered one of several examples of what lack-of-sync will do when removing a device.
> >    Mr. Cascardo found the kernel Oops that happens when a catastrophic error occurs during device
> >    removal. What if we receive a catas error while doing "init_one"?  What if we are in the middle
> >    of catas error recovery (in the init_one stage), and we get a remove_one request from higher up?
> > 
> >    There is a solution for this precise problem in the mthca driver (infiniband/hw/mthca/mthca_main.c
> >    infiniband/hw/mthca/mthca_catas.c). In the mthca driver, we DO in fact use an "mthca_device_mutex"
> >    for precisely the reason given in a. above.  I see no reason not to do the same thing here.
> > 
> >    This requires:
> > 	1. mlx4_init_one(), mlx4_remove_one() and mlx4_restart_one all grab an mlx4_device_mutex.
> >         2. new procedure __mlx4_remove_one(), which does not grab the mutex.
> > 
> >    Note that it is NOT enough to simply protect the removal operation.  The protection must wrap the
> >    ENTIRE restart operation (both removal and init), because allowing a remove in the middle of init_one
> >    or restart_one would probably also cause a kernel Oops.
> > 
> > 2. The intf_mutex is used with mlx4_un/register_device and mlx4_un/register_interface. unregister_device is
> >    used both in remove_one and in mlx4_change_port_types.  I would hesitate to grab that mutex for a more
> >    global use.  I think it is cleaner to add a device mutex (mlx4_device_mutex) for initializing/removing/
> >    restarting the device.
> > 
> > -Jack
> > 
> Another thing -- what about the desired final state of the device?  Say we do a remove one, and in the middle
> of this, we get a catas restart.  The catas restart will wait until the remove-in-progress completes, and
> then will do its remove/init -- with the net result that the device is UP rather than DOWN.
> 
> This implies that we need some sort of state machine here.
> 
> -Jack
> 
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