On Tue, Aug 12, 2014 at 4:38 PM, Doug Ledford <dledford@xxxxxxxxxx> wrote:
> We blindly assume that we can just take the rtnl lock and that will
> prevent races with downing this interface. Unfortunately, that's not
> the case. In ipoib_mcast_stop_thread() we will call flush_workqueue()
> in an attempt to clear out all remaining instances of ipoib_join_task.
> But, since this task is put on the same workqueue as the join task, the
> flush_workqueue waits on this thread too. But this thread is deadlocked
> on the rtnl lock. The better thing here is to use trylock and loop on
> that until we either get the lock or we see that FLAG_ADMIN_UP has
> been cleared, in which case we don't need to do anything anyway and we
> just return.
>
> Signed-off-by: Doug Ledford <dledford@xxxxxxxxxx>
> ---
> drivers/infiniband/ulp/ipoib/ipoib_multicast.c | 21 +++++++++++++++------
> 1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/infiniband/ulp/ipoib/ipoib_multicast.c b/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> index a0a42859f12..7e9cd39b5ef 100644
> --- a/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> +++ b/drivers/infiniband/ulp/ipoib/ipoib_multicast.c
> @@ -353,18 +353,27 @@ void ipoib_mcast_carrier_on_task(struct work_struct *work)
> carrier_on_task);
> struct ib_port_attr attr;
>
> - /*
> - * Take rtnl_lock to avoid racing with ipoib_stop() and
> - * turning the carrier back on while a device is being
> - * removed.
> - */
> if (ib_query_port(priv->ca, priv->port, &attr) ||
> attr.state != IB_PORT_ACTIVE) {
> ipoib_dbg(priv, "Keeping carrier off until IB port is active\n");
> return;
> }
>
> - rtnl_lock();
> + /*
> + * Take rtnl_lock to avoid racing with ipoib_stop() and
> + * turning the carrier back on while a device is being
> + * removed. However, ipoib_stop() will attempt to flush
> + * the workqueue while holding the rtnl lock, so loop
> + * on trylock until either we get the lock or we see
> + * FLAG_ADMIN_UP go away as that signals that we are bailing
> + * and can safely ignore the carrier on work
> + */
> + while (!rtnl_trylock()) {
> + if (!test_bit(IPOIB_FLAG_ADMIN_UP, &priv->flags))
> + return;
> + else
> + msleep(20);
> + }
I always think rtnl lock is too big for this purpose... and that 20 ms
is not ideal either. Could we have a new IPOIB private mutex used by
ipoib_stop() and this section of code ? So something like:
ipoib_stop()
{ .....
mutex_lock(&something_new);
clear_bit(IPOIB_FLAG_ADMIN_UP, &priv->flags);
...
mutex_unlock(&something_new);
return 0;
}
Then the loop would become:
// this while-loop will be very short - since we either get the mutex
quickly or "return" quickly.
while (!mutex_trylock(&something_new)) {
if (!test_bit(IPOIB_FLAG_ADMIN_UP, &priv->flags))
return;
}
> if (!ipoib_cm_admin_enabled(priv->dev))
> dev_set_mtu(priv->dev, min(priv->mcast_mtu, priv->admin_mtu));
> netif_carrier_on(priv->dev);
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