Re: [PATCH net-next] hv_netvsc: fix a race between netvsc_send() and netvsc_init_buf()

David Miller [off-list ref] writes:

From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Thu, 20 Oct 2016 10:51:04 +0200

quoted

Stephen Hemminger [off-list ref] writes:

quoted

Do we need ACCESS_ONCE() here to avoid check/use issues?

I think we don't: this is the only place in the function where we read
the variable so we'll get normal read. We're not trying to syncronize
with netvsc_init_buf() as that would require locking, if we read stale
NULL value after it was already updated on a different CPU we're fine,
we'll just return -EAGAIN.

The concern is if we race with netvsc_destroy_buf() and this pointer
becomes NULL after the test you are adding.

Thanks, this is interesting.

We may reach to netvsc_destroy_buf() by 3 pathes:

1) cleanup path in netvsc_init_buf(). It is never called with
send_section_map being not NULL so it seems we're safe.

2) from netvsc_remove() when the device is being removed. As far as I
understand we can't be on the transmit path after we call
unregister_netdev() so we're safe.

3) from netvsc_change_mtu() and netvsc_set_channels(). These pathes are
specific to netvsc driver as basically we need to remove the device and
add it back to change mtu/number of channels. The underligning 'struct
net_device' stays but the rest is being removed and added back. On both
pathes we first call netvsc_close() which does netif_tx_disable() and as
far as I understand (I may be wrong) this guarantees that we won't be in
netvsc_send().

So *I think* that we can't ever free send_section_map while in
netvsc_send() and we can't even get to netvsc_send() after it is freed
but I may have missed something.

-- 
  Vitaly

From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Fri, 21 Oct 2016 13:15:53 +0200

David Miller [off-list ref] writes:

quoted

From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Thu, 20 Oct 2016 10:51:04 +0200

quoted

Stephen Hemminger [off-list ref] writes:

quoted

Do we need ACCESS_ONCE() here to avoid check/use issues?

I think we don't: this is the only place in the function where we read
the variable so we'll get normal read. We're not trying to syncronize
with netvsc_init_buf() as that would require locking, if we read stale
NULL value after it was already updated on a different CPU we're fine,
we'll just return -EAGAIN.

The concern is if we race with netvsc_destroy_buf() and this pointer
becomes NULL after the test you are adding.

Thanks, this is interesting.

We may reach to netvsc_destroy_buf() by 3 pathes:

1) cleanup path in netvsc_init_buf(). It is never called with
send_section_map being not NULL so it seems we're safe.

2) from netvsc_remove() when the device is being removed. As far as I
understand we can't be on the transmit path after we call
unregister_netdev() so we're safe.

3) from netvsc_change_mtu() and netvsc_set_channels(). These pathes are
specific to netvsc driver as basically we need to remove the device and
add it back to change mtu/number of channels. The underligning 'struct
net_device' stays but the rest is being removed and added back. On both
pathes we first call netvsc_close() which does netif_tx_disable() and as
far as I understand (I may be wrong) this guarantees that we won't be in
netvsc_send().

So *I think* that we can't ever free send_section_map while in
netvsc_send() and we can't even get to netvsc_send() after it is freed
but I may have missed something.

Ok you may be right.

Can't the device be taken down by asynchronous events as well?  For example
if the peer end of the interface in the other guest disappears.