solos-pci: Fix race condition in tasklet RX handling

solos-pci: Fix race condition in tasklet RX handling

commit 1f6ea6e511e5ec730d8e88651da1b7b6e8fd1333 upstream.

We were seeing faults in the solos-pci receive tasklet when packets
arrived for a VCC which was currently being closed:

[18842.727906] EIP: [<e082f490>] br2684_push+0x19/0x234 [br2684] SS:ESP 0068:dfb89d14

[18845.090712] [<c13ecff3>] ? do_page_fault+0x0/0x2e1
[18845.120042] [<e082f490>] ? br2684_push+0x19/0x234 [br2684]
[18845.153530] [<e084fa13>] solos_bh+0x28b/0x7c8 [solos_pci]
[18845.186488] [<e084f711>] ? solos_irq+0x2d/0x51 [solos_pci]
[18845.219960] [<c100387b>] ? handle_irq+0x3b/0x48
[18845.247732] [<c10265cb>] ? irq_exit+0x34/0x57
[18845.274437] [<c1025720>] tasklet_action+0x42/0x69
[18845.303247] [<c102643f>] __do_softirq+0x8e/0x129
[18845.331540] [<c10264ff>] do_softirq+0x25/0x2a
[18845.358274] [<c102664c>] _local_bh_enable_ip+0x5e/0x6a
[18845.389677] [<c102666d>] local_bh_enable+0xb/0xe
[18845.417944] [<e08490a8>] ppp_unregister_channel+0x32/0xbb [ppp_generic]
[18845.458193] [<e08731ad>] pppox_unbind_sock+0x18/0x1f [pppox]

This patch uses an RCU-inspired approach to fix it. In the RX tasklet's
find_vcc() function we first refuse to use a VCC which already has the
ATM_VF_READY bit cleared. And in the VCC close function, we synchronise
with the tasklet to ensure that it can't still be using the VCC before
we continue and allow the VCC to be destroyed.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Tested-by: Nathan Williams <nathan@traverse.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>