IB/core: Fix race condition in resolving IP to MAC

Currently while resolving IP address to MAC address single delayed work
is used for resolving multiple such resolve requests. This singled work
is essentially performs two tasks.
(a) any retry needed to resolve and
(b) it executes the callback function for all completed requests

While work is executing callbacks, any new work scheduled on for this
workqueue is lost because workqueue has completed looking at all pending
requests and now looking at callbacks, but work is still under
execution. Any further retry to look at pending requests in
process_req() after executing callbacks would lead to similar race
condition (may be reduce the probably further but doesn't eliminate it).
Retrying to enqueue work that from queue_req() context is not something
rest of the kernel modules have followed.

Therefore fix in this patch utilizes kernel facility to enqueue multiple
work items to a workqueue. This ensures that no such requests
gets lost in synchronization. Request list is still maintained so that
rdma_cancel_addr() can unlink the request and get the completion with
error sooner. Neighbour update event handling continues to be handled in
same way as before.
Additionally process_req() work entry cancels any pending work for a
request that gets completed while processing those requests.

Originally ib_addr was ST workqueue, but it became MT work queue with
patch of [1]. This patch again makes it similar to ST so that
neighbour update events handler work item doesn't race with
other work items.

In one such below trace, (though on 4.5 based kernel) it can be seen
that process_req() never executed the callback, which is likely for an
event that was schedule by queue_req() when previous callback was
getting executed by workqueue.

 [<ffffffff816b0dde>] schedule+0x3e/0x90
 [<ffffffff816b3c45>] schedule_timeout+0x1b5/0x210
 [<ffffffff81618c37>] ? ip_route_output_flow+0x27/0x70
 [<ffffffffa027f9c9>] ? addr_resolve+0x149/0x1b0 [ib_addr]
 [<ffffffff816b228f>] wait_for_completion+0x10f/0x170
 [<ffffffff810b6140>] ? try_to_wake_up+0x210/0x210
 [<ffffffffa027f220>] ? rdma_copy_addr+0xa0/0xa0 [ib_addr]
 [<ffffffffa0280120>] rdma_addr_find_l2_eth_by_grh+0x1d0/0x278 [ib_addr]
 [<ffffffff81321297>] ? sub_alloc+0x77/0x1c0
 [<ffffffffa02943b7>] ib_init_ah_from_wc+0x3a7/0x5a0 [ib_core]
 [<ffffffffa0457aba>] cm_req_handler+0xea/0x580 [ib_cm]
 [<ffffffff81015982>] ? __switch_to+0x212/0x5e0
 [<ffffffffa04582fd>] cm_work_handler+0x6d/0x150 [ib_cm]
 [<ffffffff810a14c1>] process_one_work+0x151/0x4b0
 [<ffffffff810a1940>] worker_thread+0x120/0x480
 [<ffffffff816b074b>] ? __schedule+0x30b/0x890
 [<ffffffff810a1820>] ? process_one_work+0x4b0/0x4b0
 [<ffffffff810a1820>] ? process_one_work+0x4b0/0x4b0
 [<ffffffff810a6b1e>] kthread+0xce/0xf0
 [<ffffffff810a6a50>] ? kthread_freezable_should_stop+0x70/0x70
 [<ffffffff816b53a2>] ret_from_fork+0x42/0x70
 [<ffffffff810a6a50>] ? kthread_freezable_should_stop+0x70/0x70
INFO: task kworker/u144:1:156520 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
message.
kworker/u144:1  D ffff883ffe1d7600     0 156520      2 0x00000080
Workqueue: ib_addr process_req [ib_addr]
 ffff883f446fbbd8 0000000000000046 ffff881f95280000 ffff881ff24de200
 ffff883f66120000 ffff883f446f8008 ffff881f95280000 ffff883f6f9208c4
 ffff883f6f9208c8 00000000ffffffff ffff883f446fbbf8 ffffffff816b0dde

[1] http://lkml.iu.edu/hypermail/linux/kernel/1608.1/05834.html



Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>