When disabling and removing a receive context, it is possible for an
asynchronous event (i.e IRQ) to occur.  Because of this, there is a race
between cleaning up the context, and the context being used by the
asynchronous event.

cpu 0  (context cleanup)
    rc->ref_count-- (ref_count == 0)
    hfi1_rcd_free()
cpu 1  (IRQ (with rcd index))
	rcd_get_by_index()
	lock
	ref_count+++     <-- reference count race (WARNING)
	return rcd
	unlock
cpu 0
    hfi1_free_ctxtdata() <-- incorrect free location
    lock
    remove rcd from array
    unlock
    free rcd

This race will cause the following WARNING trace:

WARNING: CPU: 0 PID: 175027 at include/linux/kref.h:52 hfi1_rcd_get_by_index+0x84/0xa0 [hfi1]
CPU: 0 PID: 175027 Comm: IMB-MPI1 Kdump: loaded Tainted: G OE ------------ 3.10.0-957.el7.x86_64 #1
Hardware name: Intel Corporation S2600KP/S2600KP, BIOS SE5C610.86B.11.01.0076.C4.111920150602 11/19/2015
Call Trace:
  dump_stack+0x19/0x1b
  __warn+0xd8/0x100
  warn_slowpath_null+0x1d/0x20
  hfi1_rcd_get_by_index+0x84/0xa0 [hfi1]
  is_rcv_urgent_int+0x24/0x90 [hfi1]
  general_interrupt+0x1b6/0x210 [hfi1]
  __handle_irq_event_percpu+0x44/0x1c0
  handle_irq_event_percpu+0x32/0x80
  handle_irq_event+0x3c/0x60
  handle_edge_irq+0x7f/0x150
  handle_irq+0xe4/0x1a0
  do_IRQ+0x4d/0xf0
  common_interrupt+0x162/0x162

The race can also lead to a use after free which could be similar to:

general protection fault: 0000 1 SMP
CPU: 71 PID: 177147 Comm: IMB-MPI1 Kdump: loaded Tainted: G W OE ------------ 3.10.0-957.el7.x86_64 #1
Hardware name: Intel Corporation S2600KP/S2600KP, BIOS SE5C610.86B.11.01.0076.C4.111920150602 11/19/2015
task: ffff9962a8098000 ti: ffff99717a508000 task.ti: ffff99717a508000 __kmalloc+0x94/0x230
Call Trace:
  ? hfi1_user_sdma_process_request+0x9c8/0x1250 [hfi1]
  hfi1_user_sdma_process_request+0x9c8/0x1250 [hfi1]
  hfi1_aio_write+0xba/0x110 [hfi1]
  do_sync_readv_writev+0x7b/0xd0
  do_readv_writev+0xce/0x260
  ? handle_mm_fault+0x39d/0x9b0
  ? pick_next_task_fair+0x5f/0x1b0
  ? sched_clock_cpu+0x85/0xc0
  ? __schedule+0x13a/0x890
  vfs_writev+0x35/0x60
  SyS_writev+0x7f/0x110
  system_call_fastpath+0x22/0x27

Use the appropriate kref API to verify access.

Reorder context cleanup to ensure context removal before cleanup occurs
correctly.

Cc: stable@vger.kernel.org # v4.14.0+
Fixes: f683c80c ("IB/hfi1: Resolve kernel panics by reference counting receive contexts")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>