s390/ap: fix AP bus crash on early config change callback invocation

Fix kernel crash in AP bus code caused by very early invocation of the
config change callback function via SCLP.

After a fresh IML of the machine the crypto cards are still offline and
will get switched online only with activation of any LPAR which has the
card in it's configuration. A crypto card coming online is reported
to the LPAR via SCLP and the AP bus offers a callback function to get
this kind of information. However, it may happen that the callback is
invoked before the AP bus init function is complete. As the callback
triggers a synchronous AP bus scan, the scan may already run but some
internal states are not initialized by the AP bus init function resulting
in a crash like this:

  [   11.635859] Unable to handle kernel pointer dereference in virtual kernel address space
  [   11.635861] Failing address: 0000000000000000 TEID: 0000000000000887
  [   11.635862] Fault in home space mode while using kernel ASCE.
  [   11.635864] AS:00000000894c4007 R3:00000001fece8007 S:00000001fece7800 P:000000000000013d
  [   11.635879] Oops: 0004 ilc:1 [#1] SMP
  [   11.635882] Modules linked in:
  [   11.635884] CPU: 5 PID: 42 Comm: kworker/5:0 Not tainted 6.6.0-rc3-00003-g4dbf7cdc6b42 #12
  [   11.635886] Hardware name: IBM 3931 A01 751 (LPAR)
  [   11.635887] Workqueue: events_long ap_scan_bus
  [   11.635891] Krnl PSW : 0704c00180000000 0000000000000000 (0x0)
  [   11.635895]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
  [   11.635897] Krnl GPRS: 0000000001000a00 0000000000000000 0000000000000006 0000000089591940
  [   11.635899]            0000000080000000 0000000000000a00 0000000000000000 0000000000000000
  [   11.635901]            0000000081870c00 0000000089591000 000000008834e4e2 0000000002625a00
  [   11.635903]            0000000081734200 0000038000913c18 000000008834c6d6 0000038000913ac8
  [   11.635906] Krnl Code:>0000000000000000: 0000                illegal
  [   11.635906]            0000000000000002: 0000                illegal
  [   11.635906]            0000000000000004: 0000                illegal
  [   11.635906]            0000000000000006: 0000                illegal
  [   11.635906]            0000000000000008: 0000                illegal
  [   11.635906]            000000000000000a: 0000                illegal
  [   11.635906]            000000000000000c: 0000                illegal
  [   11.635906]            000000000000000e: 0000                illegal
  [   11.635915] Call Trace:
  [   11.635916]  [<0000000000000000>] 0x0
  [   11.635918]  [<000000008834e4e2>] ap_queue_init_state+0x82/0xb8
  [   11.635921]  [<000000008834ba1c>] ap_scan_domains+0x6fc/0x740
  [   11.635923]  [<000000008834c092>] ap_scan_adapter+0x632/0x8b0
  [   11.635925]  [<000000008834c3e4>] ap_scan_bus+0xd4/0x288
  [   11.635927]  [<00000000879a33ba>] process_one_work+0x19a/0x410
  [   11.635930] Discipline DIAG cannot be used without z/VM
  [   11.635930]  [<00000000879a3a2c>] worker_thread+0x3fc/0x560
  [   11.635933]  [<00000000879aea60>] kthread+0x120/0x128
  [   11.635936]  [<000000008792afa4>] __ret_from_fork+0x3c/0x58
  [   11.635938]  [<00000000885ebe62>] ret_from_fork+0xa/0x30
  [   11.635942] Last Breaking-Event-Address:
  [   11.635942]  [<000000008834c6d4>] ap_wait+0xcc/0x148

This patch improves the ap_bus_force_rescan() function which is
invoked by the config change callback by checking if a first
initial AP bus scan has been done. If not, the force rescan request
is simple ignored. Anyhow it does not make sense to trigger AP bus
re-scans even before the very first bus scan is complete.

Cc: stable@vger.kernel.org
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

drivers/s390/crypto/ap_bus.c

@@ -1022,6 +1022,10 @@ EXPORT_SYMBOL(ap_driver_unregister);
 
 void ap_bus_force_rescan(void)
 {
+	/* Only trigger AP bus scans after the initial scan is done */
+	if (atomic64_read(&ap_scan_bus_count) <= 0)
+		return;
+
 	/* processing a asynchronous bus rescan */
 	del_timer(&ap_config_timer);
 	queue_work(system_long_wq, &ap_scan_work);