s390/cpumf: Fix warning from check_processor_id

Function __hw_perf_event_init() used a CPU variable without
ensuring CPU preemption has been disabled. This caused the
following warning in the kernel log:

  [ 7.277085] BUG: using smp_processor_id() in preemptible
                 [00000000] code: cf-csdiag/1892
  [ 7.277111] caller is cf_diag_event_init+0x13a/0x338
  [ 7.277122] CPU: 10 PID: 1892 Comm: cf-csdiag Not tainted
                 5.0.0-20190318.rc0.git0.9e1a11e0f602.300.fc29.s390x+debug #1
  [ 7.277131] Hardware name: IBM 2964 NC9 712 (LPAR)
  [ 7.277139] Call Trace:
  [ 7.277150] ([<000000000011385a>] show_stack+0x82/0xd0)
  [ 7.277161]  [<0000000000b7a71a>] dump_stack+0x92/0xd0
  [ 7.277174]  [<00000000007b7e9c>] check_preemption_disabled+0xe4/0x100
  [ 7.277183]  [<00000000001228aa>] cf_diag_event_init+0x13a/0x338
  [ 7.277195]  [<00000000002cf3aa>] perf_try_init_event+0x72/0xf0
  [ 7.277204]  [<00000000002d0bba>] perf_event_alloc+0x6fa/0xce0
  [ 7.277214]  [<00000000002dc4a8>] __s390x_sys_perf_event_open+0x398/0xd50
  [ 7.277224]  [<0000000000b9e8f0>] system_call+0xdc/0x2d8
  [ 7.277233] 2 locks held by cf-csdiag/1892:
  [ 7.277241]  #0: 00000000976f5510 (&sig->cred_guard_mutex){+.+.},
                  at: __s390x_sys_perf_event_open+0xd2e/0xd50
  [ 7.277257]  #1: 00000000363b11bd (&pmus_srcu){....},
                  at: perf_event_alloc+0x52e/0xce0

The variable is now accessed in proper context. Use
get_cpu_var()/put_cpu_var() pair to disable
preemption during access.
As the hardware authorization settings apply to all CPUs, it
does not matter which CPU is used to check the authorization setting.

Remove the event->count assignment. It is not needed as function
perf_event_alloc() allocates memory for the event with kzalloc() and
thus count is already set to zero.

Fixes: fe5908bc ("s390/cpum_cf_diag: Add support for s390 counter facility diagnostic trace")
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/perf_cpum_cf_diag.c

@@ -196,23 +196,30 @@ static void cf_diag_perf_event_destroy(struct perf_event *event)
  */
 static int __hw_perf_event_init(struct perf_event *event)
 {
-	struct cpu_cf_events *cpuhw = this_cpu_ptr(&cpu_cf_events);
 	struct perf_event_attr *attr = &event->attr;
+	struct cpu_cf_events *cpuhw;
 	enum cpumf_ctr_set i;
 	int err = 0;
 
-	debug_sprintf_event(cf_diag_dbg, 5,
-			    "%s event %p cpu %d authorized %#x\n", __func__,
-			    event, event->cpu, cpuhw->info.auth_ctl);
+	debug_sprintf_event(cf_diag_dbg, 5, "%s event %p cpu %d\n", __func__,
+			    event, event->cpu);
 
 	event->hw.config = attr->config;
 	event->hw.config_base = 0;
-	local64_set(&event->count, 0);
 
-	/* Add all authorized counter sets to config_base */
+	/* Add all authorized counter sets to config_base. The
+	 * the hardware init function is either called per-cpu or just once
+	 * for all CPUS (event->cpu == -1).  This depends on the whether
+	 * counting is started for all CPUs or on a per workload base where
+	 * the perf event moves from one CPU to another CPU.
+	 * Checking the authorization on any CPU is fine as the hardware
+	 * applies the same authorization settings to all CPUs.
+	 */
+	cpuhw = &get_cpu_var(cpu_cf_events);
 	for (i = CPUMF_CTR_SET_BASIC; i < CPUMF_CTR_SET_MAX; ++i)
 		if (cpuhw->info.auth_ctl & cpumf_ctr_ctl[i])
 			event->hw.config_base |= cpumf_ctr_ctl[i];
+	put_cpu_var(cpu_cf_events);
 
 	/* No authorized counter sets, nothing to count/sample */
 	if (!event->hw.config_base) {