perf: Fix task_function_call() error handling

The error handling introduced by commit:

  2ed6edd3 ("perf: Add cond_resched() to task_function_call()")

looses any return value from smp_call_function_single() that is not
{0, -EINVAL}. This is a problem because it will return -EXNIO when the
target CPU is offline. Worse, in that case it'll turn into an infinite
loop.

Fixes: 2ed6edd3 ("perf: Add cond_resched() to task_function_call()")
Reported-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Barret Rhoden <brho@google.com>
Tested-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Link: https://lkml.kernel.org/r/20200827064732.20860-1-kjain@linux.ibm.com

kernel/events/core.c

@@ -99,7 +99,7 @@ static void remote_function(void *data)
  * retry due to any failures in smp_call_function_single(), such as if the
  * task_cpu() goes offline concurrently.
  *
- * returns @func return value or -ESRCH when the process isn't running
+ * returns @func return value or -ESRCH or -ENXIO when the process isn't running
  */
 static int
 task_function_call(struct task_struct *p, remote_function_f func, void *info)
@@ -115,7 +115,8 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info)
 	for (;;) {
 		ret = smp_call_function_single(task_cpu(p), remote_function,
 					       &data, 1);
-		ret = !ret ? data.ret : -EAGAIN;
+		if (!ret)
+			ret = data.ret;
 
 		if (ret != -EAGAIN)
 			break;