[RFC] perf/core: Fixes hung issue on perf stat command during cpu hotplug

[Kajol Jain]

Commit 2ed6edd33a21 ("perf: Add cond_resched() to task_function_call()")
added assignment of ret value as -EAGAIN in case function
call to 'smp_call_function_single' fails.
For non-zero ret value, it did 
'ret = !ret ? data.ret : -EAGAIN;', which always
assign -EAGAIN to ret and make second if condition useless.

In scenarios like when executing a perf stat with --per-thread option, and 
if any of the monitoring cpu goes offline, the 'smp_call_function_single'
function could return -ENXIO, and with the above check,
task_function_call hung and increases CPU
usage (because of repeated 'smp_call_function_single()')

Recration scenario:
	# perf stat -a --per-thread && (offline a CPU )

Patch here removes the tertiary condition added as part of that 
commit and added a check for NULL and -EAGAIN.

Fixes: 2ed6edd33a21("perf: Add cond_resched() to task_function_call()")
Signed-off-by: Kajol Jain <kjain@xxxxxxxxxxxxx>
Reported-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
---
 kernel/events/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5bfe8e3c6e44..330c53f7df9c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -115,9 +115,9 @@ 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 != -EAGAIN)
+		if (!ret)
+			ret = data.ret;
+		else if (ret != -EAGAIN)
 			break;
 
 		cond_resched();
-- 
2.26.2