[SRU][J][PATCH v2 1/2] UBUNTU: SAUCE: rcu-tasks: fix mismerge in trc_inspect_reader

[Krister Johansen]

BugLink: https://bugs.launchpad.net/bugs/2089373

Upstream converted the '!nesting' to 'nesting ? -EINVAL : 0' as part of
a manual conflict resolution during their merge of
9b3c4ab304("sched,rcu: Rework try_invoke_on_locked_down_task()").

When this change was pulled into the Ubuntu trees the resulting conflict
was incorrectly resolved when the patch was cherry-picked.  Continuing
to use '!nesting' results in trc_inspect_reader erroneously flagging
tasks as holdouts when they are actually quiescent.  This subsequently
results in IPIs being sent to CPUs where one is not needed.
Additionally, on platforms that have CPU hotplug slots empty, this also
results in WARNs about errors sending IPIs to invalid CPUs.  Resolving
the conflict in the same fashion as linux has addresses this problem.

Fixes: 0f17585efd61 "rcu-tasks: Add trc_inspect_reader() checks for exiting
critical section"
(backported from commit 6fedc28076bbbb32edb722e80f9406a3d1d668a8)
Signed-off-by: Krister Johansen <kjlx at templeofstupid.com>
---
 kernel/rcu/tasks.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index b9f762c2b083..5528c172570b 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -982,7 +982,7 @@ static int trc_inspect_reader(struct task_struct *t, void *arg)
 	// holdout list.
 	t->trc_reader_checked = nesting >= 0;
 	if (nesting <= 0)
-		return !nesting;  // If in QS, done, otherwise try again later.
+		return nesting ? -EINVAL : 0;  // If in QS, done, otherwise try again later.
 
 	// The task is in a read-side critical section, so set up its
 	// state so that it will awaken the grace-period kthread upon exit
-- 
2.25.1