linux-kernel - [tip:sched/core] sched/deadline: Fix wrap-around in DL heap

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Date:	Wed, 10 Aug 2016 11:00:04 -0700
From:	tip-bot for Tommaso Cucinotta <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com,
	tommaso.cucinotta@...up.it, peterz@...radead.org,
	torvalds@...ux-foundation.org, luca.abeni@...tn.it,
	juri.lelli@...il.com, mingo@...nel.org, juri.lelli@....com,
	tglx@...utronix.de
Subject: [tip:sched/core] sched/deadline: Fix wrap-around in DL heap

Commit-ID:  a23eadfae2fd45536a355b785d5a1533e1955c22
Gitweb:     http://git.kernel.org/tip/a23eadfae2fd45536a355b785d5a1533e1955c22
Author:     Tommaso Cucinotta <tommaso.cucinotta@...up.it>
AuthorDate: Tue, 19 Jul 2016 11:44:50 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 10 Aug 2016 13:32:55 +0200

sched/deadline: Fix wrap-around in DL heap

Current code in cpudeadline.c has a bug in re-heapifying when adding a
new element at the end of the heap, because a deadline value of 0 is
temporarily set in the new elem, then cpudl_change_key() is called
with the actual elem deadline as param.

However, the function compares the new deadline to set with the one
previously in the elem, which is 0.  So, if current absolute deadlines
grew so much to have negative values as s64, the comparison in
cpudl_change_key() makes the wrong decision.  Instead, as from
dl_time_before(), the kernel should handle correctly abs deadlines
wrap-arounds.

This patch fixes the problem with a minimally invasive change that
forces cpudl_change_key() to heapify up in this case.

Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Luca Abeni <luca.abeni@...tn.it>
Cc: Juri Lelli <juri.lelli@....com>
Cc: Juri Lelli <juri.lelli@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1468921493-10054-2-git-send-email-tommaso.cucinotta@sssup.it
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/cpudeadline.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 5be5882..d418449 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -168,7 +168,7 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl, int is_valid)
 
 	if (old_idx == IDX_INVALID) {
 		cp->size++;
-		cp->elements[cp->size - 1].dl = 0;
+		cp->elements[cp->size - 1].dl = dl;
 		cp->elements[cp->size - 1].cpu = cpu;
 		cp->elements[cpu].idx = cp->size - 1;
 		cpudl_change_key(cp, cp->size - 1, dl);