[tip:sched/urgent] sched: Fix crash if cpuset_cpumask_can_shrink( ) is passed an empty cpumask

[tip-bot for Mike Galbraith <tipbot@zytor.com>]

Commit-ID:  bb2bc55a694d45cdeda91b6f28ab2adec28125ef
Gitweb:     http://git.kernel.org/tip/bb2bc55a694d45cdeda91b6f28ab2adec28125ef
Author:     Mike Galbraith <umgwanakikbuti@gmail.com>
AuthorDate: Wed, 28 Jan 2015 04:53:55 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 28 Jan 2015 15:28:15 +0100

sched: Fix crash if cpuset_cpumask_can_shrink() is passed an empty cpumask

While creating an exclusive cpuset, we passed cpuset_cpumask_can_shrink()
an empty cpumask (cur), and dl_bw_of(cpumask_any(cur)) made boom with it:

 CPU: 0 PID: 6942 Comm: shield.sh Not tainted 3.19.0-master #19
 Hardware name: MEDIONPC MS-7502/MS-7502, BIOS 6.00 PG 12/26/2007
 task: ffff880224552450 ti: ffff8800caab8000 task.ti: ffff8800caab8000
 RIP: 0010:[<ffffffff81073846>]  [<ffffffff81073846>] cpuset_cpumask_can_shrink+0x56/0xb0
 [...]
 Call Trace:
  [<ffffffff810cb82a>] validate_change+0x18a/0x200
  [<ffffffff810cc877>] cpuset_write_resmask+0x3b7/0x720
  [<ffffffff810c4d58>] cgroup_file_write+0x38/0x100
  [<ffffffff811d953a>] kernfs_fop_write+0x12a/0x180
  [<ffffffff8116e1a3>] vfs_write+0xb3/0x1d0
  [<ffffffff8116ed06>] SyS_write+0x46/0xb0
  [<ffffffff8159ced6>] system_call_fastpath+0x16/0x1b

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Acked-by: Zefan Li <lizefan@huawei.com>
Fixes: f82f80426f7a ("sched/deadline: Ensure that updates to exclusive cpusets don't break AC")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1422417235.5716.5.camel@marge.simpson.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c0accc0..5c86687 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4642,6 +4642,9 @@ int cpuset_cpumask_can_shrink(const struct cpumask *cur,
 	struct dl_bw *cur_dl_b;
 	unsigned long flags;
 
+	if (!cpumask_weight(cur))
+		return ret;
+
 	rcu_read_lock_sched();
 	cur_dl_b = dl_bw_of(cpumask_any(cur));
 	trial_cpus = cpumask_weight(trial);