[PATCH] sched_groups are expected to be circular linked list, make it so right after allocation

[Igor Mammedov <imammedo@redhat.com>]

if we have one cpu that failed to boot and boot cpu gave up on waiting for it
and then another cpu is being booted, kernel might crash with following OOPS:

[  723.865765] BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
[  723.866616] IP: [<ffffffff812c3630>] __bitmap_weight+0x30/0x80
[  723.866616] PGD 7ba91067 PUD 7a205067 PMD 0
[  723.866616] Oops: 0000 [#1] SMP
[  723.898527] CPU 1
...
[  723.898527] Pid: 1221, comm: offV2.sh Tainted: G        W    3.4.0-rc4+ #213 Red Hat KVM
[  723.898527] RIP: 0010:[<ffffffff812c3630>]  [<ffffffff812c3630>] __bitmap_weight+0x30/0x80
[  723.898527] RSP: 0018:ffff88007ab9dc18  EFLAGS: 00010246
[  723.898527] RAX: 0000000000000003 RBX: 0000000000000000 RCX: 0000000000000000
[  723.898527] RDX: 0000000000000018 RSI: 0000000000000100 RDI: 0000000000000018
[  723.898527] RBP: ffff88007ab9dc18 R08: 0000000000000000 R09: 0000000000000020
[  723.898527] R10: 0000000000000004 R11: 0000000000000000 R12: ffff88007c06ed60
[  723.898527] R13: ffff880037a94000 R14: 0000000000000003 R15: ffff88007c06ed60
[  723.898527] FS:  00007f1d6a7d8700(0000) GS:ffff88007fc80000(0000) knlGS:0000000000000000
[  723.898527] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  723.898527] CR2: 0000000000000018 CR3: 000000007bb7f000 CR4: 00000000000007e0
[  723.898527] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  723.898527] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  723.898527] Process offV2.sh (pid: 1221, threadinfo ffff88007ab9c000, task ffff88007b358000)
[  723.898527] Stack:
[  723.898527]  ffff88007ab9dcc8 ffffffff8108b9b6 ffff88007ab9dc58 ffff88007b4f2a00
[  723.898527]  ffff88007c06ed60 0000000000000003 000000037ab9dc58 0000000000010008
[  723.898527]  ffffffff81a308e8 0000000000000003 ffff88007b489cc0 ffff880037b6bd20
[  723.898527] Call Trace:
[  723.898527]  [<ffffffff8108b9b6>] build_sched_domains+0x7b6/0xa50
[  723.898527]  [<ffffffff8108bea9>] partition_sched_domains+0x259/0x3f0
[  723.898527]  [<ffffffff810c4485>] cpuset_update_active_cpus+0x85/0x90
[  723.898527]  [<ffffffff81084f65>] cpuset_cpu_active+0x25/0x30
[  723.898527]  [<ffffffff81545b45>] notifier_call_chain+0x55/0x80
[  723.898527]  [<ffffffff8107e59e>] __raw_notifier_call_chain+0xe/0x10
[  723.898527]  [<ffffffff81058be0>] __cpu_notify+0x20/0x40
[  723.898527]  [<ffffffff8153af08>] _cpu_up+0xc7/0x10e
[  723.898527]  [<ffffffff8153af9b>] cpu_up+0x4c/0x5c

crash happens in  init_sched_groups_power() that expects sched_groups to be
circular linked list. However it is not always true, since sched_groups
preallocated in __sdt_alloc are initialized in build_sched_groups and it
may exit early

        if (cpu != cpumask_first(sched_domain_span(sd)))
                return 0;

without initializing sd->groups->next field.

Fix bug by initializing next field right after sched_group was allocated.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
 kernel/sched/core.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0533a68..e5212ae 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6382,6 +6382,8 @@ static int __sdt_alloc(const struct cpumask *cpu_map)
 			if (!sg)
 				return -ENOMEM;
 
+			sg->next = sg;
+
 			*per_cpu_ptr(sdd->sg, j) = sg;
 
 			sgp = kzalloc_node(sizeof(struct sched_group_power),
-- 
1.7.1