Re: [PATCH] sched: fix infinity loop in update_blocked_averages

[Vincent Guittot]

Hi Xie,

On Thu, 27 Dec 2018 at 03:57, Xie XiuQi <xiexiuqi@xxxxxxxxxx> wrote:
>
> Zhepeng Xie report a bug, there is a infinity loop in
> update_blocked_averages().
>
> PID: 14233  TASK: ffff800b2de08fc0  CPU: 1   COMMAND: "docker"
>  #0 [ffff00002213b9d0] update_blocked_averages at ffff00000811e4a8
>  #1 [ffff00002213ba60] pick_next_task_fair at ffff00000812a3b4
>  #2 [ffff00002213baf0] __schedule at ffff000008deaa88
>  #3 [ffff00002213bb70] schedule at ffff000008deb1b8
>  #4 [ffff00002213bb80] futex_wait_queue_me at ffff000008180754
>  #5 [ffff00002213bbd0] futex_wait at ffff00000818192c
>  #6 [ffff00002213bd00] do_futex at ffff000008183ee4
>  #7 [ffff00002213bde0] __arm64_sys_futex at ffff000008184398
>  #8 [ffff00002213be60] el0_svc_common at ffff0000080979ac
>  #9 [ffff00002213bea0] el0_svc_handler at ffff000008097a6c
>  #10 [ffff00002213bff0] el0_svc at ffff000008084044
>
> rq->tmp_alone_branch introduced in 4.10, used to point to
> the new beg of the list. If this cfs_rq is deleted somewhere
> else, then the tmp_alone_branch will be illegal and cause
> a list_add corruption.

shouldn't all the sequence  be protected by rq_lock ?


>
> (When enabled DEBUG_LIST, we fould this list_add corruption)
>
> [ 2546.741103] list_add corruption. next->prev should be prev
> (ffff800b4d61ad40), but was ffff800ba434fa38. (next=ffff800b6a95e740).
> [ 2546.741130] ------------[ cut here ]------------
> [ 2546.741132] kernel BUG at lib/list_debug.c:25!
> [ 2546.741136] Internal error: Oops - BUG: 0 [#1] SMP
> [ 2546.742870] CPU: 1 PID: 29428 Comm: docker-runc Kdump: loaded Tainted: G E  4.19.5-1.aarch64 #1
> [ 2546.745415] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
> [ 2546.747402] pstate: 40000085 (nZcv daIf -PAN -UAO)
> [ 2546.749015] pc : __list_add_valid+0x50/0x90
> [ 2546.750485] lr : __list_add_valid+0x50/0x90
> [ 2546.751975] sp : ffff00001b5eb910
> [ 2546.753286] x29: ffff00001b5eb910 x28: ffff800abacf0000
> [ 2546.754976] x27: ffff00001b5ebbb0 x26: ffff000009570000
> [ 2546.756665] x25: ffff00000960d000 x24: 00000250f41ca8f8
> [ 2546.758366] x23: ffff800b6a95e740 x22: ffff800b4d61ad40
> [ 2546.760066] x21: ffff800b4d61ad40 x20: ffff800ba434f080
> [ 2546.761742] x19: ffff800b4d61ac00 x18: ffffffffffffffff
> [ 2546.763425] x17: 0000000000000000 x16: 0000000000000000
> [ 2546.765089] x15: ffff000009570748 x14: 6666662073617720
> [ 2546.766755] x13: 747562202c293034 x12: 6461313664346230
> [ 2546.768429] x11: 3038666666662820 x10: 0000000000000000
> [ 2546.770124] x9 : 0000000000000001 x8 : ffff000009f34a0f
> [ 2546.771831] x7 : 0000000000000000 x6 : 000000000000250d
> [ 2546.773525] x5 : 0000000000000000 x4 : 0000000000000000
> [ 2546.775227] x3 : 0000000000000000 x2 : 70ef7f624013ca00
> [ 2546.776929] x1 : 0000000000000000 x0 : 0000000000000075
> [ 2546.778623] Process docker-runc (pid: 29428, stack limit = 0x00000000293494a2)
> [ 2546.780742] Call trace:
> [ 2546.781955]  __list_add_valid+0x50/0x90
> [ 2546.783469]  enqueue_entity+0x4a0/0x6e8
> [ 2546.784957]  enqueue_task_fair+0xac/0x610
> [ 2546.786502]  sched_move_task+0x134/0x178
> [ 2546.787993]  cpu_cgroup_attach+0x40/0x78
> [ 2546.789540]  cgroup_migrate_execute+0x378/0x3a8
> [ 2546.791169]  cgroup_migrate+0x6c/0x90
> [ 2546.792663]  cgroup_attach_task+0x148/0x238
> [ 2546.794211]  __cgroup1_procs_write.isra.2+0xf8/0x160
> [ 2546.795935]  cgroup1_procs_write+0x38/0x48
> [ 2546.797492]  cgroup_file_write+0xa0/0x170
> [ 2546.799010]  kernfs_fop_write+0x114/0x1e0
> [ 2546.800558]  __vfs_write+0x60/0x190
> [ 2546.801977]  vfs_write+0xac/0x1c0
> [ 2546.803341]  ksys_write+0x6c/0xd8
> [ 2546.804674]  __arm64_sys_write+0x24/0x30
> [ 2546.806146]  el0_svc_common+0x78/0x100
> [ 2546.807584]  el0_svc_handler+0x38/0x88
> [ 2546.809017]  el0_svc+0x8/0xc
>

Have you got more details about the sequence that generates this bug ?
Is it easily reproducible ?

> In this patch, we move rq->tmp_alone_branch point to its prev before delete it
> from list.
>
> Reported-by: Zhipeng Xie <xiezhipeng1@xxxxxxxxxx>
> Cc: Bin Li <huawei.libin@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>        [4.10+]
> Fixes: 9c2791f936ef (sched/fair: Fix hierarchical order in rq->leaf_cfs_rq_list)

If it only happens in update_blocked_averages(), the del leaf has been added by:
a9e7f6544b9c (sched/fair: Fix O(nr_cgroups) in load balance path)

> Signed-off-by: Xie XiuQi <xiexiuqi@xxxxxxxxxx>
> Tested-by: Zhipeng Xie <xiezhipeng1@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ac855b2..7a72702 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -347,6 +347,11 @@ static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
>  static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
>  {
>         if (cfs_rq->on_list) {
> +               struct rq *rq = rq_of(cfs_rq);
> +
> +               if (rq->tmp_alone_branch == &cfs_rq->leaf_cfs_rq_list)
> +                       rq->tmp_alone_branch = cfs_rq->leaf_cfs_rq_list.prev;
> +
>                 list_del_rcu(&cfs_rq->leaf_cfs_rq_list);
>                 cfs_rq->on_list = 0;
>         }
> --
> 1.8.3.1
>