Re: [PATCH -next v2 9/9] blk-iocost: fix walk_list corruption

Tejun Heo
2022-11-30T20:59:26Z

On Wed, Nov 30, 2022 at 09:21:56PM +0800, Li Nan wrote:
> From: Yu Kuai <yukuai3@huawei.com>
>
> Our test report a problem:
>
> ------------[ cut here ]------------
> list_del corruption. next->prev should be ffff888127e0c4b0, but was ffff888127e090b0
> WARNING: CPU: 2 PID: 3117789 at lib/list_debug.c:62 __list_del_entry_valid+0x119/0x130
> RIP: 0010:__list_del_entry_valid+0x119/0x130
> RIP: 0010:__list_del_entry_valid+0x119/0x130
> Call Trace:
> <IRQ>
> iocg_flush_stat.isra.0+0x11e/0x230
> ? ioc_rqos_done+0x230/0x230
> ? ioc_now+0x14f/0x180
> ioc_timer_fn+0x569/0x1640
>
> We haven't reporduced it yet, but we think this is due to parent iocg is
> freed before child iocg, and then in ioc_timer_fn, walk_list is
> corrupted.
>
> 1) Remove child cgroup can concurrent with remove parent cgroup, and
> ioc_pd_free for parent iocg can be called before child iocg. This can be
> fixed by moving the handle of walk_list to ioc_pd_offline, since that
> offline from child is ensured to be called before parent.

Which you already did in a previous patch, right?

> 2) ioc_pd_free can be triggered from both removing device and removing
> cgroup, this patch fix the problem by deleting timer before deactivating
> policy, so that free parent iocg first in this case won't matter.

Okay, so, yeah, css's pin parents but blkg's don't. I think the right thing
to do here is making sure that a child blkg pins its parent (and eventually
ioc).

> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> Signed-off-by: Li Nan <linan122@huawei.com>
> ---
> block/blk-iocost.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 710cf63a1643..d2b873908f88 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -2813,13 +2813,14 @@ static void ioc_rqos_exit(struct rq_qos *rqos)
> {
> struct ioc *ioc = rqos_to_ioc(rqos);
>
> + del_timer_sync(&ioc->timer);
> +
> blkcg_deactivate_policy(rqos->q, &blkcg_policy_iocost);
>
> spin_lock_irq(&ioc->lock);
> ioc->running = IOC_STOP;
> spin_unlock_irq(&ioc->lock);
>
> - del_timer_sync(&ioc->timer);

I don't about this workaround. Let's fix properly?

--
tejun