Re: [PATCH] sched/deadline: Fix switched_from_dl

[Daniel Bristot de Oliveira <bristot@redhat.com>]

On 07/11/2018 09:29 AM, Juri Lelli wrote:
> Mark noticed that syzkaller is able to reliably trigger the following
> 
>   dl_rq->running_bw > dl_rq->this_bw
>   WARNING: CPU: 1 PID: 153 at kernel/sched/deadline.c:124 switched_from_dl+0x454/0x608
>   Kernel panic - not syncing: panic_on_warn set ...
> 
>   CPU: 1 PID: 153 Comm: syz-executor253 Not tainted 4.18.0-rc3+ #29
>   Hardware name: linux,dummy-virt (DT)
>   Call trace:
>    dump_backtrace+0x0/0x458
>    show_stack+0x20/0x30
>    dump_stack+0x180/0x250
>    panic+0x2dc/0x4ec
>    __warn_printk+0x0/0x150
>    report_bug+0x228/0x2d8
>    bug_handler+0xa0/0x1a0
>    brk_handler+0x2f0/0x568
>    do_debug_exception+0x1bc/0x5d0
>    el1_dbg+0x18/0x78
>    switched_from_dl+0x454/0x608
>    __sched_setscheduler+0x8cc/0x2018
>    sys_sched_setattr+0x340/0x758
>    el0_svc_naked+0x30/0x34
> 
> syzkaller reproducer runs a bunch of threads that constantly switch
> between DEADLINE and NORMAL classes while interacting through futexes.
> 
> The splat above is caused by the fact that if a DEADLINE task is setattr
> back to NORMAL while in non_contending state (blocked on a futex -
> inactive timer armed), its contribution to running_bw is not removed
> before sub_rq_bw() gets called (!task_on_rq_queued() branch) and the
> latter sees running_bw > this_bw.
> 
> Fix it by removing a task contribution from running_bw if the task is
> not queued and in non_contending state while switched to a different
> class.
> 
> Reported-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>

Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>

Thanks!
-- Daniel