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SubjectRe: [PATCH] sched/deadline: Fix switched_from_dl
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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

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