Re: [PATCH] sched/deadline: correctly handle active 0-lag timers

[Juri Lelli <juri.lelli@redhat.com>]

Hi,

On 25/03/19 14:15, luca abeni wrote:
> syzbot reported the following warning:
> [  948.126369] WARNING: CPU: 4 PID: 17089 at kernel/sched/deadline.c:255 task_non_contending+0xae0/0x1950
> [  948.130198] Kernel panic - not syncing: panic_on_warn set ...
> [  948.130198]
> [  948.134221] CPU: 4 PID: 17089 Comm: syz-executor.1 Not tainted 4.19.27 #2
> [  948.139072] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
> [  948.141603] Call Trace:
> [  948.142277]  dump_stack+0xca/0x13e
> [  948.164636]  panic+0x1f7/0x543
> [  948.168704]  ? refcount_error_report+0x29d/0x29d
> [  948.172438]  ? __warn+0x1d1/0x210
> [  948.183359]  ? task_non_contending+0xae0/0x1950
> [  948.191747]  __warn+0x1ec/0x210
> [  948.196276]  ? task_non_contending+0xae0/0x1950
> [  948.202476]  report_bug+0x1ee/0x2b0
> [  948.204622]  fixup_bug.part.7+0x37/0x80
> [  948.206879]  do_error_trap+0x22c/0x290
> [  948.211340]  ? math_error+0x2f0/0x2f0
> [  948.217033]  ? trace_hardirqs_off_caller+0x40/0x190
> [  948.222477]  ? trace_hardirqs_off_thunk+0x1a/0x1c
> [  948.229877]  invalid_op+0x14/0x20
> [  948.238317] RIP: 0010:task_non_contending+0xae0/0x1950
> [  948.253825] Code: 6d 29 83 48 89 4c 24 20 48 89 54 24 10 c6 05 d0 89
> 5a 03 01 e8 11 ea ee ff 0f 0b 48 8b 4c 24 20 48 8b 54 24 10 e9 bb f7 ff
> ff <0f> 0b e9 1d f6 ff ff e8 d4 a7 09 00 85 c0 0f 85 74 f8 ff ff 48 c7
> [  948.272329] RSP: 0018:ffff8883d443f8c0 EFLAGS: 00010002
> [  948.293045] RAX: 0000000000000001 RBX: ffff8883d3572468 RCX: ffffffff813a6571
> [  948.300323] RDX: 00000000000008ab RSI: ffffc900030e4000 RDI: ffff8883e2fe6278
> [  948.305278] RBP: ffff8883e2f00000 R08: ffffed1078ea3ab2 R09: ffffed1078ea3ab2
> [  948.316441] R10: 0000000000000001 R11: ffffed1078ea3ab1 R12: 000000000002c680
> [  948.320257] R13: ffff8883d357217c R14: 0000000000000001 R15: ffff8883d3572140
> [  948.324500]  ? hrtimer_active+0x171/0x1f0
> [  948.327421]  ? dequeue_task_dl+0x38/0x970
> [  948.330572]  __schedule+0x94b/0x1a80
> [  948.333578]  ? __sched_text_start+0x8/0x8
> [  948.336141]  ? lock_downgrade+0x5e0/0x5e0
> [  948.338111]  ? plist_add+0x23e/0x480
> [  948.339706]  schedule+0x7c/0x1a0
> [  948.341395]  futex_wait_queue_me+0x319/0x600
> [  948.343329]  ? get_futex_key_refs+0xd0/0xd0
> [  948.345037]  ? lock_downgrade+0x5e0/0x5e0
> [  948.347206]  ? get_futex_key_refs+0xa4/0xd0
> [  948.353007]  futex_wait+0x1e7/0x590
> [  948.355328]  ? futex_wait_setup+0x2b0/0x2b0
> [  948.360578]  ? __lock_acquire+0x60c/0x3b70
> [  948.369186]  ? __save_stack_trace+0x92/0x100
> [  948.374344]  ? hash_futex+0x15/0x210
> [  948.376832]  ? drop_futex_key_refs+0x3c/0xd0
> [  948.378591]  ? futex_wake+0x14e/0x450
> [  948.381609]  do_futex+0x5c9/0x15e0
> [  948.384567]  ? perf_syscall_enter+0xb1/0xc80
> [  948.390307]  ? exit_robust_list+0x240/0x240
> [  948.393566]  ? ftrace_syscall_exit+0x5c0/0x5c0
> [  948.396369]  ? lock_downgrade+0x5e0/0x5e0
> [  948.401748]  ? __might_fault+0x17c/0x1c0
> [  948.404171]  __x64_sys_futex+0x296/0x380
> [  948.406472]  ? __ia32_sys_futex+0x370/0x370
> [  948.440630]  ? trace_hardirqs_on_thunk+0x1a/0x1c
> [  948.441774]  ? trace_hardirqs_off_caller+0x40/0x190
> [  948.442770]  ? do_syscall_64+0x3b/0x580
> [  948.486728]  do_syscall_64+0xc8/0x580
> [  948.489138]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> [  948.492072] RIP: 0033:0x462eb9
> [  948.492788] Code: f7 d8 64 89 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00
> 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f
> 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
> [  948.532016] RSP: 002b:00007f7ac8a67cd8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
> [  948.536811] RAX: ffffffffffffffda RBX: 000000000073bf08 RCX: 0000000000462eb9
> [  948.542138] RDX: 0000000000000000 RSI: 0000000000000080 RDI: 000000000073bf08
> [  948.548077] RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
> [  948.562535] R10: 0000000000000000 R11: 0000000000000246 R12: 000000000073bf0c
> [  948.569184] R13: 0000000000000000 R14: 000000000073bf00 R15: 00007fff106d8c10

Not sure if this stack needs a bit of cleaning for the changelog (Code:
seems mangled as well).

> line 255 of deadline.c is
> 	WARN_ON(hrtimer_active(&dl_se->inactive_timer));
> in task_non_contending().
> Unfortunately, in some cases (for example, a deadline task
> continuosly blocking and waking immediately) it can happen that
> a task blocks (and task_non_contending() is called) while the
> 0-lag timer is still active. In this case, the safest thing to
> do is to immediately decrease the running bandwidth of the task,
> without trying to re-arm the 0-lag timer.
> 
> Signed-off-by: luca abeni <luca.abeni@santannapisa.it>

But I could verify that this fixes the issue I was also able to
reproduce.

Acked-by: Juri Lelli <juri.lelli@redhat.com>

Thanks!

- Juri