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Date:   Fri, 31 Jul 2020 11:16:16 +0200
From:   daniel@ffwll.ch
Cc:     <rodrigosiqueiramelo@gmail.com>, <hamohammed.sa@gmail.com>,
        <daniel@ffwll.ch>, <airlied@linux.ie>,
        <dri-devel@lists.freedesktop.org>, <linux-kernel@vger.kernel.org>,
        <lizefan@huawei.com>, <rui.xiang@huawei.com>
Subject: Re: [PATCH] drm/vkms: Fix soft lockup.
Message-ID: <20200731091616.GT6419@phenom.ffwll.local>
Mail-Followup-To: Xu Qiang <xuqiang36@huawei.com>,
        <rodrigosiqueiramelo@gmail.com>, <hamohammed.sa@gmail.com>,
        <airlied@linux.ie>, <dri-devel@lists.freedesktop.org>,
        <linux-kernel@vger.kernel.org>, <lizefan@huawei.com>,
        <rui.xiang@huawei.com>
References: <20200729022315.18716-1-xuqiang36@huawei.com>
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On Wed, Jul 29, 2020 at 02:23:15AM +0000, Xu Qiang wrote:
> A soft deadlock occurs when call hrtimer_cancel in softirq context:
> 
> 	a) The main frequency of the machine is very slow
> 	b) output->period_ns is very small, even only 1 ns
> 
> The problem can be solved in the following way: Setting a hrtimer exit flag
> in the vkms_disable_vblank function and checking the flag in the
> vkms_vblank_simulate function. If the flag is set, the hrtimer is not added
> to hrtimer queue again,the hrtimer_cancel function can exit quickly
> without deadlock.
> 
> watchdog: BUG: soft lockup - CPU#2 stuck for 134s! [syz-executor.2:18027]
> Modules linked in:
> CPU: 2 PID: 18027 Comm: syz-executor.2 Tainted: G        W         5.8.0-rc2-csan #2
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
> RIP: 0010:csd_lock_wait kernel/smp.c:108 [inline]
> RIP: 0010:smp_call_function_many_cond+0x40c/0x470 kernel/smp.c:555
> RSP: 0018:ffffc900028d3a88 EFLAGS: 00000297
> RAX: 0000000000000002 RBX: ffff888237cacc80 RCX: ffffffff813240fc
> RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000000000000005
> RBP: 0000000000000001 R08: ffff8881f497d000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000100 R12: ffff888237cf6fc0
> R13: 0000000000000003 R14: ffff888237cacc88 R15: 0000000000000001
> FS:  00007fa6314a9700(0000) GS:ffff888237c80000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000513c80 CR3: 00000001e3df3002 CR4: 00000000000626e0
> Call Trace:
>  smp_call_function_many kernel/smp.c:577 [inline]
>  smp_call_function+0x40/0x80 kernel/smp.c:599
>  on_each_cpu+0x2a/0xc0 kernel/smp.c:717
>  __purge_vmap_area_lazy+0x7c/0xbc0 mm/vmalloc.c:1367
>  _vm_unmap_aliases.part.0+0x126/0x180 mm/vmalloc.c:1800
>  _vm_unmap_aliases mm/vmalloc.c:1769 [inline]
>  vm_unmap_aliases+0x2f/0x40 mm/vmalloc.c:1823
>  change_page_attr_set_clr+0x10a/0x4a0 arch/x86/mm/pat/set_memory.c:1732
>  change_page_attr_clear arch/x86/mm/pat/set_memory.c:1789 [inline]
>  set_memory_ro+0x2b/0x40 arch/x86/mm/pat/set_memory.c:1935
>  bpf_jit_binary_lock_ro include/linux/filter.h:815 [inline]
>  bpf_int_jit_compile+0x54f/0x663 arch/x86/net/bpf_jit_comp.c:1929
>  bpf_prog_select_runtime+0x1cc/0x2c0 kernel/bpf/core.c:1807
>  bpf_migrate_filter net/core/filter.c:1264 [inline]
>  bpf_prepare_filter net/core/filter.c:1312 [inline]
>  bpf_prepare_filter+0x518/0x620 net/core/filter.c:1278
>  __get_filter+0x107/0x160 net/core/filter.c:1481
>  sk_attach_filter+0x19/0xa0 net/core/filter.c:1496
>  sock_setsockopt+0x1208/0x1230 net/core/sock.c:1080
>  __sys_setsockopt+0x248/0x270 net/socket.c:2123
>  __do_sys_setsockopt net/socket.c:2143 [inline]
>  __se_sys_setsockopt net/socket.c:2140 [inline]
>  __x64_sys_setsockopt+0x22/0x30 net/socket.c:2140
>  do_syscall_64+0x48/0xb0 arch/x86/entry/common.c:359
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> Sending NMI from CPU 2 to CPUs 0-1,3-5:
> NMI backtrace for cpu 4 skipped: idling at native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:60
> NMI backtrace for cpu 0 skipped: idling at native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:60
> NMI backtrace for cpu 1
> NMI backtrace for cpu 3
> CPU: 3 PID: 0 Comm: swapper/3 Tainted: G        W         5.8.0-rc2-csan #2
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
> RIP: 0010:preempt_count arch/x86/include/asm/preempt.h:26 [inline]
> RIP: 0010:preempt_count_sub+0xa/0x90 kernel/sched/core.c:3874
> RSP: 0018:ffffc9000012cdd0 EFLAGS: 00000046
> RAX: 0000000000000001 RBX: ffff88822ecb8fb0 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: 0000000000000086 RDI: 0000000000000001
> RBP: ffff888237d5edc0 R08: ffff888236ddc000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 00000000ffffffff
> R13: 0000000000000086 R14: ffff888237d5edc0 R15: ffffffff85bc6d60
> FS:  0000000000000000(0000) GS:ffff888237cc0000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f35cc47adb8 CR3: 0000000005a23006 CR4: 00000000000626e0
> Call Trace:
>  <IRQ>
>  __raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:161 [inline]
>  _raw_spin_unlock_irqrestore+0x2f/0x50 kernel/locking/spinlock.c:191
>  unlock_hrtimer_base kernel/time/hrtimer.c:898 [inline]
>  hrtimer_try_to_cancel kernel/time/hrtimer.c:1171 [inline]
>  hrtimer_try_to_cancel+0xbd/0x1b0 kernel/time/hrtimer.c:1151
>  hrtimer_cancel+0x13/0x40 kernel/time/hrtimer.c:1278
>  __disable_vblank drivers/gpu/drm/drm_vblank.c:429 [inline]
>  drm_vblank_disable_and_save+0x122/0x140 drivers/gpu/drm/drm_vblank.c:470
>  vblank_disable_fn+0x96/0xa0 drivers/gpu/drm/drm_vblank.c:487
>  call_timer_fn+0x3a/0x230 kernel/time/timer.c:1404
>  expire_timers kernel/time/timer.c:1449 [inline]
>  __run_timers kernel/time/timer.c:1773 [inline]
>  __run_timers kernel/time/timer.c:1740 [inline]
>  run_timer_softirq+0x2b8/0x840 kernel/time/timer.c:1786
>  __do_softirq+0x118/0x344 kernel/softirq.c:292
>  asm_call_on_stack+0xf/0x20 arch/x86/entry/entry_64.S:711
>  </IRQ>
>  __run_on_irqstack arch/x86/include/asm/irq_stack.h:22 [inline]
>  run_on_irqstack_cond arch/x86/include/asm/irq_stack.h:48 [inline]
>  do_softirq_own_stack+0x5a/0x70 arch/x86/kernel/irq_64.c:77
>  invoke_softirq kernel/softirq.c:387 [inline]
>  __irq_exit_rcu kernel/softirq.c:417 [inline]
>  irq_exit_rcu+0xb1/0xd0 kernel/softirq.c:429
>  sysvec_apic_timer_interrupt+0x42/0xd0 arch/x86/kernel/apic/apic.c:1091
>  asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:596
> ------------[ cut here ]------------
> WARNING: CPU: 5 PID: 8121 at drivers/gpu/drm/vkms/vkms_crtc.c:21 vkms_vblank_simulate+0x19a/0x1b0 drivers/gpu/drm/vkms/vkms_crtc.c:21
> Modules linked in:
> CPU: 5 PID: 8121 Comm: syslog-ng Tainted: G        W         5.8.0-rc2-csan #2
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
> RIP: 0010:vkms_vblank_simulate+0x19a/0x1b0 drivers/gpu/drm/vkms/vkms_crtc.c:21
> RSP: 0018:ffffc90000184ee8 EFLAGS: 00010046
> RAX: ffff88822864a080 RBX: ffff88822ecb8fb0 RCX: ffffffff826cd2b0
> RDX: 0000000080010000 RSI: ffffffff826cd40a RDI: 0000000000000007
> RBP: 00000000001f058d R08: ffff88822864a080 R09: 7fffffffffffffff
> R10: 0000000000000000 R11: 0000000000000000 R12: ffff88822ecb8630
> R13: 0000000000000000 R14: 0000017998a35917 R15: ffff888237d5ed80
> FS:  00007f8dcbde0700(0000) GS:ffff888237d40000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffffffffff600400 CR3: 000000022575f002 CR4: 00000000000626e0
> Call Trace:
>  <IRQ>
>  __run_hrtimer kernel/time/hrtimer.c:1520 [inline]
>  __hrtimer_run_queues+0x14c/0x410 kernel/time/hrtimer.c:1584
>  hrtimer_interrupt+0x141/0x2c0 kernel/time/hrtimer.c:1646
>  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1080 [inline]
>  __sysvec_apic_timer_interrupt+0x6e/0x170 arch/x86/kernel/apic/apic.c:1097
>  asm_call_on_stack+0xf/0x20 arch/x86/entry/entry_64.S:711
>  </IRQ>
>  __run_on_irqstack arch/x86/include/asm/irq_stack.h:22 [inline]
>  run_on_irqstack_cond arch/x86/include/asm/irq_stack.h:48 [inline]
>  sysvec_apic_timer_interrupt+0xa9/0xd0 arch/x86/kernel/apic/apic.c:1091
>  asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:596
> RIP: 0010:con_should_update drivers/tty/vt/vt.c:291 [inline]
> RIP: 0010:do_con_write.part.0+0xad6/0xfa0 drivers/tty/vt/vt.c:2790
> ---[ end trace d6a6c6a9ecb73597 ]---
> NMI backtrace for cpu 5 skipped: idling at native_halt+0xd/0x10 arch/x86/include/asm/irqflags.h:66
> 
> Cc: stable@vger.kernel.org
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Signed-off-by: Xu Qiang <xuqiang36@huawei.com>
> ---
>  drivers/gpu/drm/vkms/vkms_crtc.c | 5 ++++-
>  drivers/gpu/drm/vkms/vkms_drv.h  | 1 +
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> index ac85e17428f8..655294f11bb8 100644
> --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> @@ -49,7 +49,7 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
>  			DRM_DEBUG_DRIVER("Composer worker already queued\n");
>  	}
>  
> -	return HRTIMER_RESTART;
> +	return output->hrtimer_exit ? HRTIMER_NORESTART : HRTIMER_RESTART;

Hm don't we need some locking for this? Or at least a bit of thought about
ordering and stuff, maybe a comment about what's going on. Or are we
covered already by a different lock (but then maybe the check here should
be moved, and a comment added about which spinlock protects this).

The other question: How did you even end up in here? I think the better
fix would be to make sure the mode we're setting is reasonable, i.e. maybe
a refresh rate between 10Hz and 1000Hz or so. Even slow machines should
then be able to stop the timer ... That would be a lot more reasonable
fix, refresh rate with just a few ns never happen on real displays :-)

Cheers, Daniel


>  }
>  
>  static int vkms_enable_vblank(struct drm_crtc *crtc)
> @@ -66,6 +66,8 @@ static int vkms_enable_vblank(struct drm_crtc *crtc)
>  	out->period_ns = ktime_set(0, vblank->framedur_ns);
>  	hrtimer_start(&out->vblank_hrtimer, out->period_ns, HRTIMER_MODE_REL);
>  
> +	out->hrtimer_exit = false;
> +
>  	return 0;
>  }
>  
> @@ -73,6 +75,7 @@ static void vkms_disable_vblank(struct drm_crtc *crtc)
>  {
>  	struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
>  
> +	out->hrtimer_exit = true;
>  	hrtimer_cancel(&out->vblank_hrtimer);
>  }
>  
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> index f4036bb0b9a8..4d4323dc2cfd 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.h
> +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> @@ -65,6 +65,7 @@ struct vkms_output {
>  	struct drm_connector connector;
>  	struct hrtimer vblank_hrtimer;
>  	ktime_t period_ns;
> +	bool	hrtimer_exit;
>  	struct drm_pending_vblank_event *event;
>  	/* ordered wq for composer_work */
>  	struct workqueue_struct *composer_workq;
> -- 
> 2.25.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch