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Subject: RE: [PATCH v2 11/13] video/hyperv_fb: Avoid taking busy spinlock on
 panic path
Thread-Topic: [PATCH v2 11/13] video/hyperv_fb: Avoid taking busy spinlock on
 panic path
Thread-Index: AQHYm6nhxD3vCNgXDkaqQNF/QdvErK2PaFrg
Date:   Mon, 25 Jul 2022 18:09:28 +0000
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 <20220719195325.402745-12-gpiccoli@igalia.com>
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From: Guilherme G. Piccoli <gpiccoli@igalia.com> Sent: Tuesday, July 19, 20=
22 12:53 PM
>=20
> The Hyper-V framebuffer code registers a panic notifier in order
> to try updating its fbdev if the kernel crashed. The notifier
> callback is straightforward, but it calls the vmbus_sendpacket()
> routine eventually, and such function takes a spinlock for the
> ring buffer operations.
>=20
> Panic path runs in atomic context, with local interrupts and
> preemption disabled, and all secondary CPUs shutdown. That said,
> taking a spinlock might cause a lockup if a secondary CPU was
> disabled with such lock taken. Fix it here by checking if the
> ring buffer spinlock is busy on Hyper-V framebuffer panic notifier;
> if so, bail-out avoiding the potential lockup scenario.
>=20
> Cc: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> Cc: Dexuan Cui <decui@microsoft.com>
> Cc: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> Cc: Michael Kelley <mikelley@microsoft.com>
> Cc: Stephen Hemminger <sthemmin@microsoft.com>
> Cc: Tianyu Lan <Tianyu.Lan@microsoft.com>
> Cc: Wei Liu <wei.liu@kernel.org>
> Tested-by: Fabio A M Martins <fabiomirmar@gmail.com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
>=20
> ---
>=20
> V2:
> - new patch, based on the discussion in [0].
> [0] https://lore.kernel.org/lkml/2787b476-6366-1c83-db80-0393da417497@iga=
lia.com/
>=20
>  drivers/hv/ring_buffer.c        | 16 ++++++++++++++++
>  drivers/video/fbdev/hyperv_fb.c |  8 +++++++-
>  include/linux/hyperv.h          |  2 ++
>  3 files changed, 25 insertions(+), 1 deletion(-)
>=20
> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> index 59a4aa86d1f3..9ceb3a7e8d19 100644
> --- a/drivers/hv/ring_buffer.c
> +++ b/drivers/hv/ring_buffer.c
> @@ -280,6 +280,22 @@ void hv_ringbuffer_cleanup(struct hv_ring_buffer_inf=
o
> *ring_info)
>  	ring_info->pkt_buffer_size =3D 0;
>  }
>=20
> +/*
> + * Check if the ring buffer spinlock is available to take or not; used o=
n
> + * atomic contexts, like panic path (see the Hyper-V framebuffer driver)=
.
> + */
> +
> +bool hv_ringbuffer_spinlock_busy(struct vmbus_channel *channel)
> +{
> +	struct hv_ring_buffer_info *rinfo =3D &channel->outbound;
> +
> +	if (spin_is_locked(&rinfo->ring_lock))
> +		return true;
> +
> +	return false;

Could simplify the code as just:

	return spin_is_locked(&rinfo->ring_lock);

> +}
> +EXPORT_SYMBOL_GPL(hv_ringbuffer_spinlock_busy);
> +
>  /* Write to the ring buffer. */
>  int hv_ringbuffer_write(struct vmbus_channel *channel,
>  			const struct kvec *kv_list, u32 kv_count,
> diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv=
_fb.c
> index 886c564787f1..e1b65a01fb96 100644
> --- a/drivers/video/fbdev/hyperv_fb.c
> +++ b/drivers/video/fbdev/hyperv_fb.c
> @@ -783,12 +783,18 @@ static void hvfb_ondemand_refresh_throttle(struct
> hvfb_par *par,
>  static int hvfb_on_panic(struct notifier_block *nb,
>  			 unsigned long e, void *p)
>  {
> +	struct hv_device *hdev;
>  	struct hvfb_par *par;
>  	struct fb_info *info;
>=20
>  	par =3D container_of(nb, struct hvfb_par, hvfb_panic_nb);
> -	par->synchronous_fb =3D true;
>  	info =3D par->info;
> +	hdev =3D device_to_hv_device(info->device);
> +
> +	if (hv_ringbuffer_spinlock_busy(hdev->channel))
> +		return NOTIFY_DONE;
> +
> +	par->synchronous_fb =3D true;
>  	if (par->need_docopy)
>  		hvfb_docopy(par, 0, dio_fb_size);
>  	synthvid_update(info, 0, 0, INT_MAX, INT_MAX);
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index 3b42264333ef..646f1da9f27e 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -1341,6 +1341,8 @@ struct hv_ring_buffer_debug_info {
>  int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info,
>  				struct hv_ring_buffer_debug_info *debug_info);
>=20
> +bool hv_ringbuffer_spinlock_busy(struct vmbus_channel *channel);
> +
>  /* Vmbus interface */
>  #define vmbus_driver_register(driver)	\
>  	__vmbus_driver_register(driver, THIS_MODULE, KBUILD_MODNAME)
> --
> 2.37.1