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Date:   Sun, 18 Sep 2022 11:12:15 -0300
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Subject: Re: [PATCH V3 09/11] video/hyperv_fb: Avoid taking busy spinlock on
 panic path
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To:     linux-hyperv@vger.kernel.org, mikelley@microsoft.com
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        will@kernel.org, xuqiang36@huawei.com,
        Andrea Parri <parri.andrea@gmail.com>,
        Dexuan Cui <decui@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Tianyu Lan <Tianyu.Lan@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>
References: <20220819221731.480795-1-gpiccoli@igalia.com>
 <20220819221731.480795-10-gpiccoli@igalia.com>
From:   "Guilherme G. Piccoli" <gpiccoli@igalia.com>
In-Reply-To: <20220819221731.480795-10-gpiccoli@igalia.com>
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On 19/08/2022 19:17, Guilherme G. Piccoli wrote:
> 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.
> 
> 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.
> 
> 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>
> 
> ---
> 
> V3:
> - simplified the code based on Michael's suggestion - thanks!
> 
> V2:
> - new patch, based on the discussion in [0].
> [0] https://lore.kernel.org/lkml/2787b476-6366-1c83-db80-0393da417497@igalia.com/
> 
> 
>  drivers/hv/ring_buffer.c        | 13 +++++++++++++
>  drivers/video/fbdev/hyperv_fb.c |  8 +++++++-
>  include/linux/hyperv.h          |  2 ++
>  3 files changed, 22 insertions(+), 1 deletion(-)
> [...]

Hi Michael, apologies for the ping.
Any reviews/comments on this one are greatly appreciated!

Cheers,


Guilherme