linux-kernel - Re: [PATCH v2] media: mediatek: vcodec: Use spinlock for context list protection lock

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Message-ID: <CAAFQd5B=0Zyyi2yJ5xD41LDBVYvAWoLu-oM_2XQORWgTDWo3uA@mail.gmail.com>
Date: Thu, 4 Sep 2025 15:39:48 +0900
From: Tomasz Figa <tfiga@...omium.org>
To: Chen-Yu Tsai <wenst@...omium.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>, Hans Verkuil <hverkuil@...all.nl>, 
	Yunfei Dong <yunfei.dong@...iatek.com>, Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org, 
	Fei Shao <fshao@...omium.org>
Subject: Re: [PATCH v2] media: mediatek: vcodec: Use spinlock for context list
 protection lock

On Thu, Sep 4, 2025 at 3:18 PM Chen-Yu Tsai <wenst@...omium.org> wrote:
>
> Ping?
>
> On Wed, Aug 20, 2025 at 6:37 PM Fei Shao <fshao@...omium.org> wrote:
> >
> > On Wed, Aug 20, 2025 at 3:54 PM Chen-Yu Tsai <wenst@...omium.org> wrote:
> > >
> > > Previously a mutex was added to protect the encoder and decoder context
> > > lists from unexpected changes originating from the SCP IP block, causing
> > > the context pointer to go invalid, resulting in a NULL pointer
> > > dereference in the IPI handler.
> > >
> > > Turns out on the MT8173, the VPU IPI handler is called from hard IRQ
> > > context. This causes a big warning from the scheduler. This was first
> > > reported downstream on the ChromeOS kernels, but is also reproducible
> > > on mainline using Fluster with the FFmpeg v4l2m2m decoders. Even though
> > > the actual capture format is not supported, the affected code paths
> > > are triggered.
> > >
>
> We really should get this in as this triggers a very large and scary
> warning every time the encoder or decoder is used.

Just to clarify, it does actually cause BUG_ON() panics, so if the
original change is already present in a stable kernel, this fix should
be merged to stable as well.

Best,
Tomasz