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Message-ID: <30e44b19731c4821fdf325689482671f90389100.camel@collabora.com>
Subject: Re: [PATCH] media: v4l2-mem2mem: fix mem order in last buf
From: Nicolas Dufresne <nicolas.dufresne@collabora.com>
To: Randy Li <ayaka@soulik.info>
Cc: mchehab@kernel.org, hverkuil-cisco@xs4all.nl, Hsia-Jun Li
 <randy.li@synaptics.com>, sebastian.fricke@collabora.com,
 alexious@zju.edu.cn,  linux-media@vger.kernel.org,
 linux-kernel@vger.kernel.org
Date: Fri, 16 Feb 2024 13:56:46 -0500
In-Reply-To: <1f80b5ea-1209-438f-b07f-3a4a308ee35d@soulik.info>
References: <20240210180414.49184-1-randy.li@synaptics.com>
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Le jeudi 15 f=C3=A9vrier 2024 =C3=A0 11:16 +0800, Randy Li a =C3=A9crit=C2=
=A0:
> On 2024/2/15 04:38, Nicolas Dufresne wrote:
> > Hi,
> >=20
> > >   media: v4l2-mem2mem: fix mem order in last buf
> > mem order ? Did you mean call order ?
> std::memory_order
> >=20
> > Le dimanche 11 f=C3=A9vrier 2024 =C3=A0 02:04 +0800, Hsia-Jun Li a =C3=
=A9crit=C2=A0:
> > > From: "Hsia-Jun(Randy) Li" <randy.li@synaptics.com>
> > >=20
> > > The has_stopped property in struct v4l2_m2m_ctx is operated
> > > without a lock protecction. Then the userspace calls to
> >                   protection   When ?                   ~~
> Access to those 3 booleans you mentioned later.

There were commenting you commit message typos, not a question.

> > > v4l2_m2m_encoder_cmd()/v4l2_m2m_decoder_cmd() may lead to
> > > a critical section issue.
> > As there is no locking, there is no critical section, perhaps a better =
phrasing
> > could help.
>=20
> "concurrent accesses to shared resources can lead to unexpected or=20
> erroneous behavior, so parts of the program where the shared resource is=
=20
> accessed need to be protected in ways that avoid the concurrent access."
>=20
> It didn't say we need a lock here.

I said it.

>=20
> > > Signed-off-by: Hsia-Jun(Randy) Li <randy.li@synaptics.com>
> > > ---
> > >   drivers/media/v4l2-core/v4l2-mem2mem.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > >=20
> > > diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v=
4l2-core/v4l2-mem2mem.c
> > > index 75517134a5e9..f1de71031e02 100644
> > > --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> > > +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> > > @@ -635,9 +635,9 @@ void v4l2_m2m_last_buffer_done(struct v4l2_m2m_ct=
x *m2m_ctx,
> > >   			       struct vb2_v4l2_buffer *vbuf)
> > >   {
> > >   	vbuf->flags |=3D V4L2_BUF_FLAG_LAST;
> > > -	vb2_buffer_done(&vbuf->vb2_buf, VB2_BUF_STATE_DONE);
> > > -
> > >   	v4l2_m2m_mark_stopped(m2m_ctx);
> > > +
> > > +	vb2_buffer_done(&vbuf->vb2_buf, VB2_BUF_STATE_DONE);
> > While it most likely fix the issue while testing, since userspace most =
likely
> > polls on that queue and don't touch the driver until the poll was signa=
lled, I
> > strongly believe this is insufficient. When I look at vicodec and wave5=
, they
> > both add a layer of locking on top of the mem2mem framework to fix this=
 issue.
>=20
> Maybe a memory barrier is enough. Since vb2_buffer_done() itself would=
=20
> invoke the (spin)lock operation.
>=20
> When the poll() returns in userspace, the future operation for those=20
> three boolean variables won't happen before the lock.
>=20
> > I think this is unfortunate, but v4l2_m2m_mark_stopped() is backed by 3=
 booleans
> > accessed in many places that aren't in any known atomic context. I thin=
k it
> > would be nice to remove the spurious locking in drivers and try and fix=
 this
> > issue in the framework itself.
> I tend to not introduce more locks here. There is a spinlock in m2m_ctx=
=20
> which is a pain in the ass, something we could reuse it to save our CPU=
=20
> but it just can't be access.

If you can find a way with memory barrier, but that is difficult to maintai=
n and
often breaks without noticing. I'm happy to review something that introduce
thread safety rather then depending on userspace call order. Can't disagree=
 with
the spinlock, its been difficult in Wave5 and there is still a bug report o=
f one
more case were we get that spinlock mixed with mutex.

Nicolas

> >=20
> > Nicolas
> >=20
> > >   }
> > >   EXPORT_SYMBOL_GPL(v4l2_m2m_last_buffer_done);
> > >  =20
>=20