linux-kernel - Re: [PATCH] media: dvb_ringbuffer: Add memory barriers

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Date:	Sat, 7 May 2016 10:26:06 -0300
From:	Mauro Carvalho Chehab <mchehab@....samsung.com>
To:	Soeren Moch <smoch@....de>
Cc:	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	Antti Palosaari <crope@....fi>
Subject: Re: [PATCH] media: dvb_ringbuffer: Add memory barriers

Em Sat, 7 May 2016 10:22:35 -0300
Mauro Carvalho Chehab <mchehab@....samsung.com> escreveu:

> Hi Soeren,
> 
> Em Sun, 7 Feb 2016 20:22:36 +0100
> Soeren Moch <smoch@....de> escreveu:
> 
> > On 27.12.2015 21:41, Soeren Moch wrote:
> > > Implement memory barriers according to Documentation/circular-buffers.txt:
> > > - use smp_store_release() to update ringbuffer read/write pointers
> > > - use smp_load_acquire() to load write pointer on reader side
> > > - use ACCESS_ONCE() to load read pointer on writer side
> > >
> > > This fixes data stream corruptions observed e.g. on an ARM Cortex-A9
> > > quad core system with different types (PCI, USB) of DVB tuners.
> > >
> > > Signed-off-by: Soeren Moch <smoch@....de>
> > > Cc: stable@...r.kernel.org # 3.14+  
> > 
> > Mauro,
> > 
> > any news or comments on this?
> > Since this is a real fix for broken behaviour, can you pick this up, please?
> 
> The problem here is that I'm very reluctant to touch at the DVB core
> without doing some tests myself, as things like locking can be
> very sensible.

In addition, it is good if other DVB developers could also test it.
Even being sent for some time, until now, nobody else tested it.

> 
> I'll try to find some time to take a look on it for Kernel 4.8,
> but I'd like to reproduce the bug locally.
> 
> Could you please provide me enough info to reproduce it (and
> eventually some test MPEG-TS where you know this would happen)?
> 
> I have two DekTek RF generators here, so I should be able to
> play such TS and see what happens with and without the patch
> on x86, arm32 and arm64.

Ah,  forgot to mention, but checkpatch.pl wants comments for the memory
barriers:

WARNING: memory barrier without comment
#52: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:58:
+	return (rbuf->pread == smp_load_acquire(&rbuf->pwrite));

WARNING: memory barrier without comment
#70: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:79:
+	avail = smp_load_acquire(&rbuf->pwrite) - rbuf->pread;

WARNING: memory barrier without comment
#79: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:89:
+	smp_store_release(&rbuf->pread, smp_load_acquire(&rbuf->pwrite));

WARNING: memory barrier without comment
#87: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:96:
+	smp_store_release(&rbuf->pread, 0);

WARNING: memory barrier without comment
#88: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:97:
+	smp_store_release(&rbuf->pwrite, 0);

WARNING: memory barrier without comment
#97: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:123:
+		smp_store_release(&rbuf->pread, 0);

WARNING: memory barrier without comment
#103: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:128:
+	smp_store_release(&rbuf->pread, (rbuf->pread + todo) % rbuf->size);

WARNING: memory barrier without comment
#112: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:143:
+		smp_store_release(&rbuf->pread, 0);

WARNING: memory barrier without comment
#117: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:147:
+	smp_store_release(&rbuf->pread, (rbuf->pread + todo) % rbuf->size);

WARNING: memory barrier without comment
#126: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:162:
+		smp_store_release(&rbuf->pwrite, 0);

WARNING: memory barrier without comment
#130: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:165:
+	smp_store_release(&rbuf->pwrite, (rbuf->pwrite + todo) % rbuf->size);

WARNING: memory barrier without comment
#139: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:185:
+		smp_store_release(&rbuf->pwrite, 0);

WARNING: memory barrier without comment
#145: FILE: drivers/media/dvb-core/dvb_ringbuffer.c:190:
+	smp_store_release(&rbuf->pwrite, (rbuf->pwrite + todo) % rbuf->size);

Thanks,
Mauro