Regression found in loopback.c patch (0cf833a)

Regression found in loopback.c patch (0cf833a)

[Neil Salstrom]

Hello,

I believe I have found a regression in drivers/net/loopback.c which was
introduced starting with the v3.7 kernel series (and also affects
v3.8).  Oddly enough, it affects DVD playback (both physical disk and
.iso files) in MythTV (I'm using v0.26 and compile from source).  I
don't know if MythTV mounts a .iso or DVD over a loopback device but for
whatever reason there is a problem.

When playing a DVD there is a constant stuttering of the video and
corresponding audio dropouts.  This is continual with mythfrontend logs
showing "Waiting for video buffers."

I found that any kernel before v3.7 did not cause this issue so I did a
git bisect between v3.6 and v3.7.  The resulting bisection was to commit
0cf833a (net: loopback: set default mtu to 64K).

I downloaded the source to v3.7.9 and reverted the line:

dev->mtu = 64 * 1024;

back to:

dev->mtu = (16 * 1024) + 20 + 20 + 12;


The resulting kernel did not cause the playback stuttering.  I have also
compiled v3.8.0 using (16 * 1024) + 20 + 20 + 12; and again had no problems.

Please let me know if you have any questions.

Thank you,

Neil Salstrom

Re: Regression found in loopback.c patch (0cf833a)

[Eric Dumazet]

On Mon, 2013-02-25 at 22:30 -0800, Neil Salstrom wrote:
> Hello,
> 
> I believe I have found a regression in drivers/net/loopback.c which was
> introduced starting with the v3.7 kernel series (and also affects
> v3.8).  Oddly enough, it affects DVD playback (both physical disk and
> .iso files) in MythTV (I'm using v0.26 and compile from source).  I
> don't know if MythTV mounts a .iso or DVD over a loopback device but for
> whatever reason there is a problem.
> 
> When playing a DVD there is a constant stuttering of the video and
> corresponding audio dropouts.  This is continual with mythfrontend logs
> showing "Waiting for video buffers."
> 
> I found that any kernel before v3.7 did not cause this issue so I did a
> git bisect between v3.6 and v3.7.  The resulting bisection was to commit
> 0cf833a (net: loopback: set default mtu to 64K).
> 
> I downloaded the source to v3.7.9 and reverted the line:
> 
> dev->mtu = 64 * 1024;
> 
> back to:
> 
> dev->mtu = (16 * 1024) + 20 + 20 + 12;
> 
> 
> The resulting kernel did not cause the playback stuttering.  I have also
> compiled v3.8.0 using (16 * 1024) + 20 + 20 + 12; and again had no problems.
> 
> Please let me know if you have any questions.
> 
> Thank you,
> 
> Neil Salstrom
> 

Hmm, I suspect an application bug.

Try to find what particular mtu value makes the application going bad,
doing a dichotomy ?

No need to recompile the kernel :

ifconfig lo mtu XXXXX