[PATCH] xfree86: detach scanout pixmaps when detaching output GPUs

[Dave Airlie]

On Thu, May 2, 2013 at 5:55 AM, Aaron Plattner <aplattner at nvidia.com> wrote:
> On 04/30/2013 11:30 PM, Dave Airlie wrote:
>> On Wed, May 1, 2013 at 7:14 AM, Aaron Plattner <aplattner at nvidia.com> wrote:
>>> Commit 8f4640bdb9d3988148e09a08d2c7e3bab1d538d6 fixed a bit of a
>>> chicken-and-egg problem by detaching GPU screens when their providers
>>> are destroyed, which happens before CloseScreen is called.  However,
>>> this created a new problem: the GPU screen tears down its RandR crtc
>>> objects during CloseScreen and if one of them is active, it tries to
>>> detach the scanout pixmap then.  This crashes because
>>> RRCrtcDetachScanoutPixmap tries to get the master screen's screen
>>> pixmap, but crtc->pScreen->current_master is already NULL at that
>>> point.
>>>
>>> It doesn't make sense for an unbound GPU screen to still be scanning
>>> out its former master screen's pixmap, so detach them first when the
>>> provider is destroyed.
>>
>> Any instructions on how to reproduce this? I've been trying to get it
>> to happen here.
>
> 1. Start X on an Intel system with a DisplayLink device attached.
> 2. xrandr --setprovideroutputsource 1 0
> 3. xrandr --output DVI-0 --off (unless you apply "xfree86: don't enable anything in xf86InitialConfiguration for GPU screens")
> 4. xrandr --auto
> 5. Terminate the X server.
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x8010be62 in RRCrtcDetachScanoutPixmap (crtc=crtc at entry=0x80259740) at ../../randr/rrcrtc.c:370
> 370     ../../randr/rrcrtc.c: No such file or directory.
> (gdb) bt
> #0  0x8010be62 in RRCrtcDetachScanoutPixmap (crtc=crtc at entry=0x80259740) at ../../randr/rrcrtc.c:370
> #1  0x8010bfab in RRCrtcDestroyResource (value=0x80259740, pid=63) at ../../randr/rrcrtc.c:678
> #2  0x80062fbe in doFreeResource (res=0x8025bc58, skip=skip at entry=0) at ../../dix/resource.c:873
> #3  0x8006415b in FreeClientResources (client=0x8023f350) at ../../dix/resource.c:1139
> #4  0x800641fc in FreeAllResources () at ../../dix/resource.c:1154
> #5  0x8002b556 in main (argc=3, argv=0xbffff704, envp=0xbffff714) at ../../dix/main.c:321
> (gdb) p crtc->pScreen->current_master
> $1 = (ScreenPtr) 0x0
> (gdb) p crtc->pScreen->myNum
> $2 = 256
>
>> I did notice that xrandr is now broken for detach? how are you detaching btw?
>> xrandr --setoutputprovidersource 0x67 0x0 should detach I would think,
>> but it fails now.
>
> Yeah, I noticed that too, and hacked it up to allow me to test it.  I'll try to clean up the change for review.
>
> The crash doesn't occur at detach time or if I pull the USB cable.

I'm having absolutely no luck reproducing this with the X server
master + my for-keithp tree pulled in.

I'm not sure what I could be doing different than you are, I've even
valgrinded it.

Dave.