panic: locking xyz against myself (linux DRM?!)

[Timo Buhrmester <fstd.lkml%gmail.com@localhost>]

For a while now, I'm getting the occasional panic which I can't directly reproduce, but it seems to correlate with long and/or memory- and/or video-intense firefox sessions.

This is a recent -current (7.99.20 on amd64, although the problem exists as of at least 7.99.10, likely earlier too), my (onboard) video chip is
> radeon0 at pci1 dev 5 function 0: vendor 1002 product 9614 (rev. 0x00)
all with
> radeondrmkmsfb0 at radeon0
> radeondrmkmsfb0: framebuffer at 0xffff800046148000, size 1280x1024, depth 32, stride 5120

Besides that, I'm using Xorg from pkgsrc-2015Q2 (i.e. X11_TYPE=modular)

Now, whenever the system is up for a few days, and I didn't think of restarting firefox for a while, it eventually crashes with:
> panic: kernel diagnostic assertion "((mutex->wwm_state != WW_OWNED) || (mutex->wwm_u.owner != curlwp))" failed: file "/usr/src/sys/external/bsd/drm2/linux/linux_ww_mutex.c", line 760 locking 0xfffffe804fc70220 against myself: 0xfffffe811c5b2840
> fatal breakpoint trap in supervisor mode
> trap type 1 code 0 rip ffffffff80193685 cs 8 rflags 246 cr2 7f7ff7e43000 ilevel 8 rsp fffffe8041791ac0
> curlwp 0xfffffe811c5b2840 pid 1237.1 lowest kstack 0xfffffe804178e2c0
> Stopped in pid 1237.1 (Xorg) at netbsd:breakpoint+0x5:  leave
> breakpoint() at netbsd:breakpoint+0x5
> vpanic() at netbsd:vpanic+0x13c
> kern_assert() at netbsd:kern_assert+0x4f
> linux_ww_mutex_trylock() at netbsd:linux_ww_mutex_trylock+0xc1
> ttm_bo_uvm_fault() at netbsd:ttm_bo_uvm_fault+0x69
> radeon_ttm_fault() at netbsd:radeon_ttm_fault+0x6a
> uvm_fault_internal() at netbsd:uvm_fault_internal+0x828
> trap() at netbsd:trap+0x32a
> --- trap (number 6) ---
> 7f7ff002e4ed:
> db{1}>

Any ideas?  Does anyone else have the same problem?

I can extract information from the crash dump if required.