drm/i915: Flush the change in debugobject before reallocation

When marking the debugobject as freed, be sure that write is flushed
before another CPU may see it on a reallocation path.

Only seen once in CI:

[  159.240873] WARNING: CPU: 3 PID: 6735 at lib/debugobjects.c:263 debug_print_object+0x87/0xb0
[  159.240897] ODEBUG: init destroyed (active state 0) object type: i915_sw_fence hint: submit_notify+0x0/0x4c [i915]
[  159.240902] Modules linked in: snd_hda_intel i915 x86_pkg_temp_thermal intel_powerclamp coretemp crct10dif_pclmul snd_hda_codec_realtek crc32_pclmul snd_hda_codec_generic snd_hda_codec_hdmi ghash_clmulni_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm mei_me lpc_ich mei e1000e ptp pps_core [last unloaded: i915]
[  159.240913] CPU: 3 PID: 6735 Comm: gem_exec_nop Tainted: G     U          4.10.0-rc3-CI-Trybot_479+ #1
[  159.240913] Hardware name: LENOVO 10AGS00601/SHARKBAY, BIOS FBKT34AUS 04/24/2013
[  159.240914] Call Trace:
[  159.240916]  dump_stack+0x67/0x92
[  159.240919]  __warn+0xc6/0xe0
[  159.240920]  warn_slowpath_fmt+0x4a/0x50
[  159.240921]  debug_print_object+0x87/0xb0
[  159.240935]  ? __i915_request_wait_for_execute+0x1d0/0x1d0 [i915]
[  159.240936]  __debug_object_init+0xb2/0x410
[  159.240950]  ? __i915_request_wait_for_execute+0x1d0/0x1d0 [i915]
[  159.240951]  debug_object_init+0x16/0x20
[  159.240962]  __i915_sw_fence_init+0x29/0x60 [i915]
[  159.240975]  i915_gem_request_alloc+0x1fb/0x450 [i915]
[  159.240987]  i915_gem_do_execbuffer.isra.15+0x798/0x1b20 [i915]
[  159.241000]  i915_gem_execbuffer2+0xc0/0x250 [i915]
[  159.241003]  drm_ioctl+0x200/0x450
[  159.241016]  ? i915_gem_execbuffer+0x330/0x330 [i915]
[  159.241018]  do_vfs_ioctl+0x90/0x6e0
[  159.241020]  ? trace_hardirqs_on_caller+0x122/0x1b0
[  159.241021]  SyS_ioctl+0x3c/0x70
[  159.241023]  entry_SYSCALL_64_fastpath+0x1c/0xb1
[  159.241024] RIP: 0033:0x7f9bc4f41357
[  159.241025] RSP: 002b:00007ffc6cd5c568 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[  159.241026] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f9bc4f41357
[  159.241026] RDX: 00007ffc6cd5c640 RSI: 0000000040406469 RDI: 0000000000000003
[  159.241027] RBP: 00007ffc6cd5c640 R08: 0000000000047508 R09: 0000000000000001
[  159.241027] R10: 000b58552d323c3d R11: 0000000000000246 R12: 0000000040406469
[  159.241028] R13: 0000000000000003 R14: 0000000000000004 R15: 0000000000000001

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170113214335.5829-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_sw_fence.c

@@ -63,6 +63,7 @@ static inline void debug_fence_destroy(struct i915_sw_fence *fence)
 static inline void debug_fence_free(struct i915_sw_fence *fence)
 {
 	debug_object_free(fence, &i915_sw_fence_debug_descr);
+	smp_wmb(); /* flush the change in state before reallocation */
 }
 
 static inline void debug_fence_assert(struct i915_sw_fence *fence)