drm/nouveau/gem: fix fence_sync race / oops

drm/nouveau/gem: fix fence_sync race / oops

Due to a race it was possible for a fence to be destroyed while another
thread was trying to synchronise with it.  If this happened in the fallback
non-semaphore path, it lead to the following oops due to fence->channel
being NULL.

BUG: unable to handle kernel NULL pointer dereference at   (null)
IP: [<fa9632ce>] nouveau_fence_update+0xe/0xe0 [nouveau]
*pde = a649c067
SMP
Modules linked in: fuse nouveau(O) ttm(O) drm_kms_helper(O) drm(O) mxm_wmi video wmi netconsole configfs lockd bnep bluetooth rfkill ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack ip6table_filter ip6_tables snd_hda_codec_realtek snd_hda_intel snd_hda_cobinfmt_misc uinput ata_generic pata_acpi pata_aet2c_algo_bit i2c_core [last unloaded: wmi]

Pid: 2255, comm: gnome-shell Tainted: G           O 3.2.0-0.rc5.git0.1.fc17.i686 #1 System manufacturer System Product Name/M2A-VM
EIP: 0060:[<fa9632ce>] EFLAGS: 00010296 CPU: 1
EIP is at nouveau_fence_update+0xe/0xe0 [nouveau]
EAX: 00000000 EBX: ddfc6dd0 ECX: dd111580 EDX: 00000000
ESI: 00003e80 EDI: dd111580 EBP: dd121d00 ESP: dd121ce8
 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
Process gnome-shell (pid: 2255, ti=dd120000 task=dd111580 task.ti=dd120000)
Stack:
 7dc86c76 00000000 00003e80 ddfc6dd0 00003e80 dd111580 dd121d0c fa96371f
 00000000 dd121d3c fa963773 dd111580 01000246 000ec53d 00000000 ddfc6dd0
 00001f40 00000000 ddfc6dd0 00000010 dc7df840 dd121d6c fa9639a0 00000000
Call Trace:
 [<fa96371f>] __nouveau_fence_signalled+0x1f/0x30 [nouveau]
 [<fa963773>] __nouveau_fence_wait+0x43/0xd0 [nouveau]
 [<fa9639a0>] nouveau_fence_sync+0x1a0/0x1c0 [nouveau]
 [<fa964046>] validate_list+0x176/0x300 [nouveau]
 [<f7d9c9c0>] ? ttm_bo_mem_put+0x30/0x30 [ttm]
 [<fa964b8a>] nouveau_gem_ioctl_pushbuf+0x48a/0xfd0 [nouveau]
 [<c0406481>] ? die+0x31/0x80
 [<f7c93d98>] drm_ioctl+0x388/0x490 [drm]
 [<c0406481>] ? die+0x31/0x80
 [<fa964700>] ? nouveau_gem_ioctl_new+0x150/0x150 [nouveau]
 [<c0635c7b>] ? file_has_perm+0xcb/0xe0
 [<f7c93a10>] ? drm_copy_field+0x80/0x80 [drm]
 [<c0564f56>] do_vfs_ioctl+0x86/0x5b0
 [<c0406481>] ? die+0x31/0x80
 [<c0635f22>] ? selinux_file_ioctl+0x62/0x130
 [<c0554f30>] ? fget_light+0x30/0x340
 [<c05654ef>] sys_ioctl+0x6f/0x80
 [<c099e3a4>] syscall_call+0x7/0xb
 [<c0406481>] ? die+0x31/0x80
 [<c0406481>] ? die+0x31/0x80

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org