drm_wait_vblank must be DRM_UNLOCKED because otherwise it will grab the drm_global_mutex and then go to sleep until the vblank event it is waiting for. That can wreck havoc in the windowing system because if one process issues this ioctl, it will block all other processes for the duration of all vblanks between the current and the one it is waiting for. In some cases it can block the entire windowing system.
v2: incorporate comments received from Daniel Vetter and Michel Daenzer.
v3: after a lengty discussion with Daniel Vetter, it was concluded we should not worry about any locking, within drm_wait_vblank function so this patch becomes a rather trivial removal of drm_global_mutex from drm_wait_vblank
Signed-off-by: Ilija Hadzic ihadzic@research.bell-labs.com --- drivers/gpu/drm/drm_drv.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c index dbabcb0..dc0eb0b 100644 --- a/drivers/gpu/drm/drm_drv.c +++ b/drivers/gpu/drm/drm_drv.c @@ -124,7 +124,7 @@ static struct drm_ioctl_desc drm_ioctls[] = { DRM_IOCTL_DEF(DRM_IOCTL_SG_ALLOC, drm_sg_alloc_ioctl, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY), DRM_IOCTL_DEF(DRM_IOCTL_SG_FREE, drm_sg_free, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
- DRM_IOCTL_DEF(DRM_IOCTL_WAIT_VBLANK, drm_wait_vblank, 0), + DRM_IOCTL_DEF(DRM_IOCTL_WAIT_VBLANK, drm_wait_vblank, DRM_UNLOCKED),
DRM_IOCTL_DEF(DRM_IOCTL_MODESET_CTL, drm_modeset_ctl, 0),
On Mon, Oct 31, 2011 at 01:10:21PM -0400, Ilija Hadzic wrote:
drm_wait_vblank must be DRM_UNLOCKED because otherwise it will grab the drm_global_mutex and then go to sleep until the vblank event it is waiting for. That can wreck havoc in the windowing system because if one process issues this ioctl, it will block all other processes for the duration of all vblanks between the current and the one it is waiting for. In some cases it can block the entire windowing system.
v2: incorporate comments received from Daniel Vetter and Michel Daenzer.
v3: after a lengty discussion with Daniel Vetter, it was concluded we should not worry about any locking, within drm_wait_vblank function so this patch becomes a rather trivial removal of drm_global_mutex from drm_wait_vblank
That commit message is a bit garbage. What about ...
"... it was concluded that the only think not yet protected with locks and atomic ops is the write to dev->last_vblank_wait. It's only used in a debug file in proc, and the current code already employs no correct locking: the proc file only takes dev->struct_mutex, whereas drm_wait_vblank implicitly took the drm_global_mutex. Given all this, it's not worth bothering to try to fix this at this time."
I think it's important to correctly document the conclusion of this discussion, because we've worried quite a bit about correct locking.
Yours, Daniel
OK, v4 coming up and your suggestion will be copied verbatim. Hopefully that does it.
This patch is probably going to become a record-holder in comment/code lines ratio ;-)
-- Ilija
On Mon, 31 Oct 2011, Daniel Vetter wrote:
On Mon, Oct 31, 2011 at 01:10:21PM -0400, Ilija Hadzic wrote:
drm_wait_vblank must be DRM_UNLOCKED because otherwise it will grab the drm_global_mutex and then go to sleep until the vblank event it is waiting for. That can wreck havoc in the windowing system because if one process issues this ioctl, it will block all other processes for the duration of all vblanks between the current and the one it is waiting for. In some cases it can block the entire windowing system.
v2: incorporate comments received from Daniel Vetter and Michel Daenzer.
v3: after a lengty discussion with Daniel Vetter, it was concluded we should not worry about any locking, within drm_wait_vblank function so this patch becomes a rather trivial removal of drm_global_mutex from drm_wait_vblank
That commit message is a bit garbage. What about ...
"... it was concluded that the only think not yet protected with locks and atomic ops is the write to dev->last_vblank_wait. It's only used in a debug file in proc, and the current code already employs no correct locking: the proc file only takes dev->struct_mutex, whereas drm_wait_vblank implicitly took the drm_global_mutex. Given all this, it's not worth bothering to try to fix this at this time."
I think it's important to correctly document the conclusion of this discussion, because we've worried quite a bit about correct locking.
Yours, Daniel
Daniel Vetter Mail: daniel@ffwll.ch Mobile: +41 (0)79 365 57 48
dri-devel@lists.freedesktop.org
-
Daniel Vetter -
Ilija Hadzic