On 4/9/21 3:40 AM, Daniel Vetter wrote:
On Thu, Apr 08, 2021 at 01:22:45PM -0400, Zack Rusin wrote:Quite often it's a little hard to tell if reservations are already held in code paths that unpin bo's. While our pinning/unpinning code should be more explicit that requires a substential amount of work so instead we can avoid the issues by making sure we try to reserve before unpinning. Because we unpin those bo's only on destruction/error paths just that check tells us if we're already reserved or not and allows to cleanly unpin. Reviewed-by: Martin Krastev <krastevm@xxxxxxxxxx> Reviewed-by: Roland Scheidegger <sroland@xxxxxxxxxx> Fixes: d1a73c641afd ("drm/vmwgfx: Make sure we unpin no longer needed buffers") Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx Signed-off-by: Zack Rusin <zackr@xxxxxxxxxx> --- drivers/gpu/drm/vmwgfx/vmwgfx_drv.h | 17 ++++++++++++++++- drivers/gpu/drm/vmwgfx/vmwgfx_mob.c | 8 ++++---- 2 files changed, 20 insertions(+), 5 deletions(-) diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h index 8087a9013455..03bef9c17e56 100644 --- a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h @@ -1517,6 +1517,21 @@ static inline struct vmw_surface *vmw_surface_reference(struct vmw_surface *srf) return srf; }+/*+ * vmw_bo_unpin_safe - currently pinning requires a reservation to be held + * but sometimes it's hard to tell if we're in a callback whose parent + * is already holding a reservation, to avoid deadloacks we have to try + * to get a reservation explicitly to also try to avoid messing up the + * internal ttm lru bo list + */ +static inline void vmw_bo_unpin_safe(struct ttm_buffer_object *bo) +{ + bool locked = dma_resv_trylock(bo->base.resv); + ttm_bo_unpin(bo); + if (locked) + dma_resv_unlock(bo->base.resv); +} + static inline void vmw_bo_unreference(struct vmw_buffer_object **buf) { struct vmw_buffer_object *tmp_buf = *buf; @@ -1524,7 +1539,7 @@ static inline void vmw_bo_unreference(struct vmw_buffer_object **buf) *buf = NULL; if (tmp_buf != NULL) { if (tmp_buf->base.pin_count > 0) - ttm_bo_unpin(&tmp_buf->base); + vmw_bo_unpin_safe(&tmp_buf->base);So in the unreference callback I understand it might be tricky and you need this, but do all the others below really don't know whether the bo is locked or not?
TBH, I just liked having all those paths going through the same functions. I agree that it wasn't really correct or particularly graceful.
Also _trylock is a bit much yolo locking here, I'd minimally put a comment there that we don't actually care about races, it's just to shut up ttm locking checks. Whether that's true or not is another question I think. And if it's just this case here, maybe inline the trylock, and for the others do a vmw_bo_unpin_unlocked which unconditionally grabs the lock?
Fair enough, I think that's a good suggestion, so I went ahead and did just that.
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