On 3/16/21 12:06 PM, Daniel Vetter wrote:
On Tue, Mar 16, 2021 at 11:38:53AM +0100, Thomas Hellström (Intel) wrote:
Hi,
On 3/16/21 10:27 AM, Daniel Vetter wrote:
On Mon, Mar 15, 2021 at 08:00:30PM +0100, Thomas Hellström (Intel) wrote:
On 3/15/21 7:47 PM, Christian König wrote:
Am 15.03.21 um 18:08 schrieb Thomas Hellström (Intel):
On 3/15/21 11:26 AM, Christian König wrote:
Am 13.03.21 um 19:29 schrieb Thomas Hellström (Intel):
Hi, Christian
On 3/12/21 10:38 AM, Christian König wrote:
We seem to have some more driver bugs than thought.
Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
include/drm/ttm/ttm_bo_api.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/include/drm/ttm/ttm_bo_api.h
b/include/drm/ttm/ttm_bo_api.h
index 4fb523dfab32..df9fe596e7c5 100644
--- a/include/drm/ttm/ttm_bo_api.h
+++ b/include/drm/ttm/ttm_bo_api.h
@@ -603,9 +603,11 @@ static inline void
ttm_bo_pin(struct ttm_buffer_object *bo)
static inline void ttm_bo_unpin(struct ttm_buffer_object *bo)
{
dma_resv_assert_held(bo->base.resv);
- WARN_ON_ONCE(!bo->pin_count);
WARN_ON_ONCE(!kref_read(&bo->kref));
- --bo->pin_count;
+ if (bo->pin_count)
+ --bo->pin_count;
+ else
+ WARN_ON_ONCE(true);
}
int ttm_mem_evict_first(struct ttm_device *bdev,
Since I now have been staring for half a year at the code of
the driver that made pinning an art, I have a couple of
suggestions here, Could we use an atomic for pin_count,
allowing unlocked unpinning but require the lock only for
pin_count transition 0->1, (but unlocked for
pin_if_already_pinned). In particular I think vmwgfx would
benefit from unlocked unpins. Also if the atomic were a
refcount_t, that would probably give you the above
behaviour?
Nope, I've considered this as well while moving the pin count into TTM.
The problem is that you not only need the BO reserved for 0->1
transitions, but also for 1->0 transitions to move the BO on the
LRU correctly.
Ah, and there is no way to have us know the correct LRU list without
reservation?
Not really, there is always the chance that CPU A is reducing the count
from 1->0 while CPU B is doing 0->1 and you end up with a LRU status
which doesn't match the pin count.
We could try to do something like a loop updating the LRU status until
it matches the pin count, but the implications of that are usually
really nasty.
OK, yeah I was more thinking along the lines of protecting the LRU status
with the global lru lock and unpin would then be
if (refcount_dec_and_lock(&bo->pin_count, &ttm_glob.lru_lock)) {
add_to_relevant_lrus(bo, bo->lru_status);
spin_unlock(&ttm_glob.lru_lock);
}
But looking at ttm_bo_move_to_lru_tail() I realize that's not really trivial
anymore. I hope it's doable at some point though.
But meanwhile, perhaps TTM needs to accept and pave over that drivers are in
fact destroying pinned buffers?
Do we have more trouble than the very fancy tricks vmwgfx does? If so I
think we could do a small helper that like ttm_dont_check_unpin() to shut
it up. Since vmwgfx drivers tend to not be loaded with any other drivers
that shouldn't reduce any test coverage. And allows vmwgfx folks to figure
out what to do - maybe you do need your own in-house pin/unpin for these
special bo?
I did try to parse your reply in the other thread, and tbh I didn't grock
it.
Not sure if you mean the description was unclear or if you thought it was a
bad idea, but in case the former, what I mean is
My unclarity is on what you explained in the vmwgfx thread about how
vmwgfx uses its pin/unpin and why. That was full of vmwgfx concepts I
don't know about. This here looks reasonably clear to me, but it does have
the race Christian sees I think.
Hmm, OK, I thought transition 0->1 under the LRU lock would have
resolved that...
static void ttm_bo_pin(struct ttm_buffer_object *bo)
{
dma_resv_assert_held() // No surprises if an evictor
determined that this object is not pinned.
if (!refcount_inc_not_zero(&bo->pin_count)) { // Could be made
ttm_bo_pin_if_pinned() and exported if there are users
spin_lock(&ttm_glob.lru_lock); // Don't race with unpin
1->0
if (refcount_read(&bo->pin_count) == 0 && bo->lru)
ttm_bo_del_from_lru(bo);
refcount_inc(&bo->pin_count);
spin_unlock(&ttm_glob.lru_lock);
}
}
static void ttm_bo_unpin(struct ttm_buffer_object *bo)
{
if (refcount_dec_and_lock(&bo->pin_count, &ttm_glob.lru_lock)) {
ttm_bo_move_to_lru_tail(bo, bo->lru_mem_type, bo->lru_prio,
NULL);
spin_unlock(&ttm_glob.lru_lock);
}
}
In addition, bo->lru_mem_type and bo->lru_prio would need to be protected by
the lru lock, and updated together with the LRU list position, which would
be the extra complexity in fastpaths. Wouldn't that resolve the pin - lru
inconsistency?
But yeah if vmwgfx is the only driver hitting trouble because of this, then
I agree let's leave it for when / if it becomes needed. Having had that
requirement in the Intel driver would have complicated the dma_resv work
quite a lot.
Yeah I know i915 does a lot of unpin in interesting places, and that's
part of why I'm worried. I've seen some bugfixes fly by that dropped
dma_resv_lock around unpin to fix really scary looking stalls
(rcu_synchronize vs other stuff and lockdep did not catch it). And once I
see that kind of stuff I'm heavily leaning towards simpler locking we can
grasp, just to be able to stay on top of the bugs. Because the bugfix did
not come with any clear explanation for why not taking dma_resv_lock
actually helps, or any other clear and in-depth locking analysis.
I think we also still have some temporary pin/unpin flying about, but
maybe those are all gone now.
No, unfortunately they aren't and yes, I now see your point. And yes, if
we can reduce the pinning to only display kinds of things this wouldn't
really matter. But if not, I'm afraid the alternative is that drivers
will have to start resorting to assuming perhaps incorrectly that an
object is isolated (only a single user), and that therefore tryreserve
for unpinning will always succeed, which is fragile at best. (This is
what vmwgfx does for the pagetables). And pinning pagetables in that
case makes sense since it reduces the number of buffers to validate and
fence by 50%...
/Thomas
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