Op 28-11-12 16:10, Thomas Hellstrom schreef:
> On 11/28/2012 03:46 PM, Maarten Lankhorst wrote:
>> Op 28-11-12 15:23, Thomas Hellstrom schreef:
>>> On 11/28/2012 02:55 PM, Maarten Lankhorst wrote:
>>>> Op 28-11-12 14:21, Thomas Hellstrom schreef:
>>>>> On 11/28/2012 01:15 PM, Maarten Lankhorst wrote:
>>>>>> Op 28-11-12 12:54, Thomas Hellstrom schreef:
>>>>>>> On 11/28/2012 12:25 PM, Maarten Lankhorst wrote:
>>>>>>>> By removing the unlocking of lru and retaking it immediately, a race is
>>>>>>>> removed where the bo is taken off the swap list or the lru list between
>>>>>>>> the unlock and relock. As such the cleanup_refs code can be simplified,
>>>>>>>> it will attempt to call ttm_bo_wait non-blockingly, and if it fails
>>>>>>>> it will drop the locks and perform a blocking wait, or return an error
>>>>>>>> if no_wait_gpu was set.
>>>>>>>>
>>>>>>>> The need for looping is also eliminated, since swapout and
>>>>>>>> evict_mem_first
>>>>>>>> will always follow the destruction path, so no new fence is allowed
>>>>>>>> to be attached. As far as I can see this may already have been the
>>>>>>>> case,
>>>>>>>> but the unlocking / relocking required a complicated loop to deal with
>>>>>>>> re-reservation.
>>>>>>>>
>>>>>>>> The downside is that ttm_bo_cleanup_memtype_use is no longer called
>>>>>>>> with
>>>>>>>> reservation held, so drivers must be aware that move_notify with a null
>>>>>>>> parameter doesn't require a reservation.
>>>>>>> Why can't we unreserve *after* ttm_bo_cleanup_memtype_use? That's not
>>>>>>> immediately clear from this patch.
>>>>>> Because we would hold the reservation while waiting and with the object
>>>>>> still
>>>>>> on swap and lru lists still, that would defeat the whole purpose of
>>>>>> keeping
>>>>>> the object on multiple lists, plus break current code that assumes bo's
>>>>>> on the
>>>>>> those lists can always be reserved.
>>>>>>
>>>>>> the if (ret && !no_wait_gpu) path has to drop the reservation and lru
>>>>>> lock, and
>>>>>> isn't guaranteed to be able to retake it. Maybe it could be guaranteed
>>>>>> now, but
>>>>>> I'm sure that would not be the case if the reservations were shared
>>>>>> across
>>>>>> devices.
>>>>> The evict path removes the BO from the LRU lists, drops the LRU lock but
>>>>> hangs on to the reservation,
>>>>> and in case the wait goes wrong, re-adds the bo to the LRU lists and
>>>>> returns an error.
>>>> If you really want to, we could hang on to the !no_wait_gpu path, wait
>>>> shouldn't ever fail there, so I suppose
>>>> leaving it off the lru lists and not re-add on any list in case of wait
>>>> fail is fine. It's still on the ddestroy list in that
>>>> case, so not adding it back to the other lists is harmless.
>>>>
>>> Well I'm a bit afraid that theoretically, other callers may have a bo
>>> reserved, while cleanup_refs_and_unlock
>>> more or less runs the whole destroy path on that buffer. Sure, we have
>>> control over those other reservers,
>>> but it may come back and bite us.
>> That's why initially I moved all the destruction to ttm_bo_release_list, to
>> have all destruction in
>> only 1 place. But even now it's serialized with the lru lock, while the
>> destruction may not happen
>> right away, it still happens before last list ref to the bo is dropped.
>>
>> But it's your call, just choose the approach you want and I'll resubmit
>> this. :-)
>>
>>> Also the wait might fail if a signal is hit, so it's definitely possible,
>>> and even likely in the case of the X server process.
>>>
>>> Anyway, I prefer if we could try to keep the reservation across the
>>> ttm_cleanup_memtype_use function, and as far
>>> as I can tell, the only thing preventing that is the reservation release in
>>> the (!no_wait_gpu) path. So if we alter that to
>>> do the same as the evict path I think without looking to deeply into the
>>> consequences that we should be safe.
>> I think returning success early without removing off ddestroy list if
>> re-reserving fails
>> with lru lock held would be better.
>>
>> We completed the wait and attempt to reserve the bo, which failed. Without
>> the lru
>> lock atomicity, this can't happen since the only places that would do it
>> call this with
>> the lru lock held.
>>
>> With the atomicity removal, the only place that could do this is
>> ttm_bo_delayed_delete
>> with remove_all set to true. And even if that happened the destruction code
>> would run
>> *anyway* since we completed the waiting part already, it would just not
>> necessarily be
>> run from this thread, but that guarantee didn't exist anyway.
>>> Then we should be able to skip patch 2 as well.
>> If my tryreserve approach sounds sane, second patch should still be
>> skippable. :-)
>
> Sure, Lets go for that approach.
Ok updated patch below, you only need to check if you like the approach or not,
since I haven't tested it yet beyond compiling. Rest of series (minus patch 2)
should still apply without modification.
drm/ttm: call ttm_bo_cleanup_refs with reservation and lru lock held, v2
By removing the unlocking of lru and retaking it immediately, a race is
removed where the bo is taken off the swap list or the lru list between
the unlock and relock. As such the cleanup_refs code can be simplified,
it will attempt to call ttm_bo_wait non-blockingly, and if it fails
it will drop the locks and perform a blocking wait, or return an error
if no_wait_gpu was set.
The need for looping is also eliminated, since swapout and evict_mem_first
will always follow the destruction path, no new fence is allowed
to be attached. As far as I can see this may already have been the case,
but the unlocking / relocking required a complicated loop to deal with
re-reservation.
Changes since v1:
- Simplify no_wait_gpu case by folding it in with empty ddestroy.
- Hold a reservation while calling ttm_bo_cleanup_memtype_use again.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst at canonical.com>
diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index 202fc20..e9f01fe 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -488,12 +488,16 @@ static void ttm_bo_cleanup_memtype_use(struct
ttm_buffer_object *bo)
ttm_bo_mem_put(bo, &bo->mem);
atomic_set(&bo->reserved, 0);
+ wake_up_all(&bo->event_queue);
/*
- * Make processes trying to reserve really pick it up.
+ * Since the final reference to this bo may not be dropped by
+ * the current task we have to put a memory barrier here to make
+ * sure the changes done in this function are always visible.
+ *
+ * This function only needs protection against the final kref_put.
*/
- smp_mb__after_atomic_dec();
- wake_up_all(&bo->event_queue);
+ smp_mb__before_atomic_dec();
}
static void ttm_bo_cleanup_refs_or_queue(struct ttm_buffer_object *bo)
@@ -543,68 +547,95 @@ static void ttm_bo_cleanup_refs_or_queue(struct
ttm_buffer_object *bo)
}
/**
- * function ttm_bo_cleanup_refs
+ * function ttm_bo_cleanup_refs_and_unlock
* If bo idle, remove from delayed- and lru lists, and unref.
* If not idle, do nothing.
*
+ * Must be called with lru_lock and reservation held, this function
+ * will drop both before returning.
+ *
* @interruptible Any sleeps should occur interruptibly.
- * @no_wait_reserve Never wait for reserve. Return -EBUSY instead.
* @no_wait_gpu Never wait for gpu. Return -EBUSY instead.
*/
-static int ttm_bo_cleanup_refs(struct ttm_buffer_object *bo,
- bool interruptible,
- bool no_wait_reserve,
- bool no_wait_gpu)
+static int ttm_bo_cleanup_refs_and_unlock(struct ttm_buffer_object *bo,
+ bool interruptible,
+ bool no_wait_gpu)
{
struct ttm_bo_device *bdev = bo->bdev;
+ struct ttm_bo_driver *driver = bdev->driver;
struct ttm_bo_global *glob = bo->glob;
int put_count;
- int ret = 0;
+ int ret;
-retry:
spin_lock(&bdev->fence_lock);
- ret = ttm_bo_wait(bo, false, interruptible, no_wait_gpu);
- spin_unlock(&bdev->fence_lock);
+ ret = ttm_bo_wait(bo, false, false, true);
- if (unlikely(ret != 0))
- return ret;
+ if (ret && !no_wait_gpu) {
+ void *sync_obj;
-retry_reserve:
- spin_lock(&glob->lru_lock);
+ /*
+ * Take a reference to the fence and unreserve,
+ * at this point the buffer should be dead, so
+ * no new sync objects can be attached.
+ */
+ sync_obj = driver->sync_obj_ref(&bo->sync_obj);
+ spin_unlock(&bdev->fence_lock);
- if (unlikely(list_empty(&bo->ddestroy))) {
+ put_count = ttm_bo_del_from_lru(bo);
+
+ atomic_set(&bo->reserved, 0);
+ wake_up_all(&bo->event_queue);
spin_unlock(&glob->lru_lock);
- return 0;
- }
- ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
+ ttm_bo_list_ref_sub(bo, put_count, true);
- if (unlikely(ret == -EBUSY)) {
- spin_unlock(&glob->lru_lock);
- if (likely(!no_wait_reserve))
- ret = ttm_bo_wait_unreserved(bo, interruptible);
- if (unlikely(ret != 0))
+ ret = driver->sync_obj_wait(sync_obj, false, interruptible);
+ driver->sync_obj_unref(&sync_obj);
+ if (ret) {
+ /*
+ * Either the wait returned -ERESTARTSYS, or -EDEADLK
+ * (radeon lockup) here. No effort is made to re-add
+ * this bo to any lru list. Instead the bo only
+ * appears on the delayed destroy list.
+ */
return ret;
+ }
- goto retry_reserve;
- }
+ /*
+ * remove sync_obj with ttm_bo_wait, the wait should be
+ * finished, and no new wait object should have been added.
+ */
+ spin_lock(&bdev->fence_lock);
+ ret = ttm_bo_wait(bo, false, false, true);
+ WARN_ON(ret);
+ spin_unlock(&bdev->fence_lock);
+ if (ret)
+ return ret;
- BUG_ON(ret != 0);
+ spin_lock(&glob->lru_lock);
+ ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
- /**
- * We can re-check for sync object without taking
- * the bo::lock since setting the sync object requires
- * also bo::reserved. A busy object at this point may
- * be caused by another thread recently starting an accelerated
- * eviction.
- */
+ /*
+ * We raced, and lost, someone else holds the reservation now,
+ * and is probably busy in ttm_bo_cleanup_memtype_use.
+ *
+ * Even if it's not the case, because we finished waiting any
+ * delayed destruction would succeed, so just return success
+ * here.
+ */
+ if (ret) {
+ spin_unlock(&glob->lru_lock);
+ return 0;
+ }
+ } else
+ spin_unlock(&bdev->fence_lock);
- if (unlikely(bo->sync_obj)) {
+ if (ret || unlikely(list_empty(&bo->ddestroy))) {
atomic_set(&bo->reserved, 0);
wake_up_all(&bo->event_queue);
spin_unlock(&glob->lru_lock);
- goto retry;
+ return ret;
}
put_count = ttm_bo_del_from_lru(bo);
@@ -647,9 +678,13 @@ static int ttm_bo_delayed_delete(struct ttm_bo_device
*bdev, bool remove_all)
kref_get(&nentry->list_kref);
}
- spin_unlock(&glob->lru_lock);
- ret = ttm_bo_cleanup_refs(entry, false, !remove_all,
- !remove_all);
+ ret = ttm_bo_reserve_locked(entry, false, !remove_all, false,
0);
+ if (!ret)
+ ret = ttm_bo_cleanup_refs_and_unlock(entry, false,
+ !remove_all);
+ else
+ spin_unlock(&glob->lru_lock);
+
kref_put(&entry->list_kref, ttm_bo_release_list);
entry = nentry;
@@ -803,9 +838,13 @@ retry:
kref_get(&bo->list_kref);
if (!list_empty(&bo->ddestroy)) {
- spin_unlock(&glob->lru_lock);
- ret = ttm_bo_cleanup_refs(bo, interruptible,
- no_wait_reserve, no_wait_gpu);
+ ret = ttm_bo_reserve_locked(bo, interruptible, no_wait_reserve,
false, 0);
+ if (!ret)
+ ret = ttm_bo_cleanup_refs_and_unlock(bo, interruptible,
+ no_wait_gpu);
+ else
+ spin_unlock(&glob->lru_lock);
+
kref_put(&bo->list_kref, ttm_bo_release_list);
return ret;
@@ -1799,8 +1838,9 @@ static int ttm_bo_swapout(struct ttm_mem_shrink *shrink)
kref_get(&bo->list_kref);
if (!list_empty(&bo->ddestroy)) {
- spin_unlock(&glob->lru_lock);
- (void) ttm_bo_cleanup_refs(bo, false, false, false);
+ ttm_bo_reserve_locked(bo, false, false, false, 0);
+ ttm_bo_cleanup_refs_and_unlock(bo, false, false);
+
kref_put(&bo->list_kref, ttm_bo_release_list);
spin_lock(&glob->lru_lock);
continue;
