[PATCH 1/6] drm/ttm: remove lru_lock around ttm_bo_reserve
Jerome Glisse
j.glisse at gmail.com
Wed Dec 5 17:19:02 PST 2012
On Fri, Nov 30, 2012 at 7:12 AM, Maarten Lankhorst
<m.b.lankhorst at gmail.com> wrote:
> There should no longer be assumptions that reserve will always succeed
> with the lru lock held, so we can safely break the whole atomic
> reserve/lru thing. As a bonus this fixes most lockdep annotations for
> reservations.
>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at canonical.com>
> ---
> drivers/gpu/drm/ttm/ttm_bo.c | 54 ++++++++++++++++++++++------------
> drivers/gpu/drm/ttm/ttm_execbuf_util.c | 2 +-
> include/drm/ttm/ttm_bo_driver.h | 19 +++---------
> 3 files changed, 40 insertions(+), 35 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> index 9028327..61b5cd0 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -213,14 +213,13 @@ int ttm_bo_del_from_lru(struct ttm_buffer_object *bo)
> return put_count;
> }
>
> -int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
> +int ttm_bo_reserve_nolru(struct ttm_buffer_object *bo,
> bool interruptible,
> bool no_wait, bool use_sequence, uint32_t sequence)
> {
> - struct ttm_bo_global *glob = bo->glob;
> int ret;
>
> - while (unlikely(atomic_read(&bo->reserved) != 0)) {
> + while (unlikely(atomic_xchg(&bo->reserved, 1) != 0)) {
> /**
> * Deadlock avoidance for multi-bo reserving.
> */
Regarding memory barrier discussion we could add a smp_rdmb here or
few line below before the read of sequence for -EAGAIN. But it's not
really important, if a CPU doesn't see new sequence value the worst
case is that the CPU will go to wait again before reaching back this
section and returning EAGAIN. So it's just gonna waste some CPU cycle
i can't see anything bad that could happen.
> @@ -241,26 +240,36 @@ int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
> if (no_wait)
> return -EBUSY;
>
> - spin_unlock(&glob->lru_lock);
> ret = ttm_bo_wait_unreserved(bo, interruptible);
> - spin_lock(&glob->lru_lock);
>
> if (unlikely(ret))
> return ret;
> }
>
> - atomic_set(&bo->reserved, 1);
> if (use_sequence) {
> + bool wake_up = false;
> /**
> * Wake up waiters that may need to recheck for deadlock,
> * if we decreased the sequence number.
> */
> if (unlikely((bo->val_seq - sequence < (1 << 31))
> || !bo->seq_valid))
> - wake_up_all(&bo->event_queue);
> + wake_up = true;
>
> + /*
> + * In the worst case with memory ordering these values can be
> + * seen in the wrong order. However since we call wake_up_all
> + * in that case, this will hopefully not pose a problem,
> + * and the worst case would only cause someone to accidentally
> + * hit -EAGAIN in ttm_bo_reserve when they see old value of
> + * val_seq. However this would only happen if seq_valid was
> + * written before val_seq was, and just means some slightly
> + * increased cpu usage
> + */
> bo->val_seq = sequence;
> bo->seq_valid = true;
If we want we could add smp_wrmb here but see above comment on
usefullness of this.
> + if (wake_up)
> + wake_up_all(&bo->event_queue);
> } else {
> bo->seq_valid = false;
> }
> @@ -289,14 +298,14 @@ int ttm_bo_reserve(struct ttm_buffer_object *bo,
> int put_count = 0;
> int ret;
>
> - spin_lock(&glob->lru_lock);
> - ret = ttm_bo_reserve_locked(bo, interruptible, no_wait, use_sequence,
> - sequence);
> - if (likely(ret == 0))
> + ret = ttm_bo_reserve_nolru(bo, interruptible, no_wait, use_sequence,
> + sequence);
> + if (likely(ret == 0)) {
> + spin_lock(&glob->lru_lock);
> put_count = ttm_bo_del_from_lru(bo);
> - spin_unlock(&glob->lru_lock);
> -
> - ttm_bo_list_ref_sub(bo, put_count, true);
> + spin_unlock(&glob->lru_lock);
> + ttm_bo_list_ref_sub(bo, put_count, true);
> + }
>
> return ret;
> }
> @@ -510,7 +519,7 @@ static void ttm_bo_cleanup_refs_or_queue(struct ttm_buffer_object *bo)
> int ret;
>
> spin_lock(&glob->lru_lock);
> - ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
> + ret = ttm_bo_reserve_nolru(bo, false, true, false, 0);
>
> spin_lock(&bdev->fence_lock);
> (void) ttm_bo_wait(bo, false, false, true);
> @@ -603,7 +612,7 @@ static int ttm_bo_cleanup_refs_and_unlock(struct ttm_buffer_object *bo,
> return ret;
>
> spin_lock(&glob->lru_lock);
> - ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
> + ret = ttm_bo_reserve_nolru(bo, false, true, false, 0);
>
> /*
> * We raced, and lost, someone else holds the reservation now,
> @@ -667,7 +676,14 @@ static int ttm_bo_delayed_delete(struct ttm_bo_device *bdev, bool remove_all)
> kref_get(&nentry->list_kref);
> }
>
> - ret = ttm_bo_reserve_locked(entry, false, !remove_all, false, 0);
> + ret = ttm_bo_reserve_nolru(entry, false, true, false, 0);
> + if (remove_all && ret) {
> + spin_unlock(&glob->lru_lock);
> + ret = ttm_bo_reserve_nolru(entry, false, false,
> + false, 0);
> + spin_lock(&glob->lru_lock);
> + }
> +
> if (!ret)
> ret = ttm_bo_cleanup_refs_and_unlock(entry, false,
> !remove_all);
> @@ -818,7 +834,7 @@ static int ttm_mem_evict_first(struct ttm_bo_device *bdev,
>
> spin_lock(&glob->lru_lock);
> list_for_each_entry(bo, &man->lru, lru) {
> - ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
> + ret = ttm_bo_reserve_nolru(bo, false, true, false, 0);
> if (!ret)
> break;
> }
> @@ -1799,7 +1815,7 @@ static int ttm_bo_swapout(struct ttm_mem_shrink *shrink)
>
> spin_lock(&glob->lru_lock);
> list_for_each_entry(bo, &glob->swap_lru, swap) {
> - ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
> + ret = ttm_bo_reserve_nolru(bo, false, true, false, 0);
> if (!ret)
> break;
> }
> diff --git a/drivers/gpu/drm/ttm/ttm_execbuf_util.c b/drivers/gpu/drm/ttm/ttm_execbuf_util.c
> index cd9e452..bd37b5c 100644
> --- a/drivers/gpu/drm/ttm/ttm_execbuf_util.c
> +++ b/drivers/gpu/drm/ttm/ttm_execbuf_util.c
> @@ -153,7 +153,7 @@ retry:
> struct ttm_buffer_object *bo = entry->bo;
>
> retry_this_bo:
> - ret = ttm_bo_reserve_locked(bo, true, true, true, val_seq);
> + ret = ttm_bo_reserve_nolru(bo, true, true, true, val_seq);
> switch (ret) {
> case 0:
> break;
> diff --git a/include/drm/ttm/ttm_bo_driver.h b/include/drm/ttm/ttm_bo_driver.h
> index e3a43a4..6fff432 100644
> --- a/include/drm/ttm/ttm_bo_driver.h
> +++ b/include/drm/ttm/ttm_bo_driver.h
> @@ -790,16 +790,7 @@ extern void ttm_mem_io_unlock(struct ttm_mem_type_manager *man);
> * to make room for a buffer already reserved. (Buffers are reserved before
> * they are evicted). The following algorithm prevents such deadlocks from
> * occurring:
> - * 1) Buffers are reserved with the lru spinlock held. Upon successful
> - * reservation they are removed from the lru list. This stops a reserved buffer
> - * from being evicted. However the lru spinlock is released between the time
> - * a buffer is selected for eviction and the time it is reserved.
> - * Therefore a check is made when a buffer is reserved for eviction, that it
> - * is still the first buffer in the lru list, before it is removed from the
> - * list. @check_lru == 1 forces this check. If it fails, the function returns
> - * -EINVAL, and the caller should then choose a new buffer to evict and repeat
> - * the procedure.
> - * 2) Processes attempting to reserve multiple buffers other than for eviction,
> + * Processes attempting to reserve multiple buffers other than for eviction,
> * (typically execbuf), should first obtain a unique 32-bit
> * validation sequence number,
> * and call this function with @use_sequence == 1 and @sequence == the unique
> @@ -832,7 +823,7 @@ extern int ttm_bo_reserve(struct ttm_buffer_object *bo,
>
>
> /**
> - * ttm_bo_reserve_locked:
> + * ttm_bo_reserve_nolru:
> *
> * @bo: A pointer to a struct ttm_buffer_object.
> * @interruptible: Sleep interruptible if waiting.
> @@ -840,9 +831,7 @@ extern int ttm_bo_reserve(struct ttm_buffer_object *bo,
> * @use_sequence: If @bo is already reserved, Only sleep waiting for
> * it to become unreserved if @sequence < (@bo)->sequence.
> *
> - * Must be called with struct ttm_bo_global::lru_lock held,
> - * and will not remove reserved buffers from the lru lists.
> - * The function may release the LRU spinlock if it needs to sleep.
> + * Will not remove reserved buffers from the lru lists.
> * Otherwise identical to ttm_bo_reserve.
> *
> * Returns:
> @@ -855,7 +844,7 @@ extern int ttm_bo_reserve(struct ttm_buffer_object *bo,
> * -EDEADLK: Bo already reserved using @sequence. This error code will only
> * be returned if @use_sequence is set to true.
> */
> -extern int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
> +extern int ttm_bo_reserve_nolru(struct ttm_buffer_object *bo,
> bool interruptible,
> bool no_wait, bool use_sequence,
> uint32_t sequence);
> --
> 1.8.0
>
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