On Mon, Aug 29, 2016 at 08:08:30AM +0100, Chris Wilson wrote:
> In order to be completely generic, we have to double check the read
> seqlock after acquiring a reference to the fence. If the driver is
> allocating fences from a SLAB_DESTROY_BY_RCU, or similar freelist, then
> within an RCU grace period a fence may be freed and reallocated. The RCU
> read side critical section does not prevent this reallocation, instead
> we have to inspect the reservation's seqlock to double check if the
> fences have been reassigned as we were acquiring our reference.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Christian König <christian.koenig@xxxxxxx>
> Cc: Alex Deucher <alexander.deucher@xxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> Cc: linux-media@xxxxxxxxxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
> ---
> drivers/dma-buf/reservation.c | 71 +++++++++++++++++++------------------------
> 1 file changed, 31 insertions(+), 40 deletions(-)
>
> diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
> index 723d8af988e5..10fd441dd4ed 100644
> --- a/drivers/dma-buf/reservation.c
> +++ b/drivers/dma-buf/reservation.c
> @@ -280,18 +280,24 @@ int reservation_object_get_fences_rcu(struct reservation_object *obj,
> unsigned *pshared_count,
> struct fence ***pshared)
> {
> - unsigned shared_count = 0;
> - unsigned retry = 1;
> - struct fence **shared = NULL, *fence_excl = NULL;
> - int ret = 0;
> + struct fence **shared = NULL;
> + struct fence *fence_excl;
> + unsigned shared_count;
> + int ret = 1;
Personally I'd go with ret = -EBUSY here, but that's a bikeshed.
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
>
> - while (retry) {
> + do {
> struct reservation_object_list *fobj;
> unsigned seq;
> + unsigned i;
>
> - seq = read_seqcount_begin(&obj->seq);
> + shared_count = i = 0;
>
> rcu_read_lock();
> + seq = read_seqcount_begin(&obj->seq);
> +
> + fence_excl = rcu_dereference(obj->fence_excl);
> + if (fence_excl && !fence_get_rcu(fence_excl))
> + goto unlock;
>
> fobj = rcu_dereference(obj->fence);
> if (fobj) {
> @@ -309,52 +315,37 @@ int reservation_object_get_fences_rcu(struct reservation_object *obj,
> }
>
> ret = -ENOMEM;
> - shared_count = 0;
> break;
> }
> shared = nshared;
> - memcpy(shared, fobj->shared, sz);
> shared_count = fobj->shared_count;
> - } else
> - shared_count = 0;
> - fence_excl = rcu_dereference(obj->fence_excl);
> -
> - retry = read_seqcount_retry(&obj->seq, seq);
> - if (retry)
> - goto unlock;
> -
> - if (!fence_excl || fence_get_rcu(fence_excl)) {
> - unsigned i;
>
> for (i = 0; i < shared_count; ++i) {
> - if (fence_get_rcu(shared[i]))
> - continue;
> -
> - /* uh oh, refcount failed, abort and retry */
> - while (i--)
> - fence_put(shared[i]);
> -
> - if (fence_excl) {
> - fence_put(fence_excl);
> - fence_excl = NULL;
> - }
> -
> - retry = 1;
> - break;
> + shared[i] = rcu_dereference(fobj->shared[i]);
> + if (!fence_get_rcu(shared[i]))
> + break;
> }
> - } else
> - retry = 1;
> + }
> +
> + if (i != shared_count || read_seqcount_retry(&obj->seq, seq)) {
> + while (i--)
> + fence_put(shared[i]);
> + fence_put(fence_excl);
> + goto unlock;
> + }
>
> + ret = 0;
> unlock:
> rcu_read_unlock();
> - }
> - *pshared_count = shared_count;
> - if (shared_count)
> - *pshared = shared;
> - else {
> - *pshared = NULL;
> + } while (ret);
> +
> + if (!shared_count) {
> kfree(shared);
> + shared = NULL;
> }
> +
> + *pshared_count = shared_count;
> + *pshared = shared;
> *pfence_excl = fence_excl;
>
> return ret;
> --
> 2.9.3
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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