On Mon, Aug 29, 2016 at 08:08:29AM +0100, Chris Wilson wrote:
> This variant of fence_get_rcu() takes an RCU protected pointer to a
> fence and carefully returns a reference to the fence ensuring that it is
> not reallocated as it does. This is required when mixing fences and
> SLAB_DESTROY_BY_RCU - although it serves a more pedagogical function atm
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> Cc: linux-media@xxxxxxxxxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
> ---
> include/linux/fence.h | 56 ++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 51 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/fence.h b/include/linux/fence.h
> index 0d763053f97a..c9c5ba98c302 100644
> --- a/include/linux/fence.h
> +++ b/include/linux/fence.h
> @@ -183,6 +183,16 @@ void fence_release(struct kref *kref);
> void fence_free(struct fence *fence);
>
> /**
> + * fence_put - decreases refcount of the fence
> + * @fence: [in] fence to reduce refcount of
> + */
> +static inline void fence_put(struct fence *fence)
> +{
> + if (fence)
> + kref_put(&fence->refcount, fence_release);
> +}
> +
> +/**
> * fence_get - increases refcount of the fence
> * @fence: [in] fence to increase refcount of
> *
> @@ -210,13 +220,49 @@ static inline struct fence *fence_get_rcu(struct fence *fence)
> }
>
> /**
> - * fence_put - decreases refcount of the fence
> - * @fence: [in] fence to reduce refcount of
> + * fence_get_rcu_safe - acquire a reference to an RCU tracked fence
> + * @fence: [in] pointer to fence to increase refcount of
> + *
> + * Function returns NULL if no refcount could be obtained, or the fence.
> + * This function handles acquiring a reference to a fence that may be
> + * reallocated within the RCU grace period (such as with SLAB_DESTROY_BY_RCU),
> + * so long as the caller is using RCU on the pointer to the fence.
> + *
> + * An alternative mechanism is to employ a seqlock to protect a bunch of
> + * fences, such as used by struct reservation_object. When using a seqlock,
> + * the seqlock must be taken before and checked after a reference to the
> + * fence is acquired (as shown here).
> + *
> + * The caller is required to hold the RCU read lock.
Would be good to cross reference the various fence_get functions a bit
better in the docs. But since the docs aren't yet pulled into the rst/html
output, that doesn't matter that much. Hence as-is:
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> */
> -static inline void fence_put(struct fence *fence)
> +static inline struct fence *fence_get_rcu_safe(struct fence * __rcu *fencep)
> {
> - if (fence)
> - kref_put(&fence->refcount, fence_release);
> + do {
> + struct fence *fence;
> +
> + fence = rcu_dereference(*fencep);
> + if (!fence || !fence_get_rcu(fence))
> + return NULL;
> +
> + /* The atomic_inc_not_zero() inside fence_get_rcu()
> + * provides a full memory barrier upon success (such as now).
> + * This is paired with the write barrier from assigning
> + * to the __rcu protected fence pointer so that if that
> + * pointer still matches the current fence, we know we
> + * have successfully acquire a reference to it. If it no
> + * longer matches, we are holding a reference to some other
> + * reallocated pointer. This is possible if the allocator
> + * is using a freelist like SLAB_DESTROY_BY_RCU where the
> + * fence remains valid for the RCU grace period, but it
> + * may be reallocated. When using such allocators, we are
> + * responsible for ensuring the reference we get is to
> + * the right fence, as below.
> + */
> + if (fence == rcu_access_pointer(*fencep))
> + return rcu_pointer_handoff(fence);
> +
> + fence_put(fence);
> + } while (1);
> }
>
> int fence_signal(struct fence *fence);
> --
> 2.9.3
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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