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dma-buf: Introduce fence_get_rcu_safe() 

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@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829070834.22296-6-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson 2016-08-29 08:08:29 +01:00 committed by Sumit Semwal
parent 998a7aa1bd
commit 4be0542073
1 changed files with 51 additions and 5 deletions
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56
include/linux/fence.h
View File

@ -182,6 +182,16 @@ void fence_init(struct fence *fence, const struct fence_ops *ops,
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.
*/
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);