| Message ID | 20160829070834.22296-6-chris@chris-wilson.co.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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@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 > --- > 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@ffwll.ch> > */ > -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 >
Am 23.09.2016 um 14:59 schrieb Daniel Vetter <daniel@ffwll.ch>: >> >> /** >> - * 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 Hi Daniel ... I'am working on ;-) * http://return42.github.io/sphkerneldoc/linux_src_doc/include/linux/fence_h.html -- Markus --
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. */ -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);
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 --- include/linux/fence.h | 56 ++++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 51 insertions(+), 5 deletions(-)