| Message ID | 20190712080314.21018-2-chris@chris-wilson.co.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/2] dma-buf: Expand reservation_list to fill allocation | expand |
On Fri, Jul 12, 2019 at 09:03:14AM +0100, Chris Wilson wrote: > As the set of shared fences is not being changed during reallocation of > the reservation list, we can skip updating the write_seqlock. > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch> > Cc: Christian König <christian.koenig@amd.com> sounds legit. Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> More seriously, I think I convinced myself that we cant see a mess of old and new fence arrays anywhere, even without the seqlock retry, so I think we should be all good. -Daniel > --- > drivers/dma-buf/reservation.c | 14 +++++++------- > 1 file changed, 7 insertions(+), 7 deletions(-) > > diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c > index 80ecc1283d15..c71b85c8c159 100644 > --- a/drivers/dma-buf/reservation.c > +++ b/drivers/dma-buf/reservation.c > @@ -157,15 +157,15 @@ int reservation_object_reserve_shared(struct reservation_object *obj, > (ksize(new) - offsetof(typeof(*new), shared)) / > sizeof(*new->shared); > > - preempt_disable(); > - write_seqcount_begin(&obj->seq); > /* > - * RCU_INIT_POINTER can be used here, > - * seqcount provides the necessary barriers > + * We are not changing the effective set of fences here so can > + * merely update the pointer to the new array; both existing > + * readers and new readers will see exactly the same set of > + * active (unsignaled) shared fences. Individual fences and the > + * old array are protected by RCU and so will not vanish under > + * the gaze of the rcu_read_lock() readers. > */ > - RCU_INIT_POINTER(obj->fence, new); > - write_seqcount_end(&obj->seq); > - preempt_enable(); > + rcu_assign_pointer(obj->fence, new); > > if (!old) > return 0; > -- > 2.22.0 >
Quoting Daniel Vetter (2019-07-16 10:21:54) > On Fri, Jul 12, 2019 at 09:03:14AM +0100, Chris Wilson wrote: > > As the set of shared fences is not being changed during reallocation of > > the reservation list, we can skip updating the write_seqlock. > > > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch> > > Cc: Christian König <christian.koenig@amd.com> > > sounds legit. > > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> > > More seriously, I think I convinced myself that we cant see a mess of old > and new fence arrays anywhere, even without the seqlock retry, so I think > we should be all good. Aye, the view remains consistent which is key. Thanks for the review, pushed to drm-misc-next. -Chris
diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c index 80ecc1283d15..c71b85c8c159 100644 --- a/drivers/dma-buf/reservation.c +++ b/drivers/dma-buf/reservation.c @@ -157,15 +157,15 @@ int reservation_object_reserve_shared(struct reservation_object *obj, (ksize(new) - offsetof(typeof(*new), shared)) / sizeof(*new->shared); - preempt_disable(); - write_seqcount_begin(&obj->seq); /* - * RCU_INIT_POINTER can be used here, - * seqcount provides the necessary barriers + * We are not changing the effective set of fences here so can + * merely update the pointer to the new array; both existing + * readers and new readers will see exactly the same set of + * active (unsignaled) shared fences. Individual fences and the + * old array are protected by RCU and so will not vanish under + * the gaze of the rcu_read_lock() readers. */ - RCU_INIT_POINTER(obj->fence, new); - write_seqcount_end(&obj->seq); - preempt_enable(); + rcu_assign_pointer(obj->fence, new); if (!old) return 0;
As the set of shared fences is not being changed during reallocation of the reservation list, we can skip updating the write_seqlock. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: Christian König <christian.koenig@amd.com> --- drivers/dma-buf/reservation.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-)