From fedf54132d2410c3949036e3f611ab8dd9dbe89e Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Mon, 29 Aug 2016 08:08:30 +0100 Subject: [PATCH] dma-buf: Restart reservation_object_get_fences_rcu() after writes MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Cc: Daniel Vetter Cc: Maarten Lankhorst Cc: Christian König Cc: Alex Deucher Cc: Sumit Semwal Cc: linux-media@vger.kernel.org Cc: dri-devel@lists.freedesktop.org Cc: linaro-mm-sig@lists.linaro.org Reviewed-by: Daniel Vetter Signed-off-by: Sumit Semwal Link: http://patchwork.freedesktop.org/patch/msgid/20160829070834.22296-7-chris@chris-wilson.co.uk --- 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..ba3e25dab95b 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 int shared_count; + int ret = 1; - while (retry) { + do { struct reservation_object_list *fobj; unsigned seq; + unsigned int 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; -- 2.20.1