At first glance it is counter intuitive to protect a constant function pointer table by RCU, but this allows modules providing the function table to unload by waiting for an RCU grace period.
v2: make one the now duplicated lockdep warnings a comment instead. v3: Add more documentation to ->wait and ->release callback. Signed-off-by: Christian König <[email protected]> --- drivers/dma-buf/dma-fence.c | 69 +++++++++++++++++++++++++------------ include/linux/dma-fence.h | 29 ++++++++++++++-- 2 files changed, 73 insertions(+), 25 deletions(-) diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index b4f5c8635276..ec21be9b089a 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -498,6 +498,7 @@ EXPORT_SYMBOL(dma_fence_signal); signed long dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout) { + const struct dma_fence_ops *ops; signed long ret; if (WARN_ON(timeout < 0)) @@ -509,15 +510,21 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout) dma_fence_enable_sw_signaling(fence); - if (trace_dma_fence_wait_start_enabled()) { - rcu_read_lock(); - trace_dma_fence_wait_start(fence); + rcu_read_lock(); + ops = rcu_dereference(fence->ops); + trace_dma_fence_wait_start(fence); + if (ops->wait) { + /* + * Implementing the wait ops is deprecated and not supported for + * issuer independent fences, so it is ok to use the ops outside + * the RCU protected section. + */ + rcu_read_unlock(); + ret = ops->wait(fence, intr, timeout); + } else { rcu_read_unlock(); - } - if (fence->ops->wait) - ret = fence->ops->wait(fence, intr, timeout); - else ret = dma_fence_default_wait(fence, intr, timeout); + } if (trace_dma_fence_wait_end_enabled()) { rcu_read_lock(); trace_dma_fence_wait_end(fence); @@ -538,6 +545,7 @@ void dma_fence_release(struct kref *kref) { struct dma_fence *fence = container_of(kref, struct dma_fence, refcount); + const struct dma_fence_ops *ops; rcu_read_lock(); trace_dma_fence_destroy(fence); @@ -569,12 +577,12 @@ void dma_fence_release(struct kref *kref) spin_unlock_irqrestore(fence->lock, flags); } - rcu_read_unlock(); - - if (fence->ops->release) - fence->ops->release(fence); + ops = rcu_dereference(fence->ops); + if (ops->release) + ops->release(fence); else dma_fence_free(fence); + rcu_read_unlock(); } EXPORT_SYMBOL(dma_fence_release); @@ -593,6 +601,7 @@ EXPORT_SYMBOL(dma_fence_free); static bool __dma_fence_enable_signaling(struct dma_fence *fence) { + const struct dma_fence_ops *ops; bool was_set; lockdep_assert_held(fence->lock); @@ -603,14 +612,18 @@ static bool __dma_fence_enable_signaling(struct dma_fence *fence) if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return false; - if (!was_set && fence->ops->enable_signaling) { + rcu_read_lock(); + ops = rcu_dereference(fence->ops); + if (!was_set && ops->enable_signaling) { trace_dma_fence_enable_signal(fence); - if (!fence->ops->enable_signaling(fence)) { + if (!ops->enable_signaling(fence)) { + rcu_read_unlock(); dma_fence_signal_locked(fence); return false; } } + rcu_read_unlock(); return true; } @@ -983,8 +996,13 @@ EXPORT_SYMBOL(dma_fence_wait_any_timeout); */ void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline) { - if (fence->ops->set_deadline && !dma_fence_is_signaled(fence)) - fence->ops->set_deadline(fence, deadline); + const struct dma_fence_ops *ops; + + rcu_read_lock(); + ops = rcu_dereference(fence->ops); + if (ops->set_deadline && !dma_fence_is_signaled(fence)) + ops->set_deadline(fence, deadline); + rcu_read_unlock(); } EXPORT_SYMBOL(dma_fence_set_deadline); @@ -1025,7 +1043,12 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, BUG_ON(!ops || !ops->get_driver_name || !ops->get_timeline_name); kref_init(&fence->refcount); - fence->ops = ops; + /* + * At first glance it is counter intuitive to protect a constant + * function pointer table by RCU, but this allows modules providing the + * function table to unload by waiting for an RCU grace period. + */ + RCU_INIT_POINTER(fence->ops, ops); INIT_LIST_HEAD(&fence->cb_list); fence->lock = lock; fence->context = context; @@ -1105,11 +1128,12 @@ EXPORT_SYMBOL(dma_fence_init64); */ const char __rcu *dma_fence_driver_name(struct dma_fence *fence) { - RCU_LOCKDEP_WARN(!rcu_read_lock_held(), - "RCU protection is required for safe access to returned string"); + const struct dma_fence_ops *ops; + /* RCU protection is required for safe access to returned string */ + ops = rcu_dereference(fence->ops); if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) - return fence->ops->get_driver_name(fence); + return ops->get_driver_name(fence); else return "detached-driver"; } @@ -1137,11 +1161,12 @@ EXPORT_SYMBOL(dma_fence_driver_name); */ const char __rcu *dma_fence_timeline_name(struct dma_fence *fence) { - RCU_LOCKDEP_WARN(!rcu_read_lock_held(), - "RCU protection is required for safe access to returned string"); + const struct dma_fence_ops *ops; + /* RCU protection is required for safe access to returned string */ + ops = rcu_dereference(fence->ops); if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) - return fence->ops->get_timeline_name(fence); + return ops->get_timeline_name(fence); else return "signaled-timeline"; } diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index 64639e104110..77f07735f556 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -66,7 +66,7 @@ struct seq_file; */ struct dma_fence { spinlock_t *lock; - const struct dma_fence_ops *ops; + const struct dma_fence_ops __rcu *ops; /* * We clear the callback list on kref_put so that by the time we * release the fence it is unused. No one should be adding to the @@ -218,6 +218,10 @@ struct dma_fence_ops { * timed out. Can also return other error values on custom implementations, * which should be treated as if the fence is signaled. For example a hardware * lockup could be reported like that. + * + * Implementing this callback prevents the BO from detaching after + * signaling and so it is mandatory for the module providing the + * dma_fence_ops to stay loaded as long as the dma_fence exists. */ signed long (*wait)(struct dma_fence *fence, bool intr, signed long timeout); @@ -229,6 +233,13 @@ struct dma_fence_ops { * Can be called from irq context. This callback is optional. If it is * NULL, then dma_fence_free() is instead called as the default * implementation. + * + * Implementing this callback prevents the BO from detaching after + * signaling and so it is mandatory for the module providing the + * dma_fence_ops to stay loaded as long as the dma_fence exists. + * + * If the callback is implemented the memory backing the dma_fence + * object must be freed RCU safe. */ void (*release)(struct dma_fence *fence); @@ -418,13 +429,19 @@ const char __rcu *dma_fence_timeline_name(struct dma_fence *fence); static inline bool dma_fence_is_signaled_locked(struct dma_fence *fence) { + const struct dma_fence_ops *ops; + if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true; - if (fence->ops->signaled && fence->ops->signaled(fence)) { + rcu_read_lock(); + ops = rcu_dereference(fence->ops); + if (ops->signaled && ops->signaled(fence)) { + rcu_read_unlock(); dma_fence_signal_locked(fence); return true; } + rcu_read_unlock(); return false; } @@ -448,13 +465,19 @@ dma_fence_is_signaled_locked(struct dma_fence *fence) static inline bool dma_fence_is_signaled(struct dma_fence *fence) { + const struct dma_fence_ops *ops; + if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true; - if (fence->ops->signaled && fence->ops->signaled(fence)) { + rcu_read_lock(); + ops = rcu_dereference(fence->ops); + if (ops->signaled && ops->signaled(fence)) { + rcu_read_unlock(); dma_fence_signal(fence); return true; } + rcu_read_unlock(); return false; } -- 2.43.0
