Linaro-mm-sig February 2026

linaro-mm-sig@lists.linaro.org

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195 discussions

[PATCH 7/8] dma-buf: use inline lock for the dma-fence-array

by Christian König

Using the inline lock is now the recommended way for dma_fence implementations. So use this approach for the framework's internal fences as well. Also saves about 4 bytes for the external spinlock. Signed-off-by: Christian König <christian.koenig(a)amd.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin(a)igalia.com> Reviewed-by: Philipp Stanner <phasta(a)kernel.org> Reviewed-by: Boris Brezillon <boris.brezillon(a)collabora.com> --- drivers/dma-buf/dma-fence-array.c | 5 ++--- include/linux/dma-fence-array.h | 1 - 2 files changed, 2 insertions(+), 4 deletions(-) diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c index 6657d4b30af9..c2119a8049fe 100644 --- a/drivers/dma-buf/dma-fence-array.c +++ b/drivers/dma-buf/dma-fence-array.c @@ -204,9 +204,8 @@ void dma_fence_array_init(struct dma_fence_array *array, array->num_fences = num_fences; - spin_lock_init(&array->lock); - dma_fence_init(&array->base, &dma_fence_array_ops, &array->lock, - context, seqno); + dma_fence_init(&array->base, &dma_fence_array_ops, NULL, context, + seqno); init_irq_work(&array->work, irq_dma_fence_array_work); atomic_set(&array->num_pending, signal_on_any ? 1 : num_fences); diff --git a/include/linux/dma-fence-array.h b/include/linux/dma-fence-array.h index 079b3dec0a16..370b3d2bba37 100644 --- a/include/linux/dma-fence-array.h +++ b/include/linux/dma-fence-array.h @@ -38,7 +38,6 @@ struct dma_fence_array_cb { struct dma_fence_array { struct dma_fence base; - spinlock_t lock; unsigned num_fences; atomic_t num_pending; struct dma_fence **fences; -- 2.43.0

1 day, 22 hours

[PATCH 6/8] dma-buf: use inline lock for the stub fence v2

by Christian König

Using the inline lock is now the recommended way for dma_fence implementations. So use this approach for the framework's internal fences as well. Also saves about 4 bytes for the external spinlock. v2: drop unnecessary changes Signed-off-by: Christian König <christian.koenig(a)amd.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin(a)igalia.com> Reviewed-by: Philipp Stanner <phasta(a)kernel.org> Reviewed-by: Boris Brezillon <boris.brezillon(a)collabora.com> --- drivers/dma-buf/dma-fence.c | 12 ++---------- 1 file changed, 2 insertions(+), 10 deletions(-) diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 39f0a4d08a2d..133cb9033b2c 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -24,7 +24,6 @@ EXPORT_TRACEPOINT_SYMBOL(dma_fence_emit); EXPORT_TRACEPOINT_SYMBOL(dma_fence_enable_signal); EXPORT_TRACEPOINT_SYMBOL(dma_fence_signaled); -static DEFINE_SPINLOCK(dma_fence_stub_lock); static struct dma_fence dma_fence_stub; /* @@ -123,12 +122,9 @@ static const struct dma_fence_ops dma_fence_stub_ops = { static int __init dma_fence_init_stub(void) { - dma_fence_init(&dma_fence_stub, &dma_fence_stub_ops, - &dma_fence_stub_lock, 0, 0); - + dma_fence_init(&dma_fence_stub, &dma_fence_stub_ops, NULL, 0, 0); set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &dma_fence_stub.flags); - dma_fence_signal(&dma_fence_stub); return 0; } @@ -160,11 +156,7 @@ struct dma_fence *dma_fence_allocate_private_stub(ktime_t timestamp) if (fence == NULL) return NULL; - dma_fence_init(fence, - &dma_fence_stub_ops, - &dma_fence_stub_lock, - 0, 0); - + dma_fence_init(fence, &dma_fence_stub_ops, NULL, 0, 0); set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags); -- 2.43.0

1 day, 22 hours

[PATCH 5/8] dma-buf/selftests: test RCU ops and inline lock v2

by Christian König

Drop the mock_fence and the kmem_cache, instead use the inline lock and test if the ops are properly dropped after signaling. v2: move the RCU check to the end of the test Signed-off-by: Christian König <christian.koenig(a)amd.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin(a)igalia.com> --- drivers/dma-buf/st-dma-fence.c | 44 ++++++++-------------------------- 1 file changed, 10 insertions(+), 34 deletions(-) diff --git a/drivers/dma-buf/st-dma-fence.c b/drivers/dma-buf/st-dma-fence.c index 5d0d9abc6e21..0d9d524d79b6 100644 --- a/drivers/dma-buf/st-dma-fence.c +++ b/drivers/dma-buf/st-dma-fence.c @@ -14,43 +14,26 @@ #include "selftest.h" -static struct kmem_cache *slab_fences; - -static struct mock_fence { - struct dma_fence base; - struct spinlock lock; -} *to_mock_fence(struct dma_fence *f) { - return container_of(f, struct mock_fence, base); -} - static const char *mock_name(struct dma_fence *f) { return "mock"; } -static void mock_fence_release(struct dma_fence *f) -{ - kmem_cache_free(slab_fences, to_mock_fence(f)); -} - static const struct dma_fence_ops mock_ops = { .get_driver_name = mock_name, .get_timeline_name = mock_name, - .release = mock_fence_release, }; static struct dma_fence *mock_fence(void) { - struct mock_fence *f; + struct dma_fence *f; - f = kmem_cache_alloc(slab_fences, GFP_KERNEL); + f = kmalloc(sizeof(*f), GFP_KERNEL); if (!f) return NULL; - spin_lock_init(&f->lock); - dma_fence_init(&f->base, &mock_ops, &f->lock, 0, 0); - - return &f->base; + dma_fence_init(f, &mock_ops, NULL, 0, 0); + return f; } static int sanitycheck(void *arg) @@ -100,6 +83,11 @@ static int test_signaling(void *arg) goto err_free; } + if (rcu_dereference_protected(f->ops, true)) { + pr_err("Fence ops not cleared on signal\n"); + goto err_free; + } + err = 0; err_free: dma_fence_put(f); @@ -540,19 +528,7 @@ int dma_fence(void) SUBTEST(test_stub), SUBTEST(race_signal_callback), }; - int ret; pr_info("sizeof(dma_fence)=%zu\n", sizeof(struct dma_fence)); - - slab_fences = KMEM_CACHE(mock_fence, - SLAB_TYPESAFE_BY_RCU | - SLAB_HWCACHE_ALIGN); - if (!slab_fences) - return -ENOMEM; - - ret = subtests(tests, NULL); - - kmem_cache_destroy(slab_fences); - - return ret; + return subtests(tests, NULL); } -- 2.43.0

1 day, 22 hours

[PATCH 4/8] dma-buf: inline spinlock for fence protection v5

by Christian König

Implement per-fence spinlocks, allowing implementations to not give an external spinlock to protect the fence internal state. Instead a spinlock embedded into the fence structure itself is used in this case. Shared spinlocks have the problem that implementations need to guarantee that the lock lives at least as long all fences referencing them. Using a per-fence spinlock allows completely decoupling spinlock producer and consumer life times, simplifying the handling in most use cases. v2: improve naming, coverage and function documentation v3: fix one additional locking in the selftests v4: separate out some changes to make the patch smaller, fix one amdgpu crash found by CI systems v5: improve comments Signed-off-by: Christian König <christian.koenig(a)amd.com> Reviewed-by: Boris Brezillon <boris.brezillon(a)collabora.com> --- drivers/dma-buf/dma-fence.c | 21 ++++++++++++++++----- drivers/dma-buf/sync_debug.h | 2 +- drivers/gpu/drm/drm_crtc.c | 2 +- drivers/gpu/drm/drm_writeback.c | 2 +- drivers/gpu/drm/nouveau/nouveau_fence.c | 3 ++- drivers/gpu/drm/qxl/qxl_release.c | 3 ++- drivers/gpu/drm/vmwgfx/vmwgfx_fence.c | 3 ++- drivers/gpu/drm/xe/xe_hw_fence.c | 3 ++- include/linux/dma-fence.h | 19 +++++++++++++------ 9 files changed, 40 insertions(+), 18 deletions(-) diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index bd4ec7e26dae..39f0a4d08a2d 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -343,7 +343,6 @@ void __dma_fence_might_wait(void) } #endif - /** * dma_fence_signal_timestamp_locked - signal completion of a fence * @fence: the fence to signal @@ -1068,7 +1067,6 @@ static void __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, spinlock_t *lock, u64 context, u64 seqno, unsigned long flags) { - BUG_ON(!lock); BUG_ON(!ops || !ops->get_driver_name || !ops->get_timeline_name); kref_init(&fence->refcount); @@ -1080,10 +1078,15 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, */ RCU_INIT_POINTER(fence->ops, ops); INIT_LIST_HEAD(&fence->cb_list); - fence->lock = lock; fence->context = context; fence->seqno = seqno; fence->flags = flags | BIT(DMA_FENCE_FLAG_INITIALIZED_BIT); + if (lock) { + fence->extern_lock = lock; + } else { + spin_lock_init(&fence->inline_lock); + fence->flags |= BIT(DMA_FENCE_FLAG_INLINE_LOCK_BIT); + } fence->error = 0; trace_dma_fence_init(fence); @@ -1093,7 +1096,7 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, * dma_fence_init - Initialize a custom fence. * @fence: the fence to initialize * @ops: the dma_fence_ops for operations on this fence - * @lock: the irqsafe spinlock to use for locking this fence + * @lock: optional irqsafe spinlock to use for locking this fence * @context: the execution context this fence is run on * @seqno: a linear increasing sequence number for this context * @@ -1103,6 +1106,10 @@ __dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, * * context and seqno are used for easy comparison between fences, allowing * to check which fence is later by simply using dma_fence_later(). + * + * It is strongly discouraged to provide an external lock because this couples + * lock and fence life time. This is only allowed for legacy use cases when + * multiple fences need to be prevented from signaling out of order. */ void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, @@ -1116,7 +1123,7 @@ EXPORT_SYMBOL(dma_fence_init); * dma_fence_init64 - Initialize a custom fence with 64-bit seqno support. * @fence: the fence to initialize * @ops: the dma_fence_ops for operations on this fence - * @lock: the irqsafe spinlock to use for locking this fence + * @lock: optional irqsafe spinlock to use for locking this fence * @context: the execution context this fence is run on * @seqno: a linear increasing sequence number for this context * @@ -1126,6 +1133,10 @@ EXPORT_SYMBOL(dma_fence_init); * * Context and seqno are used for easy comparison between fences, allowing * to check which fence is later by simply using dma_fence_later(). + * + * It is strongly discouraged to provide an external lock because this couples + * lock and fence life time. This is only allowed for legacy use cases when + * multiple fences need to be prevented from signaling out of order. */ void dma_fence_init64(struct dma_fence *fence, const struct dma_fence_ops *ops, diff --git a/drivers/dma-buf/sync_debug.h b/drivers/dma-buf/sync_debug.h index 02af347293d0..c49324505b20 100644 --- a/drivers/dma-buf/sync_debug.h +++ b/drivers/dma-buf/sync_debug.h @@ -47,7 +47,7 @@ struct sync_timeline { static inline struct sync_timeline *dma_fence_parent(struct dma_fence *fence) { - return container_of(fence->lock, struct sync_timeline, lock); + return container_of(fence->extern_lock, struct sync_timeline, lock); } /** diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c index a7797d260f1e..17472915842f 100644 --- a/drivers/gpu/drm/drm_crtc.c +++ b/drivers/gpu/drm/drm_crtc.c @@ -159,7 +159,7 @@ static const struct dma_fence_ops drm_crtc_fence_ops; static struct drm_crtc *fence_to_crtc(struct dma_fence *fence) { BUG_ON(fence->ops != &drm_crtc_fence_ops); - return container_of(fence->lock, struct drm_crtc, fence_lock); + return container_of(fence->extern_lock, struct drm_crtc, fence_lock); } static const char *drm_crtc_fence_get_driver_name(struct dma_fence *fence) diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c index 95b8a2e4bda6..624a4e8b6c99 100644 --- a/drivers/gpu/drm/drm_writeback.c +++ b/drivers/gpu/drm/drm_writeback.c @@ -81,7 +81,7 @@ * From userspace, this property will always read as zero. */ -#define fence_to_wb_connector(x) container_of(x->lock, \ +#define fence_to_wb_connector(x) container_of(x->extern_lock, \ struct drm_writeback_connector, \ fence_lock) diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c index 4a193b7d6d9e..c282c94138b2 100644 --- a/drivers/gpu/drm/nouveau/nouveau_fence.c +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c @@ -41,7 +41,8 @@ static const struct dma_fence_ops nouveau_fence_ops_legacy; static inline struct nouveau_fence_chan * nouveau_fctx(struct nouveau_fence *fence) { - return container_of(fence->base.lock, struct nouveau_fence_chan, lock); + return container_of(fence->base.extern_lock, struct nouveau_fence_chan, + lock); } static bool diff --git a/drivers/gpu/drm/qxl/qxl_release.c b/drivers/gpu/drm/qxl/qxl_release.c index 06b0b2aa7953..37d4ae0faf0d 100644 --- a/drivers/gpu/drm/qxl/qxl_release.c +++ b/drivers/gpu/drm/qxl/qxl_release.c @@ -62,7 +62,8 @@ static long qxl_fence_wait(struct dma_fence *fence, bool intr, struct qxl_device *qdev; unsigned long cur, end = jiffies + timeout; - qdev = container_of(fence->lock, struct qxl_device, release_lock); + qdev = container_of(fence->extern_lock, struct qxl_device, + release_lock); if (!wait_event_timeout(qdev->release_event, (dma_fence_is_signaled(fence) || diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c b/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c index 85795082fef9..d251eec57df9 100644 --- a/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c @@ -47,7 +47,8 @@ struct vmw_event_fence_action { static struct vmw_fence_manager * fman_from_fence(struct vmw_fence_obj *fence) { - return container_of(fence->base.lock, struct vmw_fence_manager, lock); + return container_of(fence->base.extern_lock, struct vmw_fence_manager, + lock); } static void vmw_fence_obj_destroy(struct dma_fence *f) diff --git a/drivers/gpu/drm/xe/xe_hw_fence.c b/drivers/gpu/drm/xe/xe_hw_fence.c index ae8ed15b64c5..14720623ad00 100644 --- a/drivers/gpu/drm/xe/xe_hw_fence.c +++ b/drivers/gpu/drm/xe/xe_hw_fence.c @@ -124,7 +124,8 @@ static struct xe_hw_fence *to_xe_hw_fence(struct dma_fence *fence); static struct xe_hw_fence_irq *xe_hw_fence_irq(struct xe_hw_fence *fence) { - return container_of(fence->dma.lock, struct xe_hw_fence_irq, lock); + return container_of(fence->dma.extern_lock, struct xe_hw_fence_irq, + lock); } static const char *xe_hw_fence_get_driver_name(struct dma_fence *dma_fence) diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index 086324af96c9..9ed359e38d4e 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -34,7 +34,8 @@ struct seq_file; * @ops: dma_fence_ops associated with this fence * @rcu: used for releasing fence with kfree_rcu * @cb_list: list of all callbacks to call - * @lock: spin_lock_irqsave used for locking + * @extern_lock: external spin_lock_irqsave used for locking (deprecated) + * @inline_lock: alternative internal spin_lock_irqsave used for locking * @context: execution context this fence belongs to, returned by * dma_fence_context_alloc() * @seqno: the sequence number of this fence inside the execution context, @@ -49,6 +50,7 @@ struct seq_file; * of the time. * * DMA_FENCE_FLAG_INITIALIZED_BIT - fence was initialized + * DMA_FENCE_FLAG_INLINE_LOCK_BIT - use inline spinlock instead of external one * DMA_FENCE_FLAG_SIGNALED_BIT - fence is already signaled * DMA_FENCE_FLAG_TIMESTAMP_BIT - timestamp recorded for fence signaling * DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT - enable_signaling might have been called @@ -66,7 +68,10 @@ struct seq_file; * been completed, or never called at all. */ struct dma_fence { - spinlock_t *lock; + union { + spinlock_t *extern_lock; + spinlock_t inline_lock; + }; const struct dma_fence_ops __rcu *ops; /* * We clear the callback list on kref_put so that by the time we @@ -100,6 +105,7 @@ struct dma_fence { enum dma_fence_flag_bits { DMA_FENCE_FLAG_INITIALIZED_BIT, + DMA_FENCE_FLAG_INLINE_LOCK_BIT, DMA_FENCE_FLAG_SEQNO64_BIT, DMA_FENCE_FLAG_SIGNALED_BIT, DMA_FENCE_FLAG_TIMESTAMP_BIT, @@ -381,11 +387,12 @@ dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep) * dma_fence_spinlock - return pointer to the spinlock protecting the fence * @fence: the fence to get the lock from * - * Return the pointer to the extern lock. + * Return either the pointer to the embedded or the external spin lock. */ static inline spinlock_t *dma_fence_spinlock(struct dma_fence *fence) { - return fence->lock; + return test_bit(DMA_FENCE_FLAG_INLINE_LOCK_BIT, &fence->flags) ? + &fence->inline_lock : fence->extern_lock; } /** @@ -396,7 +403,7 @@ static inline spinlock_t *dma_fence_spinlock(struct dma_fence *fence) * Lock the fence, preventing it from changing to the signaled state. */ #define dma_fence_lock_irqsave(fence, flags) \ - spin_lock_irqsave(fence->lock, flags) + spin_lock_irqsave(dma_fence_spinlock(fence), flags) /** * dma_fence_unlock_irqrestore - unlock the fence and irqrestore @@ -406,7 +413,7 @@ static inline spinlock_t *dma_fence_spinlock(struct dma_fence *fence) * Unlock the fence, allowing it to change it's state to signaled again. */ #define dma_fence_unlock_irqrestore(fence, flags) \ - spin_unlock_irqrestore(fence->lock, flags) + spin_unlock_irqrestore(dma_fence_spinlock(fence), flags) /** * dma_fence_assert_held - lockdep assertion that fence is locked -- 2.43.0

1 day, 22 hours

[PATCH 3/8] dma-buf: abstract fence locking v2

by Christian König

Add dma_fence_lock_irqsafe() and dma_fence_unlock_irqrestore() wrappers and mechanically apply them everywhere. Just a pre-requisite cleanup for a follow up patch. v2: add some missing i915 bits, add abstraction for lockdep assertion as well v3: one more suggestion by Tvrtko Signed-off-by: Christian König <christian.koenig(a)amd.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin(a)igalia.com> --- drivers/dma-buf/dma-fence.c | 48 ++++++++++----------- drivers/dma-buf/st-dma-fence.c | 6 ++- drivers/dma-buf/sw_sync.c | 14 +++--- drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c | 4 +- drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 4 +- drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h | 2 +- drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 2 +- drivers/gpu/drm/i915/i915_active.c | 19 ++++---- drivers/gpu/drm/nouveau/nouveau_drm.c | 5 ++- drivers/gpu/drm/scheduler/sched_fence.c | 6 +-- drivers/gpu/drm/xe/xe_sched_job.c | 4 +- include/linux/dma-fence.h | 38 ++++++++++++++++ 12 files changed, 96 insertions(+), 56 deletions(-) diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index ae77f900c267..bd4ec7e26dae 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -365,7 +365,7 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence, struct dma_fence_cb *cur, *tmp; struct list_head cb_list; - lockdep_assert_held(fence->lock); + dma_fence_assert_held(fence); if (unlikely(test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))) @@ -412,9 +412,9 @@ void dma_fence_signal_timestamp(struct dma_fence *fence, ktime_t timestamp) if (WARN_ON(!fence)) return; - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); dma_fence_signal_timestamp_locked(fence, timestamp); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); } EXPORT_SYMBOL(dma_fence_signal_timestamp); @@ -473,9 +473,9 @@ bool dma_fence_check_and_signal(struct dma_fence *fence) unsigned long flags; bool ret; - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); ret = dma_fence_check_and_signal_locked(fence); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); return ret; } @@ -501,9 +501,9 @@ void dma_fence_signal(struct dma_fence *fence) tmp = dma_fence_begin_signalling(); - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); dma_fence_signal_timestamp_locked(fence, ktime_get()); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); dma_fence_end_signalling(tmp); } @@ -604,10 +604,10 @@ void dma_fence_release(struct kref *kref) * don't leave chains dangling. We set the error flag first * so that the callbacks know this signal is due to an error. */ - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); fence->error = -EDEADLK; dma_fence_signal_locked(fence); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); } ops = rcu_dereference(fence->ops); @@ -637,7 +637,7 @@ static bool __dma_fence_enable_signaling(struct dma_fence *fence) const struct dma_fence_ops *ops; bool was_set; - lockdep_assert_held(fence->lock); + dma_fence_assert_held(fence); was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags); @@ -673,9 +673,9 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence) { unsigned long flags; - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); __dma_fence_enable_signaling(fence); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); } EXPORT_SYMBOL(dma_fence_enable_sw_signaling); @@ -715,8 +715,7 @@ int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb, return -ENOENT; } - spin_lock_irqsave(fence->lock, flags); - + dma_fence_lock_irqsave(fence, flags); if (__dma_fence_enable_signaling(fence)) { cb->func = func; list_add_tail(&cb->node, &fence->cb_list); @@ -724,8 +723,7 @@ int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb, INIT_LIST_HEAD(&cb->node); ret = -ENOENT; } - - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); return ret; } @@ -748,9 +746,9 @@ int dma_fence_get_status(struct dma_fence *fence) unsigned long flags; int status; - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); status = dma_fence_get_status_locked(fence); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); return status; } @@ -780,13 +778,11 @@ dma_fence_remove_callback(struct dma_fence *fence, struct dma_fence_cb *cb) unsigned long flags; bool ret; - spin_lock_irqsave(fence->lock, flags); - + dma_fence_lock_irqsave(fence, flags); ret = !list_empty(&cb->node); if (ret) list_del_init(&cb->node); - - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); return ret; } @@ -825,7 +821,7 @@ dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout) unsigned long flags; signed long ret = timeout ? timeout : 1; - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); if (dma_fence_test_signaled_flag(fence)) goto out; @@ -849,11 +845,11 @@ dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout) __set_current_state(TASK_INTERRUPTIBLE); else __set_current_state(TASK_UNINTERRUPTIBLE); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); ret = schedule_timeout(ret); - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); if (ret > 0 && intr && signal_pending(current)) ret = -ERESTARTSYS; } @@ -863,7 +859,7 @@ dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout) __set_current_state(TASK_RUNNING); out: - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); return ret; } EXPORT_SYMBOL(dma_fence_default_wait); diff --git a/drivers/dma-buf/st-dma-fence.c b/drivers/dma-buf/st-dma-fence.c index 73ed6fd48a13..5d0d9abc6e21 100644 --- a/drivers/dma-buf/st-dma-fence.c +++ b/drivers/dma-buf/st-dma-fence.c @@ -410,8 +410,10 @@ struct race_thread { static void __wait_for_callbacks(struct dma_fence *f) { - spin_lock_irq(f->lock); - spin_unlock_irq(f->lock); + unsigned long flags; + + dma_fence_lock_irqsave(f, flags); + dma_fence_unlock_irqrestore(f, flags); } static int thread_signal_callback(void *arg) diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c index 6f09d13be6b6..4c81a37dd682 100644 --- a/drivers/dma-buf/sw_sync.c +++ b/drivers/dma-buf/sw_sync.c @@ -156,12 +156,12 @@ static void timeline_fence_release(struct dma_fence *fence) struct sync_timeline *parent = dma_fence_parent(fence); unsigned long flags; - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); if (!list_empty(&pt->link)) { list_del(&pt->link); rb_erase(&pt->node, &parent->pt_tree); } - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); sync_timeline_put(parent); dma_fence_free(fence); @@ -179,7 +179,7 @@ static void timeline_fence_set_deadline(struct dma_fence *fence, ktime_t deadlin struct sync_pt *pt = dma_fence_to_sync_pt(fence); unsigned long flags; - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); if (test_bit(SW_SYNC_HAS_DEADLINE_BIT, &fence->flags)) { if (ktime_before(deadline, pt->deadline)) pt->deadline = deadline; @@ -187,7 +187,7 @@ static void timeline_fence_set_deadline(struct dma_fence *fence, ktime_t deadlin pt->deadline = deadline; __set_bit(SW_SYNC_HAS_DEADLINE_BIT, &fence->flags); } - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); } static const struct dma_fence_ops timeline_fence_ops = { @@ -431,13 +431,13 @@ static int sw_sync_ioctl_get_deadline(struct sync_timeline *obj, unsigned long a goto put_fence; } - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); if (!test_bit(SW_SYNC_HAS_DEADLINE_BIT, &fence->flags)) { ret = -ENOENT; goto unlock; } data.deadline_ns = ktime_to_ns(pt->deadline); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); dma_fence_put(fence); @@ -450,7 +450,7 @@ static int sw_sync_ioctl_get_deadline(struct sync_timeline *obj, unsigned long a return 0; unlock: - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); put_fence: dma_fence_put(fence); diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c index b82357c65723..1404e1fe62a4 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c @@ -479,10 +479,10 @@ bool amdgpu_ring_soft_recovery(struct amdgpu_ring *ring, unsigned int vmid, if (amdgpu_sriov_vf(ring->adev) || !ring->funcs->soft_recovery || !fence) return false; - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); if (!dma_fence_is_signaled_locked(fence)) dma_fence_set_error(fence, -ENODATA); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); while (!dma_fence_is_signaled(fence) && ktime_to_ns(ktime_sub(deadline, ktime_get())) > 0) diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c index 6a2ea200d90c..4761e7486811 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c @@ -2802,8 +2802,8 @@ void amdgpu_vm_fini(struct amdgpu_device *adev, struct amdgpu_vm *vm) dma_fence_put(vm->last_unlocked); dma_fence_wait(vm->last_tlb_flush, false); /* Make sure that all fence callbacks have completed */ - spin_lock_irqsave(vm->last_tlb_flush->lock, flags); - spin_unlock_irqrestore(vm->last_tlb_flush->lock, flags); + dma_fence_lock_irqsave(vm->last_tlb_flush, flags); + dma_fence_unlock_irqrestore(vm->last_tlb_flush, flags); dma_fence_put(vm->last_tlb_flush); list_for_each_entry_safe(mapping, tmp, &vm->freed, list) { diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h index 139642eacdd0..d5c41e24fb51 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h @@ -638,7 +638,7 @@ static inline uint64_t amdgpu_vm_tlb_seq(struct amdgpu_vm *vm) * sure that the dma_fence structure isn't freed up. */ rcu_read_lock(); - lock = vm->last_tlb_flush->lock; + lock = dma_fence_spinlock(vm->last_tlb_flush); rcu_read_unlock(); spin_lock_irqsave(lock, flags); diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c index bf6117d5fc57..78ea2d9ccedf 100644 --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c @@ -148,7 +148,7 @@ __dma_fence_signal__notify(struct dma_fence *fence, { struct dma_fence_cb *cur, *tmp; - lockdep_assert_held(fence->lock); + dma_fence_assert_held(fence); list_for_each_entry_safe(cur, tmp, list, node) { INIT_LIST_HEAD(&cur->node); diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c index 6b0c1162505a..9d41e052ab65 100644 --- a/drivers/gpu/drm/i915/i915_active.c +++ b/drivers/gpu/drm/i915/i915_active.c @@ -1045,9 +1045,10 @@ __i915_active_fence_set(struct i915_active_fence *active, * nesting rules for the fence->lock; the inner lock is always the * older lock. */ - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); if (prev) - spin_lock_nested(prev->lock, SINGLE_DEPTH_NESTING); + spin_lock_nested(dma_fence_spinlock(prev), + SINGLE_DEPTH_NESTING); /* * A does the cmpxchg first, and so it sees C or NULL, as before, or @@ -1061,17 +1062,18 @@ __i915_active_fence_set(struct i915_active_fence *active, */ while (cmpxchg(__active_fence_slot(active), prev, fence) != prev) { if (prev) { - spin_unlock(prev->lock); + spin_unlock(dma_fence_spinlock(prev)); dma_fence_put(prev); } - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); prev = i915_active_fence_get(active); GEM_BUG_ON(prev == fence); - spin_lock_irqsave(fence->lock, flags); + dma_fence_lock_irqsave(fence, flags); if (prev) - spin_lock_nested(prev->lock, SINGLE_DEPTH_NESTING); + spin_lock_nested(dma_fence_spinlock(prev), + SINGLE_DEPTH_NESTING); } /* @@ -1088,10 +1090,11 @@ __i915_active_fence_set(struct i915_active_fence *active, */ if (prev) { __list_del_entry(&active->cb.node); - spin_unlock(prev->lock); /* serialise with prev->cb_list */ + /* serialise with prev->cb_list */ + spin_unlock(dma_fence_spinlock(prev)); } list_add_tail(&active->cb.node, &fence->cb_list); - spin_unlock_irqrestore(fence->lock, flags); + dma_fence_unlock_irqrestore(fence, flags); return prev; } diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c b/drivers/gpu/drm/nouveau/nouveau_drm.c index f2e04a048ac2..e6805b8b9ffa 100644 --- a/drivers/gpu/drm/nouveau/nouveau_drm.c +++ b/drivers/gpu/drm/nouveau/nouveau_drm.c @@ -156,12 +156,13 @@ nouveau_name(struct drm_device *dev) static inline bool nouveau_cli_work_ready(struct dma_fence *fence) { + unsigned long flags; bool ret = true; - spin_lock_irq(fence->lock); + dma_fence_lock_irqsave(fence, flags); if (!dma_fence_is_signaled_locked(fence)) ret = false; - spin_unlock_irq(fence->lock); + dma_fence_unlock_irqrestore(fence, flags); if (ret == true) dma_fence_put(fence); diff --git a/drivers/gpu/drm/scheduler/sched_fence.c b/drivers/gpu/drm/scheduler/sched_fence.c index 9391d6f0dc01..724d77694246 100644 --- a/drivers/gpu/drm/scheduler/sched_fence.c +++ b/drivers/gpu/drm/scheduler/sched_fence.c @@ -156,19 +156,19 @@ static void drm_sched_fence_set_deadline_finished(struct dma_fence *f, struct dma_fence *parent; unsigned long flags; - spin_lock_irqsave(&fence->lock, flags); + dma_fence_lock_irqsave(f, flags); /* If we already have an earlier deadline, keep it: */ if (test_bit(DRM_SCHED_FENCE_FLAG_HAS_DEADLINE_BIT, &f->flags) && ktime_before(fence->deadline, deadline)) { - spin_unlock_irqrestore(&fence->lock, flags); + dma_fence_unlock_irqrestore(f, flags); return; } fence->deadline = deadline; set_bit(DRM_SCHED_FENCE_FLAG_HAS_DEADLINE_BIT, &f->flags); - spin_unlock_irqrestore(&fence->lock, flags); + dma_fence_unlock_irqrestore(f, flags); /* * smp_load_aquire() to ensure that if we are racing another diff --git a/drivers/gpu/drm/xe/xe_sched_job.c b/drivers/gpu/drm/xe/xe_sched_job.c index 3927666fe556..ae5b38b2a884 100644 --- a/drivers/gpu/drm/xe/xe_sched_job.c +++ b/drivers/gpu/drm/xe/xe_sched_job.c @@ -190,11 +190,11 @@ static bool xe_fence_set_error(struct dma_fence *fence, int error) unsigned long irq_flags; bool signaled; - spin_lock_irqsave(fence->lock, irq_flags); + dma_fence_lock_irqsave(fence, irq_flags); signaled = test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags); if (!signaled) dma_fence_set_error(fence, error); - spin_unlock_irqrestore(fence->lock, irq_flags); + dma_fence_unlock_irqrestore(fence, irq_flags); return signaled; } diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index 80db7ede91de..086324af96c9 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -377,6 +377,44 @@ dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep) } while (1); } +/** + * dma_fence_spinlock - return pointer to the spinlock protecting the fence + * @fence: the fence to get the lock from + * + * Return the pointer to the extern lock. + */ +static inline spinlock_t *dma_fence_spinlock(struct dma_fence *fence) +{ + return fence->lock; +} + +/** + * dma_fence_lock_irqsave - irqsave lock the fence + * @fence: the fence to lock + * @flags: where to store the CPU flags. + * + * Lock the fence, preventing it from changing to the signaled state. + */ +#define dma_fence_lock_irqsave(fence, flags) \ + spin_lock_irqsave(fence->lock, flags) + +/** + * dma_fence_unlock_irqrestore - unlock the fence and irqrestore + * @fence: the fence to unlock + * @flags the CPU flags to restore + * + * Unlock the fence, allowing it to change it's state to signaled again. + */ +#define dma_fence_unlock_irqrestore(fence, flags) \ + spin_unlock_irqrestore(fence->lock, flags) + +/** + * dma_fence_assert_held - lockdep assertion that fence is locked + * @fence: the fence which should be locked + */ +#define dma_fence_assert_held(fence) \ + lockdep_assert_held(dma_fence_spinlock(fence)); + #ifdef CONFIG_LOCKDEP bool dma_fence_begin_signalling(void); void dma_fence_end_signalling(bool cookie); -- 2.43.0

1 day, 22 hours

[PATCH 2/8] dma-buf: detach fence ops on signal v2

by Christian König

When neither a release nor a wait backend ops is specified it is possible to let the dma_fence live on independently of the module who issued it. This makes it possible to unload drivers and only wait for all their fences to signal. v2: fix typo in comment Signed-off-by: Christian König <christian.koenig(a)amd.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin(a)igalia.com> Reviewed-by: Philipp Stanner <phasta(a)kernel.org> Reviewed-by: Boris Brezillon <boris.brezillon(a)collabora.com> --- drivers/dma-buf/dma-fence.c | 16 ++++++++++++---- include/linux/dma-fence.h | 4 ++-- 2 files changed, 14 insertions(+), 6 deletions(-) diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index d3c4e23bf297..ae77f900c267 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -371,6 +371,14 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence, &fence->flags))) return; + /* + * When neither a release nor a wait operation is specified set the ops + * pointer to NULL to allow the fence structure to become independent + * from who originally issued it. + */ + if (!fence->ops->release && !fence->ops->wait) + RCU_INIT_POINTER(fence->ops, NULL); + /* Stash the cb_list before replacing it with the timestamp */ list_replace(&fence->cb_list, &cb_list); @@ -537,7 +545,7 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout) rcu_read_lock(); ops = rcu_dereference(fence->ops); trace_dma_fence_wait_start(fence); - if (ops->wait) { + if (ops && ops->wait) { /* * Implementing the wait ops is deprecated and not supported for * issuers of fences who need their lifetime to be independent @@ -603,7 +611,7 @@ void dma_fence_release(struct kref *kref) } ops = rcu_dereference(fence->ops); - if (ops->release) + if (ops && ops->release) ops->release(fence); else dma_fence_free(fence); @@ -639,7 +647,7 @@ static bool __dma_fence_enable_signaling(struct dma_fence *fence) rcu_read_lock(); ops = rcu_dereference(fence->ops); - if (!was_set && ops->enable_signaling) { + if (!was_set && ops && ops->enable_signaling) { trace_dma_fence_enable_signal(fence); if (!ops->enable_signaling(fence)) { @@ -1025,7 +1033,7 @@ void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline) rcu_read_lock(); ops = rcu_dereference(fence->ops); - if (ops->set_deadline && !dma_fence_is_signaled(fence)) + if (ops && ops->set_deadline && !dma_fence_is_signaled(fence)) ops->set_deadline(fence, deadline); rcu_read_unlock(); } diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index 9d8a4ebe8bf7..80db7ede91de 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -472,7 +472,7 @@ dma_fence_is_signaled_locked(struct dma_fence *fence) rcu_read_lock(); ops = rcu_dereference(fence->ops); - if (ops->signaled && ops->signaled(fence)) { + if (ops && ops->signaled && ops->signaled(fence)) { rcu_read_unlock(); dma_fence_signal_locked(fence); return true; @@ -508,7 +508,7 @@ dma_fence_is_signaled(struct dma_fence *fence) rcu_read_lock(); ops = rcu_dereference(fence->ops); - if (ops->signaled && ops->signaled(fence)) { + if (ops && ops->signaled && ops->signaled(fence)) { rcu_read_unlock(); dma_fence_signal(fence); return true; -- 2.43.0

1 day, 22 hours

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1 day, 23 hours

Re: [PATCH 2/8] dma-buf: detach fence ops on signal v2

by Christian König

On 2/13/26 15:22, Boris Brezillon wrote: >> --- >> drivers/dma-buf/dma-fence.c | 16 ++++++++++++---- >> include/linux/dma-fence.h | 4 ++-- >> 2 files changed, 14 insertions(+), 6 deletions(-) >> >> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c >> index de9bf18be3d4..ba02321bef0b 100644 >> --- a/drivers/dma-buf/dma-fence.c >> +++ b/drivers/dma-buf/dma-fence.c >> @@ -371,6 +371,14 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence, >> &fence->flags))) >> return; >> >> + /* >> + * When neither a release nor a wait operation is specified set the ops >> + * pointer to NULL to allow the fence structure to become independent >> + * from who originally issued it. > > I think this deserves some comment in the dma_fence_ops doc, so that > people know what to expect when they implement this interface. There was already a warning added like ~5years ago that implementations shouldn't use the wait callback. Completely independent of this patch set here we already had tons of trouble with it because it can't take into account when userpsace waits for multiple fences from different implementations. It potentially was never a good idea to have in the first place, we basically only had it because radeon (and IIRC nouveau at that point) depended on it. Regards, Christian.

2 days, 1 hour

Re: [PATCH 1/8] dma-buf: protected fence ops by RCU v5

by Christian König

On 2/19/26 11:35, Philipp Stanner wrote: > On Thu, 2026-02-19 at 11:23 +0100, Christian König wrote: >> On 2/12/26 09:56, Philipp Stanner wrote: >>>>>> @@ -454,13 +465,19 @@ dma_fence_test_signaled_flag(struct dma_fence *fence) >>>>>> static inline bool >>>>>> dma_fence_is_signaled_locked(struct dma_fence *fence) >>>>>> { >>>>>> + const struct dma_fence_ops *ops; >>>>>> + >>>>>> if (dma_fence_test_signaled_flag(fence)) >>>>>> return true; >>>>>> >>>>>> - if (fence->ops->signaled && fence->ops->signaled(fence)) { >>>>>> + rcu_read_lock(); >>>>>> + ops = rcu_dereference(fence->ops); >>>>>> + if (ops->signaled && ops->signaled(fence)) { >>>>> >>>>> Maybe you can educate me a bit about RCU here – couldn't this still >>>>> race? If the ops were unloaded before you take rcu_read_lock(), >>>>> rcu_dereference() would give you an invalid pointer here since you >>>>> don't check for !ops, no? >>>> >>>> Perfectly correct thinking, yes. >>>> >>>> But the check for !ops is added in patch #2 when we actually start to set ops = NULL when the fence signals. >>>> >>>> I intentionally separated that because it is basically the second step in making the solution to detach the fence ops from the module by RCU work. >>>> >>>> We could merge the two patches together, but I think the separation actually makes sense should anybody start to complain about the additional RCU overhead. >>>> >>> >>> Alright, makes sense. However the above does not read correct.. >>> >>> But then my question would be: What's the purpose of this patch, what >>> does it solve or address atomically? >> >> Adding the RCU annotation and related logic, e.g. rcu_read_lock()/rcu_read_unlock()/rcu_dereference() etc... >> >> This allows the automated statically RCU checker to validate what we do here and point out potential mistakes. >> >> Additional to that should adding the rcu_read_lock() protection cause performance problems it will bisect to this patch here alone. > > Alright, thx for the info. Very useful > >> >>> Adding RCU here does not yet change behavior and it does not solve the >>> unloading problem, does it? >> >> Nope, no functional behavior change. It's purely to get the automated checkers going. >> >>> If it's a mere preperational step and the patches should not be merged, >>> I'd guard the above with a simple comment like "Cleanup preparation. >>> 'ops' can yet not be NULL, but this will be the case subsequently." >> >> A comment added in this patch and removed in the next one? Na, that sounds like overkill to me. > > ACK. > But then lets do a normalkill by adding the info you provided above > into the commit message, shall we? ^_^ > > "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." > > This doesn't reveal what the patch is actually about, just that > something is counter-intuitive to someone already very familiar with > the series' intent and the code's deeper background :) > > "This or that about dma_fence shall be cleaned up in subsequent > patches. To prepare for that, add … which allows the RCU checker to > validate …" I've already added the sentence "...As first step to solve this issue protect the fence ops by RCU." in the commit message to make it clear that this is not a full solution to the issue. > *Philipp reads that*: ["Ah, this patch is in preparation and allows the > RCU checker to validate everything!"] Yeah, mentioning the RCU checker is clearly a good idea. Going to add that. Christian. > > ;p > > P.

2 days, 2 hours

Re: [PATCH 1/8] dma-buf: protected fence ops by RCU v5

by Christian König

On 2/12/26 09:56, Philipp Stanner wrote: >>>> @@ -454,13 +465,19 @@ dma_fence_test_signaled_flag(struct dma_fence *fence) >>>> static inline bool >>>> dma_fence_is_signaled_locked(struct dma_fence *fence) >>>> { >>>> + const struct dma_fence_ops *ops; >>>> + >>>> if (dma_fence_test_signaled_flag(fence)) >>>> return true; >>>> >>>> - if (fence->ops->signaled && fence->ops->signaled(fence)) { >>>> + rcu_read_lock(); >>>> + ops = rcu_dereference(fence->ops); >>>> + if (ops->signaled && ops->signaled(fence)) { >>> >>> Maybe you can educate me a bit about RCU here – couldn't this still >>> race? If the ops were unloaded before you take rcu_read_lock(), >>> rcu_dereference() would give you an invalid pointer here since you >>> don't check for !ops, no? >> >> Perfectly correct thinking, yes. >> >> But the check for !ops is added in patch #2 when we actually start to set ops = NULL when the fence signals. >> >> I intentionally separated that because it is basically the second step in making the solution to detach the fence ops from the module by RCU work. >> >> We could merge the two patches together, but I think the separation actually makes sense should anybody start to complain about the additional RCU overhead. >> > > Alright, makes sense. However the above does not read correct.. > > But then my question would be: What's the purpose of this patch, what > does it solve or address atomically? Adding the RCU annotation and related logic, e.g. rcu_read_lock()/rcu_read_unlock()/rcu_dereference() etc... This allows the automated statically RCU checker to validate what we do here and point out potential mistakes. Additional to that should adding the rcu_read_lock() protection cause performance problems it will bisect to this patch here alone. > Adding RCU here does not yet change behavior and it does not solve the > unloading problem, does it? Nope, no functional behavior change. It's purely to get the automated checkers going. > If it's a mere preperational step and the patches should not be merged, > I'd guard the above with a simple comment like "Cleanup preparation. > 'ops' can yet not be NULL, but this will be the case subsequently." A comment added in this patch and removed in the next one? Na, that sounds like overkill to me. Christian. > > > P. >

2 days, 4 hours

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Linaro-mm-sig February 2026