vk: more locks for rsh

Plus add a note for Metal.

Remove and leave out threading/rsh details for now as things are not
quite clear yet when it comes to QRhis operating in parallel with
one or more shared resource.

src/rhi/qrhi.cpp

@@ -224,7 +224,7 @@ QT_BEGIN_NAMESPACE
     QRhiTexture::nativeHandles(). Most importantly, passing pointers in structs
     and via setters does not transfer ownership.
 
-    \section3 Threading
+    \section3 Threading and resource sharing
 
     A QRhi instance can be created and used on any thread but all usage must be
     limited to that one single thread.
@@ -240,12 +240,7 @@ QT_BEGIN_NAMESPACE
     from \l{QRhiResource::isShareable()}{isShareable()} is usable with other
     QRhi instances as well, as long as they are associated with the same
     QRhiResourceSharingHost. Resources can also outlive their creating QRhi in
-    this case, as long as there is at least one QRhi left in the "sharing
-    group". When the QRhi instances live and operate on different threads,
-    additional synchronization may be required. When writing to resources,
-    additional synchronization may be required even when the QRhi instances
-    live on the same thread. See QRhiResourceSharingHost for further
-    discussion.
+    this case, as long as there is at least one QRhi left in the sharing group.
 
     \sa {Qt Shader Tools}, QRhiResourceSharingHost
  */

src/rhi/qrhimetal.mm

@@ -52,7 +52,10 @@ QT_BEGIN_NAMESPACE
     are Managed on macOS and Shared on iOS/tvOS, and still duplicated.
     "Immutable" is like "static" but with only one native buffer underneath.
     Textures are Private (device local) and a host visible staging buffer is
-    used to upload data to them.
+    used to upload data to them. Does not rely on strong objects refs from
+    command buffers (hence uses commandBufferWithUnretainedReferences), but
+    does rely on automatic dependency tracking between encoders (hence no
+    MTLResourceHazardTrackingModeUntracked atm).
 */
 
 #if __has_feature(objc_arc)

src/rhi/qrhivulkan.cpp

@@ -439,6 +439,12 @@ bool QRhiVulkan::create(QRhi::Flags flags)
     }
 
     if (gfxQueueFamilyIdx != -1) {
+        // Will use one queue always, including when multiple QRhis use the
+        // same device. This has significant consequences, and cannot easily be
+        // changed (e.g. think pipeline barriers which create a dependency
+        // between commands submitted to a queue - with multiple queues
+        // additional synchronization would be needed)
+
         if (!gfxQueue)
             df->vkGetDeviceQueue(dev, gfxQueueFamilyIdx, 0, &gfxQueue);
 
@@ -1563,7 +1569,7 @@ void QRhiVulkan::waitCommandCompletion(int frameSlot)
 
 QRhi::FrameOpResult QRhiVulkan::beginNonWrapperFrame(QRhiSwapChain *swapChain)
 {
-    QMutexLocker lock(rsh ? &rsh->mtx : nullptr); // externally synchronized fun for VkQueue & co.
+    QMutexLocker lock(rsh ? &rsh->mtx : nullptr);
 
     QVkSwapChain *swapChainD = QRHI_RES(QVkSwapChain, swapChain);
     QVkSwapChain::FrameResources &frame(swapChainD->frameRes[swapChainD->currentFrameSlot]);
@@ -1678,7 +1684,7 @@ QRhi::FrameOpResult QRhiVulkan::beginNonWrapperFrame(QRhiSwapChain *swapChain)
 
 QRhi::FrameOpResult QRhiVulkan::endNonWrapperFrame(QRhiSwapChain *swapChain)
 {
-    QMutexLocker lock(rsh ? &rsh->mtx : nullptr); // externally synchronized fun for VkQueue & co.
+    QMutexLocker lock(rsh ? &rsh->mtx : nullptr);
 
     Q_ASSERT(inFrame);
     inFrame = false;
@@ -1875,6 +1881,8 @@ QRhi::FrameOpResult QRhiVulkan::finish()
 
 void QRhiVulkan::activateTextureRenderTarget(QRhiCommandBuffer *, QRhiTextureRenderTarget *rt)
 {
+    QMutexLocker lock(rsh ? &rsh->mtx : nullptr);
+
     QVkTextureRenderTarget *rtD = QRHI_RES(QVkTextureRenderTarget, rt);
     rtD->lastActiveFrameSlot = currentFrameSlot;
     rtD->d.rp->lastActiveFrameSlot = currentFrameSlot;
@@ -1900,6 +1908,8 @@ void QRhiVulkan::activateTextureRenderTarget(QRhiCommandBuffer *, QRhiTextureRen
 
 void QRhiVulkan::deactivateTextureRenderTarget(QRhiCommandBuffer *, QRhiTextureRenderTarget *rt)
 {
+    QMutexLocker lock(rsh ? &rsh->mtx : nullptr);
+
     QVkTextureRenderTarget *rtD = QRHI_RES(QVkTextureRenderTarget, rt);
     // already in the right layout when the renderpass ends
     const QVector<QRhiColorAttachment> colorAttachments = rtD->m_desc.colorAttachments();
@@ -2269,6 +2279,8 @@ void QRhiVulkan::enqueueResourceUpdates(QRhiCommandBuffer *cb, QRhiResourceUpdat
     QRhiResourceUpdateBatchPrivate *ud = QRhiResourceUpdateBatchPrivate::get(resourceUpdates);
     QRhiProfilerPrivate *rhiP = profilerPrivateOrNull();
 
+    QMutexLocker lock(rsh ? &rsh->mtx : nullptr);
+
     for (const QRhiResourceUpdateBatchPrivate::DynamicBufferUpdate &u : ud->dynamicBufferUpdates) {
         QVkBuffer *bufD = QRHI_RES(QVkBuffer, u.buf);
         Q_ASSERT(bufD->m_type == QRhiBuffer::Dynamic);
@@ -2706,6 +2718,8 @@ void QRhiVulkan::executeBufferHostWritesForCurrentFrame(QVkBuffer *bufD)
     if (updates.isEmpty())
         return;
 
+    QMutexLocker lock(rsh ? &rsh->mtx : nullptr);
+
     Q_ASSERT(bufD->m_type == QRhiBuffer::Dynamic);
     void *p = nullptr;
     VmaAllocation a = toVmaAllocation(bufD->allocations[currentFrameSlot]);

todo.txt

-event
 res.sh.: example to show read/write same texture
-threading with rsh with resource - more external sync needed?
+event/fence for res.sh.?
 advanced blend modes
 gl: tex formats (texture, readback)
 gl: srgb? (glEnable and co.)