[core] Use internal locking in CommandAllocator.

Move the `Mutex` in `Device::command_allocator` inside the
`CommandAllocator` type itself, allowing it to be passed by shared
reference instead of mutable reference.

Passing `CommandAllocator` to functions like
`PendingWrites::post_submit` by mutable reference requires the caller
to acquire and hold the mutex for the entire time the callee runs, but
`CommandAllocator` is just a recycling pool, with very simple
invariants; there's no reason to hold the lock for a long time.

wgpu-core/src/command/allocator.rs

@@ -2,6 +2,8 @@ use crate::hal_api::HalApi;
 use crate::resource_log;
 use hal::Device as _;
 
+use parking_lot::Mutex;
+
 /// A pool of free [`wgpu_hal::CommandEncoder`]s, owned by a `Device`.
 ///
 /// Each encoder in this list is in the "closed" state.
@@ -12,13 +14,13 @@ use hal::Device as _;
 /// [ce]: wgpu_hal::CommandEncoder
 /// [cb]: wgpu_hal::Api::CommandBuffer
 pub(crate) struct CommandAllocator<A: HalApi> {
-    free_encoders: Vec<A::CommandEncoder>,
+    free_encoders: Mutex<Vec<A::CommandEncoder>>,
 }
 
 impl<A: HalApi> CommandAllocator<A> {
     pub(crate) fn new() -> Self {
         Self {
-            free_encoders: Vec::new(),
+            free_encoders: Mutex::new(Vec::new()),
         }
     }
 
@@ -27,11 +29,12 @@ impl<A: HalApi> CommandAllocator<A> {
     /// If we have free encoders in the pool, take one of those. Otherwise,
     /// create a new one on `device`.
     pub(crate) fn acquire_encoder(
-        &mut self,
+        &self,
         device: &A::Device,
         queue: &A::Queue,
     ) -> Result<A::CommandEncoder, hal::DeviceError> {
-        match self.free_encoders.pop() {
+        let mut free_encoders = self.free_encoders.lock();
+        match free_encoders.pop() {
             Some(encoder) => Ok(encoder),
             None => unsafe {
                 let hal_desc = hal::CommandEncoderDescriptor { label: None, queue };
@@ -41,19 +44,18 @@ impl<A: HalApi> CommandAllocator<A> {
     }
 
     /// Add `encoder` back to the free pool.
-    pub(crate) fn release_encoder(&mut self, encoder: A::CommandEncoder) {
-        self.free_encoders.push(encoder);
+    pub(crate) fn release_encoder(&self, encoder: A::CommandEncoder) {
+        let mut free_encoders = self.free_encoders.lock();
+        free_encoders.push(encoder);
     }
 
     /// Free the pool of command encoders.
     ///
     /// This is only called when the `Device` is dropped.
-    pub(crate) fn dispose(self, device: &A::Device) {
-        resource_log!(
-            "CommandAllocator::dispose encoders {}",
-            self.free_encoders.len()
-        );
-        for cmd_encoder in self.free_encoders {
+    pub(crate) fn dispose(&self, device: &A::Device) {
+        let mut free_encoders = self.free_encoders.lock();
+        resource_log!("CommandAllocator::dispose encoders {}", free_encoders.len());
+        for cmd_encoder in free_encoders.drain(..) {
             unsafe {
                 device.destroy_command_encoder(cmd_encoder);
             }

wgpu-core/src/device/global.rs

@@ -1351,9 +1351,6 @@ impl Global {
             };
             let encoder = match device
                 .command_allocator
-                .lock()
-                .as_mut()
-                .unwrap()
                 .acquire_encoder(device.raw(), queue.raw.as_ref().unwrap())
             {
                 Ok(raw) => raw,

wgpu-core/src/device/life.rs

@@ -361,7 +361,7 @@ impl<A: HalApi> LifetimeTracker<A> {
     pub fn triage_submissions(
         &mut self,
         last_done: SubmissionIndex,
-        command_allocator: &mut crate::command::CommandAllocator<A>,
+        command_allocator: &crate::command::CommandAllocator<A>,
     ) -> SmallVec<[SubmittedWorkDoneClosure; 1]> {
         profiling::scope!("triage_submissions");
 

wgpu-core/src/device/queue.rs

@@ -258,7 +258,7 @@ impl<A: HalApi> PendingWrites<A> {
     #[must_use]
     fn post_submit(
         &mut self,
-        command_allocator: &mut CommandAllocator<A>,
+        command_allocator: &CommandAllocator<A>,
         device: &A::Device,
         queue: &A::Queue,
     ) -> Option<EncoderInFlight<A>> {
@@ -1530,7 +1530,7 @@ impl Global {
 
             profiling::scope!("cleanup");
             if let Some(pending_execution) = pending_writes.post_submit(
-                device.command_allocator.lock().as_mut().unwrap(),
+                &device.command_allocator,
                 device.raw(),
                 queue.raw.as_ref().unwrap(),
             ) {

wgpu-core/src/device/resource.rs

@@ -97,7 +97,7 @@ pub struct Device<A: HalApi> {
     pub(crate) zero_buffer: Option<A::Buffer>,
     pub(crate) info: ResourceInfo<Device<A>>,
 
-    pub(crate) command_allocator: Mutex<Option<command::CommandAllocator<A>>>,
+    pub(crate) command_allocator: command::CommandAllocator<A>,
     //Note: The submission index here corresponds to the last submission that is done.
     pub(crate) active_submission_index: AtomicU64, //SubmissionIndex,
     // NOTE: if both are needed, the `snatchable_lock` must be consistently acquired before the
@@ -165,7 +165,7 @@ impl<A: HalApi> Drop for Device<A> {
         let raw = self.raw.take().unwrap();
         let pending_writes = self.pending_writes.lock().take().unwrap();
         pending_writes.dispose(&raw);
-        self.command_allocator.lock().take().unwrap().dispose(&raw);
+        self.command_allocator.dispose(&raw);
         unsafe {
             raw.destroy_buffer(self.zero_buffer.take().unwrap());
             raw.destroy_fence(self.fence.write().take().unwrap());
@@ -223,7 +223,7 @@ impl<A: HalApi> Device<A> {
         let fence =
             unsafe { raw_device.create_fence() }.map_err(|_| CreateDeviceError::OutOfMemory)?;
 
-        let mut com_alloc = command::CommandAllocator::new();
+        let com_alloc = command::CommandAllocator::new();
         let pending_encoder = com_alloc
             .acquire_encoder(&raw_device, raw_queue)
             .map_err(|_| CreateDeviceError::OutOfMemory)?;
@@ -269,7 +269,7 @@ impl<A: HalApi> Device<A> {
             queue_to_drop: OnceCell::new(),
             zero_buffer: Some(zero_buffer),
             info: ResourceInfo::new("<device>", None),
-            command_allocator: Mutex::new(Some(com_alloc)),
+            command_allocator: com_alloc,
             active_submission_index: AtomicU64::new(0),
             fence: RwLock::new(Some(fence)),
             snatchable_lock: unsafe { SnatchLock::new() },
@@ -423,10 +423,8 @@ impl<A: HalApi> Device<A> {
         };
 
         let mut life_tracker = self.lock_life();
-        let submission_closures = life_tracker.triage_submissions(
-            last_done_index,
-            self.command_allocator.lock().as_mut().unwrap(),
-        );
+        let submission_closures =
+            life_tracker.triage_submissions(last_done_index, &self.command_allocator);
 
         {
             // Normally, `temp_suspected` exists only to save heap
@@ -3483,10 +3481,9 @@ impl<A: HalApi> Device<A> {
                     .map_err(DeviceError::from)?
             };
             drop(guard);
-            let closures = self.lock_life().triage_submissions(
-                submission_index,
-                self.command_allocator.lock().as_mut().unwrap(),
-            );
+            let closures = self
+                .lock_life()
+                .triage_submissions(submission_index, &self.command_allocator);
             assert!(
                 closures.is_empty(),
                 "wait_for_submit is not expected to work with closures"
@@ -3614,10 +3611,7 @@ impl<A: HalApi> Device<A> {
             log::error!("failed to wait for the device: {error}");
         }
         let mut life_tracker = self.lock_life();
-        let _ = life_tracker.triage_submissions(
-            current_index,
-            self.command_allocator.lock().as_mut().unwrap(),
-        );
+        let _ = life_tracker.triage_submissions(current_index, &self.command_allocator);
         if let Some(device_lost_closure) = life_tracker.device_lost_closure.take() {
             // It's important to not hold the lock while calling the closure.
             drop(life_tracker);