Kernel/USB: Make UHCI descriptor pool thread-safe

Right now the TD and QH descriptor pools look to be susceptible
to a race condition in the event they are accessed simultaneously
by separate threads making USB transfers. This fix does not seem to
add any noticeable overhead.

Kernel/Bus/USB/UHCI/UHCIDescriptorPool.h

@@ -9,6 +9,7 @@
 #include <AK/NonnullOwnPtr.h>
 #include <AK/OwnPtr.h>
 #include <AK/Stack.h>
+#include <Kernel/Locking/Spinlock.h>
 #include <Kernel/Memory/MemoryManager.h>
 #include <Kernel/Memory/Region.h>
 #include <Kernel/StdLib.h>
@@ -38,6 +39,8 @@ public:
 
     [[nodiscard]] T* try_take_free_descriptor()
     {
+        SpinlockLocker locker(m_pool_lock);
+
         // We're out of descriptors!
         if (m_free_descriptor_stack.is_empty())
             return nullptr;
@@ -51,6 +54,8 @@ public:
 
     void release_to_pool(T* ptr)
     {
+        SpinlockLocker locker(m_pool_lock);
+
         dbgln_if(UHCI_VERBOSE_DEBUG, "Returning descriptor @ {} to pool {}", ptr, m_pool_name);
         if (!m_free_descriptor_stack.push(ptr))
             dbgln("Failed to return descriptor to pool {}. Stack overflow!", m_pool_name);
@@ -78,5 +83,7 @@ private:
     StringView m_pool_name;                                   // Name of this pool
     NonnullOwnPtr<Memory::Region> m_pool_region;              // Memory region where descriptors actually reside
     Stack<T*, PAGE_SIZE / sizeof(T)> m_free_descriptor_stack; // Stack of currently free descriptor pointers
+    Spinlock m_pool_lock;
 };
+
 }