zephyr: Replace critical-section implementation

There is a fairly fundamental incompatibility between Zephyr spin locks
and the Critical Section specification.  Zephyr spin locks do not allow
nesting from within a single spin lock.  The critical section API only
has an `acquire` and `release` entry, and provides no way (such as a
stack frame) to have a unique context for different invocation places.

Unfortunately, this means we cannot use spin locks for critical
sections.

Instead, this change implements critical sections using irq locking.
The implementation of these macros on Zephyr does try to make them SMP
safe, with a simple atomic lock, but there is still something preventing
the riscv SMP from working.

Also, these entries cannot be called from user mode.  There are various
other reasons we don't support usermode, so at this time, just have a
compile time assertion that usermode is not enabled in the build.  If it
is needed, we will have to come up with another way to implement this.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

zephyr/src/sys.rs

@@ -39,42 +39,39 @@ pub fn uptime_get() -> i64 {
     unsafe { crate::raw::k_uptime_get() }
 }
 
+// The below implementation, based on interrupt locking has only been tested on single CPU.  The
+// implementation suggests it should work on SMP, and can be tested.  The docs for irq_lock()
+// explicitly state that it cannot be used from userspace. Unfortunately, spinlocks have
+// incompatible semantics with critical sections, so to work with userspace we'd need probably a
+// syscall.
+#[cfg(CONFIG_USERSPACE)]
+compile_error!("Critical-section implementation does not work with CONFIG_USERSPACE");
+
 pub mod critical {
     //! Zephyr implementation of critical sections.
     //!
-    //! Critical sections from Rust are handled with a single Zephyr spinlock.  This doesn't allow
-    //! any nesting, but neither does the `critical-section` crate.
-    //!
-    //! This provides the underlying critical section crate, which is useful for external crates
-    //! that want this interface.  However, it isn't a particularly hygienic interface to use.  For
-    //! something a bit nicer, please see [`sync::SpinMutex`].
-    //!
-    //! [`sync::SpinMutex`]: crate::sync::SpinMutex
+    //! The critical-section crate explicitly states that critical sections can be nested.
+    //! Unfortunately, Zephyr spinlocks cannot be nested.  It is possible to nest different ones,
+    //! but the critical-section implementation API doesn't give access to the stack.
 
-    use core::{ffi::c_int, ptr::addr_of_mut};
+    use core::{ffi::c_int, sync::atomic::{fence, Ordering}};
 
     use critical_section::RawRestoreState;
-    use zephyr_sys::{k_spin_lock, k_spin_unlock, k_spinlock, k_spinlock_key_t};
+    use zephyr_sys::{zr_irq_lock, zr_irq_unlock};
 
     struct ZephyrCriticalSection;
     critical_section::set_impl!(ZephyrCriticalSection);
 
-    // The critical section shares a single spinlock.
-    static mut LOCK: k_spinlock = unsafe { core::mem::zeroed() };
-
     unsafe impl critical_section::Impl for ZephyrCriticalSection {
         unsafe fn acquire() -> RawRestoreState {
-            let res = k_spin_lock(addr_of_mut!(LOCK));
-            res.key as RawRestoreState
+            let res = zr_irq_lock();
+            fence(Ordering::Acquire);
+            res as RawRestoreState
         }
 
         unsafe fn release(token: RawRestoreState) {
-            k_spin_unlock(
-                addr_of_mut!(LOCK),
-                k_spinlock_key_t {
-                    key: token as c_int,
-                },
-            );
+            fence(Ordering::Release);
+            zr_irq_unlock(token as c_int);
         }
     }
 }