Auto merge of #76645 - fusion-engineering-forks:windows-lock, r=kennytm

 Small cleanups in Windows Mutex.

 - Move `held` into the boxed part, since the SRW lock implementation does not use this. This makes the Mutex 50% smaller.
 - Use `Cell` instead of `UnsafeCell` for `held`, such that `.replace()` can be used.
 - Add some comments.
 - Avoid creating multiple `&mut`s to the critical section object in `ReentrantMutex`.

2 files changed, 42 insertions, 43 deletions

diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index b834361b750..5333d75ec1b 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -315,6 +315,7 @@
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(unsafe_block_in_unsafe_fn)]
+#![feature(unsafe_cell_raw_get)]
 #![feature(untagged_unions)]
 #![feature(unwind_attributes)]
 #![feature(vec_into_raw_parts)]
diff --git a/library/std/src/sys/windows/mutex.rs b/library/std/src/sys/windows/mutex.rs
index 63dfc640908..1e09b95c872 100644
--- a/library/std/src/sys/windows/mutex.rs
+++ b/library/std/src/sys/windows/mutex.rs
@@ -19,20 +19,25 @@
 //! CriticalSection is used and we keep track of who's holding the mutex to
 //! detect recursive locks.
 
-use crate::cell::UnsafeCell;
+use crate::cell::{Cell, UnsafeCell};
 use crate::mem::{self, MaybeUninit};
 use crate::sync::atomic::{AtomicUsize, Ordering};
 use crate::sys::c;
 use crate::sys::compat;
 
 pub struct Mutex {
+    // This is either directly an SRWLOCK (if supported), or a Box<Inner> otherwise.
     lock: AtomicUsize,
-    held: UnsafeCell<bool>,
 }
 
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}
 
+struct Inner {
+    remutex: ReentrantMutex,
+    held: Cell<bool>,
+}
+
 #[derive(Clone, Copy)]
 enum Kind {
     SRWLock = 1,
@@ -51,7 +56,6 @@ impl Mutex {
             // This works because SRWLOCK_INIT is 0 (wrapped in a struct), so we are also properly
             // initializing an SRWLOCK here.
             lock: AtomicUsize::new(0),
-            held: UnsafeCell::new(false),
         }
     }
     #[inline]
@@ -60,10 +64,11 @@ impl Mutex {
         match kind() {
             Kind::SRWLock => c::AcquireSRWLockExclusive(raw(self)),
             Kind::CriticalSection => {
-                let re = self.remutex();
-                (*re).lock();
-                if !self.flag_locked() {
-                    (*re).unlock();
+                let inner = &*self.inner();
+                inner.remutex.lock();
+                if inner.held.replace(true) {
+                    // It was already locked, so we got a recursive lock which we do not want.
+                    inner.remutex.unlock();
                     panic!("cannot recursively lock a mutex");
                 }
             }
@@ -73,23 +78,27 @@ impl Mutex {
         match kind() {
             Kind::SRWLock => c::TryAcquireSRWLockExclusive(raw(self)) != 0,
             Kind::CriticalSection => {
-                let re = self.remutex();
-                if !(*re).try_lock() {
+                let inner = &*self.inner();
+                if !inner.remutex.try_lock() {
                     false
-                } else if self.flag_locked() {
-                    true
-                } else {
-                    (*re).unlock();
+                } else if inner.held.replace(true) {
+                    // It was already locked, so we got a recursive lock which we do not want.
+                    inner.remutex.unlock();
                     false
+                } else {
+                    true
                 }
             }
         }
     }
     pub unsafe fn unlock(&self) {
-        *self.held.get() = false;
         match kind() {
             Kind::SRWLock => c::ReleaseSRWLockExclusive(raw(self)),
-            Kind::CriticalSection => (*self.remutex()).unlock(),
+            Kind::CriticalSection => {
+                let inner = &*(self.lock.load(Ordering::SeqCst) as *const Inner);
+                inner.held.set(false);
+                inner.remutex.unlock();
+            }
         }
     }
     pub unsafe fn destroy(&self) {
@@ -97,38 +106,27 @@ impl Mutex {
             Kind::SRWLock => {}
             Kind::CriticalSection => match self.lock.load(Ordering::SeqCst) {
                 0 => {}
-                n => {
-                    Box::from_raw(n as *mut ReentrantMutex).destroy();
-                }
+                n => Box::from_raw(n as *mut Inner).remutex.destroy(),
             },
         }
     }
 
-    unsafe fn remutex(&self) -> *mut ReentrantMutex {
+    unsafe fn inner(&self) -> *const Inner {
         match self.lock.load(Ordering::SeqCst) {
             0 => {}
-            n => return n as *mut _,
+            n => return n as *const _,
         }
-        let re = box ReentrantMutex::uninitialized();
-        re.init();
-        let re = Box::into_raw(re);
-        match self.lock.compare_and_swap(0, re as usize, Ordering::SeqCst) {
-            0 => re,
+        let inner = box Inner { remutex: ReentrantMutex::uninitialized(), held: Cell::new(false) };
+        inner.remutex.init();
+        let inner = Box::into_raw(inner);
+        match self.lock.compare_and_swap(0, inner as usize, Ordering::SeqCst) {
+            0 => inner,
             n => {
-                Box::from_raw(re).destroy();
-                n as *mut _
+                Box::from_raw(inner).remutex.destroy();
+                n as *const _
             }
         }
     }
-
-    unsafe fn flag_locked(&self) -> bool {
-        if *self.held.get() {
-            false
-        } else {
-            *self.held.get() = true;
-            true
-        }
-    }
 }
 
 fn kind() -> Kind {
@@ -150,7 +148,7 @@ fn kind() -> Kind {
 }
 
 pub struct ReentrantMutex {
-    inner: UnsafeCell<MaybeUninit<c::CRITICAL_SECTION>>,
+    inner: MaybeUninit<UnsafeCell<c::CRITICAL_SECTION>>,
 }
 
 unsafe impl Send for ReentrantMutex {}
@@ -158,27 +156,27 @@ unsafe impl Sync for ReentrantMutex {}
 
 impl ReentrantMutex {
     pub const fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex { inner: UnsafeCell::new(MaybeUninit::uninit()) }
+        ReentrantMutex { inner: MaybeUninit::uninit() }
     }
 
     pub unsafe fn init(&self) {
-        c::InitializeCriticalSection((&mut *self.inner.get()).as_mut_ptr());
+        c::InitializeCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr()));
     }
 
     pub unsafe fn lock(&self) {
-        c::EnterCriticalSection((&mut *self.inner.get()).as_mut_ptr());
+        c::EnterCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr()));
     }
 
     #[inline]
     pub unsafe fn try_lock(&self) -> bool {
-        c::TryEnterCriticalSection((&mut *self.inner.get()).as_mut_ptr()) != 0
+        c::TryEnterCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr())) != 0
     }
 
     pub unsafe fn unlock(&self) {
-        c::LeaveCriticalSection((&mut *self.inner.get()).as_mut_ptr());
+        c::LeaveCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr()));
     }
 
     pub unsafe fn destroy(&self) {
-        c::DeleteCriticalSection((&mut *self.inner.get()).as_mut_ptr());
+        c::DeleteCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr()));
     }
 }