Use a single ReentrantMutex implementation on all platforms.

12 files changed, 91 insertions, 540 deletions

diff --git a/library/std/src/sys/hermit/mutex.rs b/library/std/src/sys/hermit/mutex.rs
index 415cbba101c..97b4c49896f 100644
--- a/library/std/src/sys/hermit/mutex.rs
+++ b/library/std/src/sys/hermit/mutex.rs
@@ -1,6 +1,5 @@
 use crate::cell::UnsafeCell;
 use crate::collections::VecDeque;
-use crate::ffi::c_void;
 use crate::hint;
 use crate::ops::{Deref, DerefMut, Drop};
 use crate::ptr;
@@ -220,38 +219,3 @@ impl Mutex {
     #[inline]
     pub unsafe fn destroy(&self) {}
 }
-
-pub struct ReentrantMutex {
-    inner: *const c_void,
-}
-
-impl ReentrantMutex {
-    pub const unsafe fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex { inner: ptr::null() }
-    }
-
-    #[inline]
-    pub unsafe fn init(&self) {
-        let _ = abi::recmutex_init(&self.inner as *const *const c_void as *mut _);
-    }
-
-    #[inline]
-    pub unsafe fn lock(&self) {
-        let _ = abi::recmutex_lock(self.inner);
-    }
-
-    #[inline]
-    pub unsafe fn try_lock(&self) -> bool {
-        true
-    }
-
-    #[inline]
-    pub unsafe fn unlock(&self) {
-        let _ = abi::recmutex_unlock(self.inner);
-    }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {
-        let _ = abi::recmutex_destroy(self.inner);
-    }
-}
diff --git a/library/std/src/sys/itron/mutex.rs b/library/std/src/sys/itron/mutex.rs
index e01f595ac54..5ee231882bb 100644
--- a/library/std/src/sys/itron/mutex.rs
+++ b/library/std/src/sys/itron/mutex.rs
@@ -5,7 +5,6 @@ use super::{
     error::{expect_success, expect_success_aborting, fail, ItronError},
     spin::SpinIdOnceCell,
 };
-use crate::cell::UnsafeCell;
 
 pub struct Mutex {
     /// The ID of the underlying mutex object
@@ -89,95 +88,3 @@ impl Drop for MutexGuard<'_> {
         unsafe { self.0.unlock() };
     }
 }
-
-// All empty stubs because this platform does not yet support threads, so lock
-// acquisition always succeeds.
-pub struct ReentrantMutex {
-    /// The ID of the underlying mutex object
-    mtx: abi::ID,
-    /// The lock count.
-    count: UnsafeCell<usize>,
-}
-
-unsafe impl Send for ReentrantMutex {}
-unsafe impl Sync for ReentrantMutex {}
-
-impl ReentrantMutex {
-    pub const unsafe fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex { mtx: 0, count: UnsafeCell::new(0) }
-    }
-
-    pub unsafe fn init(&mut self) {
-        self.mtx = expect_success(
-            unsafe {
-                abi::acre_mtx(&abi::T_CMTX {
-                    // Priority inheritance mutex
-                    mtxatr: abi::TA_INHERIT,
-                    // Unused
-                    ceilpri: 0,
-                })
-            },
-            &"acre_mtx",
-        );
-    }
-
-    pub unsafe fn lock(&self) {
-        match unsafe { abi::loc_mtx(self.mtx) } {
-            abi::E_OBJ => {
-                // Recursive lock
-                unsafe {
-                    let count = &mut *self.count.get();
-                    if let Some(new_count) = count.checked_add(1) {
-                        *count = new_count;
-                    } else {
-                        // counter overflow
-                        rtabort!("lock count overflow");
-                    }
-                }
-            }
-            er => {
-                expect_success(er, &"loc_mtx");
-            }
-        }
-    }
-
-    pub unsafe fn unlock(&self) {
-        unsafe {
-            let count = &mut *self.count.get();
-            if *count > 0 {
-                *count -= 1;
-                return;
-            }
-        }
-
-        expect_success_aborting(unsafe { abi::unl_mtx(self.mtx) }, &"unl_mtx");
-    }
-
-    pub unsafe fn try_lock(&self) -> bool {
-        let er = unsafe { abi::ploc_mtx(self.mtx) };
-        if er == abi::E_OBJ {
-            // Recursive lock
-            unsafe {
-                let count = &mut *self.count.get();
-                if let Some(new_count) = count.checked_add(1) {
-                    *count = new_count;
-                } else {
-                    // counter overflow
-                    rtabort!("lock count overflow");
-                }
-            }
-            true
-        } else if er == abi::E_TMOUT {
-            // Locked by another thread
-            false
-        } else {
-            expect_success(er, &"ploc_mtx");
-            // Top-level lock by the current thread
-            true
-        }
-    }
-
-    pub unsafe fn destroy(&self) {
-        expect_success_aborting(unsafe { abi::del_mtx(self.mtx) }, &"del_mtx");
-    }
-}
diff --git a/library/std/src/sys/sgx/mutex.rs b/library/std/src/sys/sgx/mutex.rs
index 0b2d1f4487f..98a390c4c2b 100644
--- a/library/std/src/sys/sgx/mutex.rs
+++ b/library/std/src/sys/sgx/mutex.rs
@@ -1,8 +1,4 @@
-use fortanix_sgx_abi::Tcs;
-
-use super::abi::thread;
-
-use super::waitqueue::{try_lock_or_false, NotifiedTcs, SpinMutex, WaitQueue, WaitVariable};
+use super::waitqueue::{try_lock_or_false, SpinMutex, WaitQueue, WaitVariable};
 
 pub struct Mutex {
     inner: SpinMutex<WaitVariable<bool>>,
@@ -60,84 +56,3 @@ impl Mutex {
     #[inline]
     pub unsafe fn destroy(&self) {}
 }
-
-struct ReentrantLock {
-    owner: Option<Tcs>,
-    count: usize,
-}
-
-pub struct ReentrantMutex {
-    inner: SpinMutex<WaitVariable<ReentrantLock>>,
-}
-
-impl ReentrantMutex {
-    pub const fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex {
-            inner: SpinMutex::new(WaitVariable::new(ReentrantLock { owner: None, count: 0 })),
-        }
-    }
-
-    #[inline]
-    pub unsafe fn init(&self) {}
-
-    #[inline]
-    pub unsafe fn lock(&self) {
-        let mut guard = self.inner.lock();
-        match guard.lock_var().owner {
-            Some(tcs) if tcs != thread::current() => {
-                // Another thread has the lock, wait
-                WaitQueue::wait(guard, || {});
-                // Another thread has passed the lock to us
-            }
-            _ => {
-                // We are just now obtaining the lock
-                guard.lock_var_mut().owner = Some(thread::current());
-                guard.lock_var_mut().count += 1;
-            }
-        }
-    }
-
-    #[inline]
-    pub unsafe fn unlock(&self) {
-        let mut guard = self.inner.lock();
-        if guard.lock_var().count > 1 {
-            guard.lock_var_mut().count -= 1;
-        } else {
-            match WaitQueue::notify_one(guard) {
-                Err(mut guard) => {
-                    // No other waiters, unlock
-                    guard.lock_var_mut().count = 0;
-                    guard.lock_var_mut().owner = None;
-                }
-                Ok(mut guard) => {
-                    // There was a thread waiting, just pass the lock
-                    if let NotifiedTcs::Single(tcs) = guard.notified_tcs() {
-                        guard.lock_var_mut().owner = Some(tcs)
-                    } else {
-                        unreachable!() // called notify_one
-                    }
-                }
-            }
-        }
-    }
-
-    #[inline]
-    pub unsafe fn try_lock(&self) -> bool {
-        let mut guard = try_lock_or_false!(self.inner);
-        match guard.lock_var().owner {
-            Some(tcs) if tcs != thread::current() => {
-                // Another thread has the lock
-                false
-            }
-            _ => {
-                // We are just now obtaining the lock
-                guard.lock_var_mut().owner = Some(thread::current());
-                guard.lock_var_mut().count += 1;
-                true
-            }
-        }
-    }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
-}
diff --git a/library/std/src/sys/unix/locks/futex.rs b/library/std/src/sys/unix/locks/futex.rs
index b166e7c453c..7a63af1ad7c 100644
--- a/library/std/src/sys/unix/locks/futex.rs
+++ b/library/std/src/sys/unix/locks/futex.rs
@@ -1,6 +1,5 @@
-use crate::cell::UnsafeCell;
 use crate::sync::atomic::{
-    AtomicU32, AtomicUsize,
+    AtomicU32,
     Ordering::{Acquire, Relaxed, Release},
 };
 use crate::sys::futex::{futex_wait, futex_wake, futex_wake_all};
@@ -163,98 +162,3 @@ impl Condvar {
         r
     }
 }
-
-/// A reentrant mutex. Used by stdout().lock() and friends.
-///
-/// The 'owner' field tracks which thread has locked the mutex.
-///
-/// We use current_thread_unique_ptr() as the thread identifier,
-/// which is just the address of a thread local variable.
-///
-/// If `owner` is set to the identifier of the current thread,
-/// we assume the mutex is already locked and instead of locking it again,
-/// we increment `lock_count`.
-///
-/// When unlocking, we decrement `lock_count`, and only unlock the mutex when
-/// it reaches zero.
-///
-/// `lock_count` is protected by the mutex and only accessed by the thread that has
-/// locked the mutex, so needs no synchronization.
-///
-/// `owner` can be checked by other threads that want to see if they already
-/// hold the lock, so needs to be atomic. If it compares equal, we're on the
-/// same thread that holds the mutex and memory access can use relaxed ordering
-/// since we're not dealing with multiple threads. If it compares unequal,
-/// synchronization is left to the mutex, making relaxed memory ordering for
-/// the `owner` field fine in all cases.
-pub struct ReentrantMutex {
-    mutex: Mutex,
-    owner: AtomicUsize,
-    lock_count: UnsafeCell<u32>,
-}
-
-unsafe impl Send for ReentrantMutex {}
-unsafe impl Sync for ReentrantMutex {}
-
-impl ReentrantMutex {
-    #[inline]
-    pub const unsafe fn uninitialized() -> Self {
-        Self { mutex: Mutex::new(), owner: AtomicUsize::new(0), lock_count: UnsafeCell::new(0) }
-    }
-
-    #[inline]
-    pub unsafe fn init(&self) {}
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
-
-    pub unsafe fn try_lock(&self) -> bool {
-        let this_thread = current_thread_unique_ptr();
-        if self.owner.load(Relaxed) == this_thread {
-            self.increment_lock_count();
-            true
-        } else if self.mutex.try_lock() {
-            self.owner.store(this_thread, Relaxed);
-            debug_assert_eq!(*self.lock_count.get(), 0);
-            *self.lock_count.get() = 1;
-            true
-        } else {
-            false
-        }
-    }
-
-    pub unsafe fn lock(&self) {
-        let this_thread = current_thread_unique_ptr();
-        if self.owner.load(Relaxed) == this_thread {
-            self.increment_lock_count();
-        } else {
-            self.mutex.lock();
-            self.owner.store(this_thread, Relaxed);
-            debug_assert_eq!(*self.lock_count.get(), 0);
-            *self.lock_count.get() = 1;
-        }
-    }
-
-    unsafe fn increment_lock_count(&self) {
-        *self.lock_count.get() = (*self.lock_count.get())
-            .checked_add(1)
-            .expect("lock count overflow in reentrant mutex");
-    }
-
-    pub unsafe fn unlock(&self) {
-        *self.lock_count.get() -= 1;
-        if *self.lock_count.get() == 0 {
-            self.owner.store(0, Relaxed);
-            self.mutex.unlock();
-        }
-    }
-}
-
-/// Get an address that is unique per running thread.
-///
-/// This can be used as a non-null usize-sized ID.
-pub fn current_thread_unique_ptr() -> usize {
-    // Use a non-drop type to make sure it's still available during thread destruction.
-    thread_local! { static X: u8 = const { 0 } }
-    X.with(|x| <*const _>::addr(x))
-}
diff --git a/library/std/src/sys/unix/locks/mod.rs b/library/std/src/sys/unix/locks/mod.rs
index e0404f40c69..17796f8894b 100644
--- a/library/std/src/sys/unix/locks/mod.rs
+++ b/library/std/src/sys/unix/locks/mod.rs
@@ -5,15 +5,13 @@ cfg_if::cfg_if! {
     ))] {
         mod futex;
         mod futex_rwlock;
-        pub use futex::{Mutex, MovableMutex, Condvar, MovableCondvar, ReentrantMutex};
+        pub use futex::{Mutex, MovableMutex, Condvar, MovableCondvar};
         pub use futex_rwlock::{RwLock, MovableRwLock};
     } else {
         mod pthread_mutex;
-        mod pthread_remutex;
         mod pthread_rwlock;
         mod pthread_condvar;
         pub use pthread_mutex::{Mutex, MovableMutex};
-        pub use pthread_remutex::ReentrantMutex;
         pub use pthread_rwlock::{RwLock, MovableRwLock};
         pub use pthread_condvar::{Condvar, MovableCondvar};
     }
diff --git a/library/std/src/sys/unix/locks/pthread_remutex.rs b/library/std/src/sys/unix/locks/pthread_remutex.rs
deleted file mode 100644
index b006181ee3a..00000000000
--- a/library/std/src/sys/unix/locks/pthread_remutex.rs
+++ /dev/null
@@ -1,46 +0,0 @@
-use super::pthread_mutex::PthreadMutexAttr;
-use crate::cell::UnsafeCell;
-use crate::mem::MaybeUninit;
-use crate::sys::cvt_nz;
-
-pub struct ReentrantMutex {
-    inner: UnsafeCell<libc::pthread_mutex_t>,
-}
-
-unsafe impl Send for ReentrantMutex {}
-unsafe impl Sync for ReentrantMutex {}
-
-impl ReentrantMutex {
-    pub const unsafe fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex { inner: UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER) }
-    }
-
-    pub unsafe fn init(&self) {
-        let mut attr = MaybeUninit::<libc::pthread_mutexattr_t>::uninit();
-        cvt_nz(libc::pthread_mutexattr_init(attr.as_mut_ptr())).unwrap();
-        let attr = PthreadMutexAttr(&mut attr);
-        cvt_nz(libc::pthread_mutexattr_settype(attr.0.as_mut_ptr(), libc::PTHREAD_MUTEX_RECURSIVE))
-            .unwrap();
-        cvt_nz(libc::pthread_mutex_init(self.inner.get(), attr.0.as_ptr())).unwrap();
-    }
-
-    pub unsafe fn lock(&self) {
-        let result = libc::pthread_mutex_lock(self.inner.get());
-        debug_assert_eq!(result, 0);
-    }
-
-    #[inline]
-    pub unsafe fn try_lock(&self) -> bool {
-        libc::pthread_mutex_trylock(self.inner.get()) == 0
-    }
-
-    pub unsafe fn unlock(&self) {
-        let result = libc::pthread_mutex_unlock(self.inner.get());
-        debug_assert_eq!(result, 0);
-    }
-
-    pub unsafe fn destroy(&self) {
-        let result = libc::pthread_mutex_destroy(self.inner.get());
-        debug_assert_eq!(result, 0);
-    }
-}
diff --git a/library/std/src/sys/unsupported/locks/mutex.rs b/library/std/src/sys/unsupported/locks/mutex.rs
index b3203c16c50..cad991aae5e 100644
--- a/library/std/src/sys/unsupported/locks/mutex.rs
+++ b/library/std/src/sys/unsupported/locks/mutex.rs
@@ -36,26 +36,3 @@ impl Mutex {
     #[inline]
     pub unsafe fn destroy(&self) {}
 }
-
-// All empty stubs because this platform does not yet support threads, so lock
-// acquisition always succeeds.
-pub struct ReentrantMutex {}
-
-impl ReentrantMutex {
-    pub const unsafe fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex {}
-    }
-
-    pub unsafe fn init(&self) {}
-
-    pub unsafe fn lock(&self) {}
-
-    #[inline]
-    pub unsafe fn try_lock(&self) -> bool {
-        true
-    }
-
-    pub unsafe fn unlock(&self) {}
-
-    pub unsafe fn destroy(&self) {}
-}
diff --git a/library/std/src/sys/wasm/atomics/mutex.rs b/library/std/src/sys/wasm/atomics/mutex.rs
index 3a09f0bf9bb..1acc8392444 100644
--- a/library/std/src/sys/wasm/atomics/mutex.rs
+++ b/library/std/src/sys/wasm/atomics/mutex.rs
@@ -1,8 +1,6 @@
 use crate::arch::wasm32;
-use crate::cell::UnsafeCell;
 use crate::mem;
-use crate::sync::atomic::{AtomicU32, AtomicUsize, Ordering::SeqCst};
-use crate::sys::thread;
+use crate::sync::atomic::{AtomicUsize, Ordering::SeqCst};
 
 pub struct Mutex {
     locked: AtomicUsize,
@@ -64,93 +62,3 @@ impl Mutex {
         self.locked.as_mut_ptr() as *mut i32
     }
 }
-
-pub struct ReentrantMutex {
-    owner: AtomicU32,
-    recursions: UnsafeCell<u32>,
-}
-
-unsafe impl Send for ReentrantMutex {}
-unsafe impl Sync for ReentrantMutex {}
-
-// Reentrant mutexes are similarly implemented to mutexes above except that
-// instead of "1" meaning unlocked we use the id of a thread to represent
-// whether it has locked a mutex. That way we have an atomic counter which
-// always holds the id of the thread that currently holds the lock (or 0 if the
-// lock is unlocked).
-//
-// Once a thread acquires a lock recursively, which it detects by looking at
-// the value that's already there, it will update a local `recursions` counter
-// in a nonatomic fashion (as we hold the lock). The lock is then fully
-// released when this recursion counter reaches 0.
-
-impl ReentrantMutex {
-    pub const unsafe fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex { owner: AtomicU32::new(0), recursions: UnsafeCell::new(0) }
-    }
-
-    pub unsafe fn init(&self) {
-        // nothing to do...
-    }
-
-    pub unsafe fn lock(&self) {
-        let me = thread::my_id();
-        while let Err(owner) = self._try_lock(me) {
-            // SAFETY: the caller must guarantee that `self.ptr()` and `owner` are valid i32.
-            let val = unsafe { wasm32::memory_atomic_wait32(self.ptr(), owner as i32, -1) };
-            debug_assert!(val == 0 || val == 1);
-        }
-    }
-
-    #[inline]
-    pub unsafe fn try_lock(&self) -> bool {
-        unsafe { self._try_lock(thread::my_id()).is_ok() }
-    }
-
-    #[inline]
-    unsafe fn _try_lock(&self, id: u32) -> Result<(), u32> {
-        let id = id.checked_add(1).unwrap();
-        match self.owner.compare_exchange(0, id, SeqCst, SeqCst) {
-            // we transitioned from unlocked to locked
-            Ok(_) => {
-                debug_assert_eq!(*self.recursions.get(), 0);
-                Ok(())
-            }
-
-            // we currently own this lock, so let's update our count and return
-            // true.
-            Err(n) if n == id => {
-                *self.recursions.get() += 1;
-                Ok(())
-            }
-
-            // Someone else owns the lock, let our caller take care of it
-            Err(other) => Err(other),
-        }
-    }
-
-    pub unsafe fn unlock(&self) {
-        // If we didn't ever recursively lock the lock then we fully unlock the
-        // mutex and wake up a waiter, if any. Otherwise we decrement our
-        // recursive counter and let some one else take care of the zero.
-        match *self.recursions.get() {
-            0 => {
-                self.owner.swap(0, SeqCst);
-                // SAFETY: the caller must guarantee that `self.ptr()` is valid i32.
-                unsafe {
-                    wasm32::memory_atomic_notify(self.ptr() as *mut i32, 1);
-                } // wake up one waiter, if any
-            }
-            ref mut n => *n -= 1,
-        }
-    }
-
-    pub unsafe fn destroy(&self) {
-        // nothing to do...
-    }
-
-    #[inline]
-    fn ptr(&self) -> *mut i32 {
-        self.owner.as_mut_ptr() as *mut i32
-    }
-}
diff --git a/library/std/src/sys/windows/c.rs b/library/std/src/sys/windows/c.rs
index 336264d99f9..5f14edaf067 100644
--- a/library/std/src/sys/windows/c.rs
+++ b/library/std/src/sys/windows/c.rs
@@ -536,15 +536,6 @@ pub struct CONDITION_VARIABLE {
 pub struct SRWLOCK {
     pub ptr: LPVOID,
 }
-#[repr(C)]
-pub struct CRITICAL_SECTION {
-    CriticalSectionDebug: LPVOID,
-    LockCount: LONG,
-    RecursionCount: LONG,
-    OwningThread: HANDLE,
-    LockSemaphore: HANDLE,
-    SpinCount: ULONG_PTR,
-}
 
 #[repr(C)]
 pub struct REPARSE_MOUNTPOINT_DATA_BUFFER {
@@ -875,11 +866,6 @@ if #[cfg(target_vendor = "uwp")] {
 #[link(name = "kernel32")]
 extern "system" {
     pub fn GetCurrentProcessId() -> DWORD;
-    pub fn InitializeCriticalSection(CriticalSection: *mut CRITICAL_SECTION);
-    pub fn EnterCriticalSection(CriticalSection: *mut CRITICAL_SECTION);
-    pub fn TryEnterCriticalSection(CriticalSection: *mut CRITICAL_SECTION) -> BOOL;
-    pub fn LeaveCriticalSection(CriticalSection: *mut CRITICAL_SECTION);
-    pub fn DeleteCriticalSection(CriticalSection: *mut CRITICAL_SECTION);
 
     pub fn GetSystemDirectoryW(lpBuffer: LPWSTR, uSize: UINT) -> UINT;
     pub fn RemoveDirectoryW(lpPathName: LPCWSTR) -> BOOL;
diff --git a/library/std/src/sys/windows/locks/mod.rs b/library/std/src/sys/windows/locks/mod.rs
index 35bd5913034..d412ff152a0 100644
--- a/library/std/src/sys/windows/locks/mod.rs
+++ b/library/std/src/sys/windows/locks/mod.rs
@@ -2,5 +2,5 @@ mod condvar;
 mod mutex;
 mod rwlock;
 pub use condvar::{Condvar, MovableCondvar};
-pub use mutex::{MovableMutex, Mutex, ReentrantMutex};
+pub use mutex::{MovableMutex, Mutex};
 pub use rwlock::{MovableRwLock, RwLock};
diff --git a/library/std/src/sys/windows/locks/mutex.rs b/library/std/src/sys/windows/locks/mutex.rs
index 56f91ebe582..9fa280b8b76 100644
--- a/library/std/src/sys/windows/locks/mutex.rs
+++ b/library/std/src/sys/windows/locks/mutex.rs
@@ -15,7 +15,6 @@
 //!    is that there are no guarantees of fairness.
 
 use crate::cell::UnsafeCell;
-use crate::mem::MaybeUninit;
 use crate::sys::c;
 
 pub struct Mutex {
@@ -60,37 +59,3 @@ impl Mutex {
         // SRWLock does not need to be destroyed.
     }
 }
-
-pub struct ReentrantMutex {
-    inner: MaybeUninit<UnsafeCell<c::CRITICAL_SECTION>>,
-}
-
-unsafe impl Send for ReentrantMutex {}
-unsafe impl Sync for ReentrantMutex {}
-
-impl ReentrantMutex {
-    pub const fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex { inner: MaybeUninit::uninit() }
-    }
-
-    pub unsafe fn init(&self) {
-        c::InitializeCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr()));
-    }
-
-    pub unsafe fn lock(&self) {
-        c::EnterCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr()));
-    }
-
-    #[inline]
-    pub unsafe fn try_lock(&self) -> bool {
-        c::TryEnterCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr())) != 0
-    }
-
-    pub unsafe fn unlock(&self) {
-        c::LeaveCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr()));
-    }
-
-    pub unsafe fn destroy(&self) {
-        c::DeleteCriticalSection(UnsafeCell::raw_get(self.inner.as_ptr()));
-    }
-}
diff --git a/library/std/src/sys_common/remutex.rs b/library/std/src/sys_common/remutex.rs
index 801c9c28dd3..8f252308de7 100644
--- a/library/std/src/sys_common/remutex.rs
+++ b/library/std/src/sys_common/remutex.rs
@@ -1,10 +1,12 @@
 #[cfg(all(test, not(target_os = "emscripten")))]
 mod tests;
 
+use crate::cell::UnsafeCell;
 use crate::marker::PhantomPinned;
 use crate::ops::Deref;
 use crate::panic::{RefUnwindSafe, UnwindSafe};
 use crate::pin::Pin;
+use crate::sync::atomic::{AtomicUsize, Ordering::Relaxed};
 use crate::sys::locks as sys;
 
 /// A re-entrant mutual exclusion
@@ -12,8 +14,36 @@ use crate::sys::locks as sys;
 /// This mutex will block *other* threads waiting for the lock to become
 /// available. The thread which has already locked the mutex can lock it
 /// multiple times without blocking, preventing a common source of deadlocks.
+///
+/// This is used by stdout().lock() and friends.
+///
+/// ## Implementation details
+///
+/// The 'owner' field tracks which thread has locked the mutex.
+///
+/// We use current_thread_unique_ptr() as the thread identifier,
+/// which is just the address of a thread local variable.
+///
+/// If `owner` is set to the identifier of the current thread,
+/// we assume the mutex is already locked and instead of locking it again,
+/// we increment `lock_count`.
+///
+/// When unlocking, we decrement `lock_count`, and only unlock the mutex when
+/// it reaches zero.
+///
+/// `lock_count` is protected by the mutex and only accessed by the thread that has
+/// locked the mutex, so needs no synchronization.
+///
+/// `owner` can be checked by other threads that want to see if they already
+/// hold the lock, so needs to be atomic. If it compares equal, we're on the
+/// same thread that holds the mutex and memory access can use relaxed ordering
+/// since we're not dealing with multiple threads. If it compares unequal,
+/// synchronization is left to the mutex, making relaxed memory ordering for
+/// the `owner` field fine in all cases.
 pub struct ReentrantMutex<T> {
-    inner: sys::ReentrantMutex,
+    mutex: sys::Mutex,
+    owner: AtomicUsize,
+    lock_count: UnsafeCell<u32>,
     data: T,
     _pinned: PhantomPinned,
 }
@@ -53,7 +83,9 @@ impl<T> ReentrantMutex<T> {
     /// lock/unlock methods safe.
     pub const unsafe fn new(t: T) -> ReentrantMutex<T> {
         ReentrantMutex {
-            inner: sys::ReentrantMutex::uninitialized(),
+            mutex: sys::Mutex::new(),
+            owner: AtomicUsize::new(0),
+            lock_count: UnsafeCell::new(0),
             data: t,
             _pinned: PhantomPinned,
         }
@@ -66,7 +98,7 @@ impl<T> ReentrantMutex<T> {
     /// Unsafe to call more than once, and must be called after this will no
     /// longer move in memory.
     pub unsafe fn init(self: Pin<&mut Self>) {
-        self.get_unchecked_mut().inner.init()
+        self.get_unchecked_mut().mutex.init()
     }
 
     /// Acquires a mutex, blocking the current thread until it is able to do so.
@@ -82,7 +114,19 @@ impl<T> ReentrantMutex<T> {
     /// this call will return failure if the mutex would otherwise be
     /// acquired.
     pub fn lock(self: Pin<&Self>) -> ReentrantMutexGuard<'_, T> {
-        unsafe { self.inner.lock() }
+        let this_thread = current_thread_unique_ptr();
+        // Safety: We only touch lock_count when we own the lock,
+        // and since self is pinned we can safely call the lock() on the mutex.
+        unsafe {
+            if self.owner.load(Relaxed) == this_thread {
+                self.increment_lock_count();
+            } else {
+                self.mutex.lock();
+                self.owner.store(this_thread, Relaxed);
+                debug_assert_eq!(*self.lock_count.get(), 0);
+                *self.lock_count.get() = 1;
+            }
+        }
         ReentrantMutexGuard { lock: self }
     }
 
@@ -99,20 +143,35 @@ impl<T> ReentrantMutex<T> {
     /// this call will return failure if the mutex would otherwise be
     /// acquired.
     pub fn try_lock(self: Pin<&Self>) -> Option<ReentrantMutexGuard<'_, T>> {
-        if unsafe { self.inner.try_lock() } {
-            Some(ReentrantMutexGuard { lock: self })
-        } else {
-            None
+        let this_thread = current_thread_unique_ptr();
+        // Safety: We only touch lock_count when we own the lock,
+        // and since self is pinned we can safely call the try_lock on the mutex.
+        unsafe {
+            if self.owner.load(Relaxed) == this_thread {
+                self.increment_lock_count();
+                Some(ReentrantMutexGuard { lock: self })
+            } else if self.mutex.try_lock() {
+                self.owner.store(this_thread, Relaxed);
+                debug_assert_eq!(*self.lock_count.get(), 0);
+                *self.lock_count.get() = 1;
+                Some(ReentrantMutexGuard { lock: self })
+            } else {
+                None
+            }
         }
     }
+
+    unsafe fn increment_lock_count(&self) {
+        *self.lock_count.get() = (*self.lock_count.get())
+            .checked_add(1)
+            .expect("lock count overflow in reentrant mutex");
+    }
 }
 
 impl<T> Drop for ReentrantMutex<T> {
     fn drop(&mut self) {
-        // This is actually safe b/c we know that there is no further usage of
-        // this mutex (it's up to the user to arrange for a mutex to get
-        // dropped, that's not our job)
-        unsafe { self.inner.destroy() }
+        // Safety: We're the unique owner of this mutex and not going to use it afterwards.
+        unsafe { self.mutex.destroy() }
     }
 }
 
@@ -127,8 +186,22 @@ impl<T> Deref for ReentrantMutexGuard<'_, T> {
 impl<T> Drop for ReentrantMutexGuard<'_, T> {
     #[inline]
     fn drop(&mut self) {
+        // Safety: We own the lock, and the lock is pinned.
         unsafe {
-            self.lock.inner.unlock();
+            *self.lock.lock_count.get() -= 1;
+            if *self.lock.lock_count.get() == 0 {
+                self.lock.owner.store(0, Relaxed);
+                self.lock.mutex.unlock();
+            }
         }
     }
 }
+
+/// Get an address that is unique per running thread.
+///
+/// This can be used as a non-null usize-sized ID.
+pub fn current_thread_unique_ptr() -> usize {
+    // Use a non-drop type to make sure it's still available during thread destruction.
+    thread_local! { static X: u8 = const { 0 } }
+    X.with(|x| <*const _>::addr(x))
+}