RedBear-OS/src/header/pthread/rwlock.rs

use super::*;

use core::sync::atomic::{AtomicI32 as AtomicInt, Ordering};

use crate::header::errno::EBUSY;

// PTHREAD_RWLOCK_INITIALIZER

// TODO: Optimize for short waits and long waits, using AtomicLock::wait_until, but still
// supporting timeouts.
// TODO: Add futex ops that use bitmasks.

const EXCLUSIVE: u32 = (1 << (u32::BITS - 1)) - 1;
// Separate "waiting for wrlocks" and "waiting for rdlocks"?
//const WAITING: u32 = 1 << (u32::BITS - 1);

#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_destroy(rwlock: *mut pthread_rwlock_t) -> c_int {
    // (Informing the compiler that this pointer is valid, might improve optimizations.)
    let _rwlock: &pthread_rwlock_t = &*rwlock;
    0
}
#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_init(rwlock: *mut pthread_rwlock_t, attr: *const pthread_rwlockattr_t) -> c_int {
    let attr = attr.cast::<RlctRwlockAttr>().as_ref();

    rwlock.cast::<RlctRwlock>().write(RlctRwlock {
        state: AtomicInt::new(0),
    });

    0
}
#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_rdlock(rwlock: *mut pthread_rwlock_t) -> c_int {
    pthread_rwlock_timedrdlock(rwlock, core::ptr::null())
}
#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_timedrdlock(rwlock: *mut pthread_rwlock_t, timeout: *const timespec) -> c_int {
    let rwlock = &*rwlock.cast::<RlctRwlock>();
    let timeout = timeout.as_ref();

    loop {
        if pthread_rwlock_tryrdlock(rwlock as *const _ as *mut _) == EBUSY {
            crate::sync::futex_wait(&rwlock.state, EXCLUSIVE as i32, timeout);
        }
        return 0;
    }
}
#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_timedwrlock(rwlock: *mut pthread_rwlock_t, timeout: *const timespec) -> c_int {
    let rwlock = &*rwlock.cast::<RlctRwlock>();
    let timeout = timeout.as_ref();

    /*loop {
        if pthread_rwlock_trywrlock(rwlock as *const _ as *mut _) == EBUSY {
            crate::sync::futex_wait(&rwlock.state, EXCLUSIVE as i32, timeout);
        }
        return 0;
    }*/
    loop {
        match rwlock.state.compare_exchange(0, EXCLUSIVE as i32, Ordering::Acquire, Ordering::Relaxed) {
            Ok(_) => return 0,
            Err(value) => {
                // TODO: More than just forwarding the timeout.
                crate::sync::futex_wait(&rwlock.state, value, timeout);
            }
        }
    }
}
#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_tryrdlock(rwlock: *mut pthread_rwlock_t) -> c_int {
    let rwlock = &*rwlock.cast::<RlctRwlock>();

    let mut cached = rwlock.state.load(Ordering::Acquire) as u32;

    loop {
        let old = if cached == EXCLUSIVE { 0 } else { cached };
        let new = old + 1;

        assert_ne!(new, EXCLUSIVE, "overflow");

        match rwlock.state.compare_exchange_weak(cached as i32, new as i32, Ordering::Acquire, Ordering::Relaxed) {
            Ok(_) => return 0,
            Err(value) if value as u32 == EXCLUSIVE => return EBUSY,
            Err(value) => {
                cached = value as u32;
                core::hint::spin_loop();
            }
        }
    }
}
#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_trywrlock(rwlock: *mut pthread_rwlock_t) -> c_int {
    let rwlock = &*rwlock.cast::<RlctRwlock>();

    match rwlock.state.compare_exchange(0, EXCLUSIVE as i32, Ordering::Acquire, Ordering::Relaxed) {
        Ok(_) => 0,
        Err(_) => EBUSY,
    }
}
#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_unlock(rwlock: *const pthread_rwlock_t) -> c_int {
    let rwlock = &*rwlock.cast::<RlctRwlock>();

    let old = rwlock.state.swap(0, Ordering::Release) as u32;

    if old == EXCLUSIVE {
        crate::sync::futex_wake(&rwlock.state, i32::MAX);
    }

    0
}
#[no_mangle]
pub unsafe extern "C" fn pthread_rwlock_wrlock(rwlock: *mut pthread_rwlock_t) -> c_int {
    pthread_rwlock_timedwrlock(rwlock, core::ptr::null())
}

#[no_mangle]
pub unsafe extern "C" fn pthread_rwlockattr_destroy(attr: *mut pthread_rwlockattr_t) -> c_int {
    let _attr = &mut *attr.cast::<RlctRwlockAttr>();

    // No-op
    0
}

#[no_mangle]
pub unsafe extern "C" fn pthread_rwlockattr_getpshared(attr: *const pthread_rwlockattr_t, pshared: *mut c_int) -> c_int {
    core::ptr::write(pshared, (*attr.cast::<RlctRwlockAttr>()).pshared);

    0
}

#[no_mangle]
pub unsafe extern "C" fn pthread_rwlockattr_init(attr: *mut pthread_rwlockattr_t) -> c_int {
    attr.cast::<RlctRwlockAttr>().write(RlctRwlockAttr {
        // Default according to POSIX.
        pshared: PTHREAD_PROCESS_PRIVATE,
    });

    0
}

#[no_mangle]
pub unsafe extern "C" fn pthread_rwlockattr_setpshared(attr: *mut pthread_rwlockattr_t, pshared: c_int) -> c_int {
    (*attr.cast::<RlctRwlockAttr>()).pshared = pshared;
    0
}

pub(crate) struct RlctRwlockAttr {
    pub pshared: c_int,
}
pub(crate) struct RlctRwlock {
    pub state: AtomicInt,
}