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/*
* Copyright (c) 2020, Andrew Kaster <andrewdkaster@gmail.com>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include <AK/Atomic.h>
#include <serenity.h>
#include <stdint.h>
#include <sys/types.h>
// Static variable guard logic, using futexes
// Written while looking at libc++ from a distance ;) This is super tricky...
// From itanium spec: Get passed a pointer to a 64 bit guard "object"
// [ 1 guard byte (checked by compiler), 7 bytes to work with ]
class CxaGuard {
public:
enum GuardState : uint8_t {
NotInitialized = 0,
DoneInit = 1,
InitInProgress = 2,
WaitingOnInit = 4
};
CxaGuard(uint64_t* guard_pointer)
: m_full_guard_var(guard_pointer)
, m_compiler_guard_byte((uint8_t*)guard_pointer)
, m_local_state_byte(&((uint8_t*)guard_pointer)[1])
{
}
int acquire()
{
// Someone else already initailzied this by storing to the 'really done' slot
if (GuardState::NotInitialized != m_compiler_guard_byte.load(AK::memory_order_acquire))
return 0;
// Time to begin the waiting game..
while (true) {
// Try to claim initialization for this thread from the initial state
uint8_t last_state = GuardState::NotInitialized;
if (m_local_state_byte.compare_exchange_strong(last_state, GuardState::InitInProgress, AK::memory_order_acq_rel))
return 1; // Tell compiler-generated code to init the variable
// Someone else set the state to done, we're done here
if (last_state == DoneInit)
return 0;
// Someone else set InitInProgress, time to wait (maybe)
if (last_state & GuardState::InitInProgress) {
// Try to set the in progress + done bits
if ((last_state & GuardState::WaitingOnInit) == 0) {
if (!m_local_state_byte.compare_exchange_strong(last_state, (GuardState::InitInProgress | GuardState::WaitingOnInit), AK::memory_order_acq_rel)) {
if (last_state == GuardState::DoneInit)
return 0;
if (last_state == GuardState::NotInitialized)
continue; // start over, and try to initialize in this thread
// else wait, someone else set the wait bit before us
}
}
// State is InitInProgress | WaitingOnInit, time to sleep
wait_on_futex();
}
}
}
void release()
{
// Store the value the compiler will look at, we're done! yay
m_compiler_guard_byte.store(GuardState::DoneInit, AK::memory_order_release);
// Set the value other threads chilling in our loop will look at to done
uint8_t old_local_state = m_local_state_byte.exchange(GuardState::DoneInit, AK::memory_order_acq_rel);
// Wake up sleepy heads, the variable is initialized!
if (old_local_state & GuardState::WaitingOnInit)
wake_futex();
}
void abort()
{
uint8_t old_local_state = m_local_state_byte.exchange(GuardState::NotInitialized, AK::memory_order_acq_rel);
// Wake up sleepy heads, someone else gets to try...
if (old_local_state & GuardState::WaitingOnInit)
wake_futex();
}
private:
void wait_on_futex()
{
int futex_value = (GuardState::InitInProgress | GuardState::WaitingOnInit) << 8;
futex((int32_t*)m_full_guard_var, FUTEX_WAIT, futex_value, nullptr);
}
void wake_futex()
{
futex((int32_t*)m_full_guard_var, FUTEX_WAKE, INT32_MAX, nullptr);
}
void* m_full_guard_var;
AtomicRef<uint8_t> m_compiler_guard_byte;
AtomicRef<uint8_t> m_local_state_byte; // we'll load and set this atomically
// Note that we're not using 6/8 bytes of the guard object...
};
extern "C" {
int __cxa_guard_acquire(uint64_t* guard_object)
{
CxaGuard g(guard_object);
return g.acquire();
}
void __cxa_guard_release(uint64_t* guard_object)
{
CxaGuard g(guard_object);
g.release();
}
void __cxa_guard_abort(uint64_t* guard_object)
{
CxaGuard g(guard_object);
g.abort();
}
} // extern "C"
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