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AK: Make Weakable non-atomic

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Let's not punish single-threaded workloads with the performance cost of
atomic weakables. The kernel keeps using LockWeakable.
This commit is contained in:
Andreas Kling 2022-09-01 18:37:58 +02:00
parent 159f9688dc
commit 53c0038d2c
1 changed files with 6 additions and 40 deletions
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46
AK/Weakable.h
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@ -7,11 +7,9 @@
#pragma once #pragma once
#include <AK/Assertions.h> #include <AK/Assertions.h>
#include <AK/Atomic.h>
#include <AK/RefCounted.h> #include <AK/RefCounted.h>
#include <AK/RefPtr.h> #include <AK/RefPtr.h>
#include <AK/StdLibExtras.h> #include <AK/StdLibExtras.h>
#include <sched.h>
namespace AK { namespace AK {
@ -31,50 +29,18 @@ public:
RefPtr<T> strong_ref() const RefPtr<T> strong_ref() const
requires(IsBaseOf<RefCountedBase, T>) requires(IsBaseOf<RefCountedBase, T>)
{ {
RefPtr<T> ref; return static_cast<T*>(m_ptr);
{
if (!(m_consumers.fetch_add(1u << 1, AK::MemoryOrder::memory_order_acquire) & 1u)) {
T* ptr = (T*)m_ptr.load(AK::MemoryOrder::memory_order_acquire);
if (ptr && ptr->try_ref())
ref = adopt_ref(*ptr);
}
m_consumers.fetch_sub(1u << 1, AK::MemoryOrder::memory_order_release);
}
return ref;
} }
template<typename T> template<typename T>
T* unsafe_ptr() const T* unsafe_ptr() const
{ {
if (m_consumers.load(AK::MemoryOrder::memory_order_relaxed) & 1u) return static_cast<T*>(m_ptr);
return nullptr;
// NOTE: This may return a non-null pointer even if revocation
// has been triggered as there is a possible race! But it's "unsafe"
// anyway because we return a raw pointer without ensuring a
// reference...
return (T*)m_ptr.load(AK::MemoryOrder::memory_order_acquire);
} }
bool is_null() const bool is_null() const { return m_ptr == nullptr; }
{
return unsafe_ptr<void>() == nullptr;
}
void revoke() void revoke() { m_ptr = nullptr; }
{
auto current_consumers = m_consumers.fetch_or(1u, AK::MemoryOrder::memory_order_relaxed);
VERIFY(!(current_consumers & 1u));
// We flagged revocation, now wait until everyone trying to obtain
// a strong reference is done
while (current_consumers > 0) {
sched_yield();
current_consumers = m_consumers.load(AK::MemoryOrder::memory_order_acquire) & ~1u;
}
// No one is trying to use it (anymore)
m_ptr.store(nullptr, AK::MemoryOrder::memory_order_release);
}
private: private:
template<typename T> template<typename T>
@ -82,8 +48,8 @@ private:
: m_ptr(&weakable) : m_ptr(&weakable)
{ {
} }
mutable Atomic<void*> m_ptr;
mutable Atomic<unsigned> m_consumers; // LSB indicates revocation in progress mutable void* m_ptr { nullptr };
}; };
template<typename T> template<typename T>