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AK: Make RefPtr, NonnullRefPtr, WeakPtr thread safe

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This makes most operations thread safe, especially so that they
can safely be used in the Kernel. This includes obtaining a strong
reference from a weak reference, which now requires an explicit
call to WeakPtr::strong_ref(). Another major change is that
Weakable::make_weak_ref() may require the explicit target type.
Previously we used reinterpret_cast in WeakPtr, assuming that it
can be properly converted. But WeakPtr does not necessarily have
the knowledge to be able to do this. Instead, we now ask the class
itself to deliver a WeakPtr to the type that we want.

Also, WeakLink is no longer specific to a target type. The reason
for this is that we want to be able to safely convert e.g. WeakPtr<T>
to WeakPtr<U>, and before this we just reinterpret_cast the internal
WeakLink<T> to WeakLink<U>, which is a bold assumption that it would
actually produce the correct code. Instead, WeakLink now operates
on just a raw pointer and we only make those constructors/operators
available if we can verify that it can be safely cast.

In order to guarantee thread safety, we now use the least significant
bit in the pointer for locking purposes. This also means that only
properly aligned pointers can be used.
This commit is contained in:
Tom 2020-09-29 16:26:13 -06:00 • committed by Andreas Kling
parent 3c1ef744f6
commit 75f61fe3d9
50 changed files with 819 additions and 322 deletions
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329
AK/RefPtr.h
View file

@ -26,11 +26,15 @@
#pragma once
#include <AK/Atomic.h>
#include <AK/LogStream.h>
#include <AK/NonnullRefPtr.h>
#include <AK/StdLibExtras.h>
#include <AK/Traits.h>
#include <AK/Types.h>
#ifdef KERNEL
# include <Kernel/Arch/i386/CPU.h>
#endif
namespace AK {
@ -39,19 +43,87 @@ class OwnPtr;
template<typename T>
struct RefPtrTraits {
static T* as_ptr(FlatPtr bits)
ALWAYS_INLINE static T* as_ptr(FlatPtr bits)
{
return (T*)bits;
return (T*)(bits & ~(FlatPtr)1);
}
static FlatPtr as_bits(T* ptr)
ALWAYS_INLINE static FlatPtr as_bits(T* ptr)
{
ASSERT(!((FlatPtr)ptr & 1));
return (FlatPtr)ptr;
}
static bool is_null(FlatPtr bits)
template<typename U, typename PtrTraits>
ALWAYS_INLINE static FlatPtr convert_from(FlatPtr bits)
{
return !bits;
if (PtrTraits::is_null(bits))
return default_null_value;
return as_bits(PtrTraits::as_ptr(bits));
}
ALWAYS_INLINE static bool is_null(FlatPtr bits)
{
return !(bits & ~(FlatPtr)1);
}
ALWAYS_INLINE static FlatPtr exchange(Atomic<FlatPtr>& atomic_var, FlatPtr new_value)
{
// Only exchange when lock is not held
ASSERT(!(new_value & 1));
FlatPtr expected = atomic_var.load(AK::MemoryOrder::memory_order_relaxed);
for (;;) {
expected &= ~(FlatPtr)1; // only if lock bit is not set
if (atomic_var.compare_exchange_strong(expected, new_value, AK::MemoryOrder::memory_order_acq_rel))
break;
#ifdef KERNEL
Kernel::Processor::wait_check();
#endif
}
return expected;
}
ALWAYS_INLINE static bool exchange_if_null(Atomic<FlatPtr>& atomic_var, FlatPtr new_value)
{
// Only exchange when lock is not held
ASSERT(!(new_value & 1));
for (;;) {
FlatPtr expected = default_null_value; // only if lock bit is not set
if (atomic_var.compare_exchange_strong(expected, new_value, AK::MemoryOrder::memory_order_acq_rel))
break;
if (!is_null(expected))
return false;
#ifdef KERNEL
Kernel::Processor::wait_check();
#endif
}
return true;
}
ALWAYS_INLINE static FlatPtr lock(Atomic<FlatPtr>& atomic_var)
{
// This sets the lock bit atomically, preventing further modifications.
// This is important when e.g. copying a RefPtr where the source
// might be released and freed too quickly. This allows us
// to temporarily lock the pointer so we can add a reference, then
// unlock it
FlatPtr bits;
for (;;) {
bits = atomic_var.fetch_or(1, AK::MemoryOrder::memory_order_acq_rel);
if (!(bits & 1))
break;
#ifdef KERNEL
Kernel::Processor::wait_check();
#endif
}
ASSERT(!(bits & 1));
return bits;
}
ALWAYS_INLINE static void unlock(Atomic<FlatPtr>& atomic_var, FlatPtr new_value)
{
ASSERT(!(new_value & 1));
atomic_var.store(new_value, AK::MemoryOrder::memory_order_release);
}
static constexpr FlatPtr default_null_value = 0;
@ -63,6 +135,9 @@ template<typename T, typename PtrTraits>
class RefPtr {
template<typename U, typename P>
friend class RefPtr;
template<typename U>
friend class WeakPtr;
public:
enum AdoptTag {
Adopt
@ -79,62 +154,55 @@ public:
{
T* ptr = const_cast<T*>(&object);
ASSERT(ptr);
ASSERT(!ptr == PtrTraits::is_null(m_bits));
ASSERT(!is_null());
ptr->ref();
}
RefPtr(AdoptTag, T& object)
: m_bits(PtrTraits::as_bits(&object))
{
ASSERT(&object);
ASSERT(!PtrTraits::is_null(m_bits));
ASSERT(!is_null());
}
RefPtr(RefPtr&& other)
: m_bits(other.leak_ref_raw())
{
}
ALWAYS_INLINE RefPtr(const NonnullRefPtr<T>& other)
: m_bits(PtrTraits::as_bits(const_cast<T*>(other.ptr())))
: m_bits(PtrTraits::as_bits(const_cast<T*>(other.add_ref())))
{
ASSERT(!PtrTraits::is_null(m_bits));
PtrTraits::as_ptr(m_bits)->ref();
}
template<typename U>
ALWAYS_INLINE RefPtr(const NonnullRefPtr<U>& other)
: m_bits(PtrTraits::as_bits(const_cast<U*>(other.ptr())))
: m_bits(PtrTraits::as_bits(const_cast<U*>(other.add_ref())))
{
ASSERT(!PtrTraits::is_null(m_bits));
PtrTraits::as_ptr(m_bits)->ref();
}
template<typename U>
ALWAYS_INLINE RefPtr(NonnullRefPtr<U>&& other)
: m_bits(PtrTraits::as_bits(&other.leak_ref()))
{
ASSERT(!PtrTraits::is_null(m_bits));
ASSERT(!is_null());
}
template<typename U, typename P = RefPtrTraits<U>>
RefPtr(RefPtr<U, P>&& other)
: m_bits(other.leak_ref_raw())
: m_bits(PtrTraits::template convert_from<U, P>(other.leak_ref_raw()))
{
}
RefPtr(const RefPtr& other)
: m_bits(PtrTraits::as_bits(const_cast<T*>(other.ptr())))
: m_bits(other.add_ref_raw())
{
ref_if_not_null(const_cast<T*>(other.ptr()));
}
template<typename U, typename P = RefPtrTraits<U>>
RefPtr(const RefPtr<U, P>& other)
: m_bits(PtrTraits::as_bits(const_cast<U*>(other.ptr())))
: m_bits(other.add_ref_raw())
{
ref_if_not_null(const_cast<U*>(other.ptr()));
}
ALWAYS_INLINE ~RefPtr()
{
clear();
#ifdef SANITIZE_PTRS
if constexpr (sizeof(T*) == 8)
m_bits = 0xe0e0e0e0e0e0e0e0;
m_bits.store(0xe0e0e0e0e0e0e0e0, AK::MemoryOrder::memory_order_relaxed);
else
m_bits = 0xe0e0e0e0;
m_bits.store(0xe0e0e0e0, AK::MemoryOrder::memory_order_relaxed);
#endif
}
RefPtr(std::nullptr_t) { }
@ -144,79 +212,85 @@ public:
template<typename U>
RefPtr& operator=(const OwnPtr<U>&) = delete;
template<typename U>
void swap(RefPtr<U, PtrTraits>& other)
void swap(RefPtr& other)
{
::swap(m_bits, other.m_bits);
if (this == &other)
return;
// NOTE: swap is not atomic!
FlatPtr other_bits = PtrTraits::exchange(other.m_bits, PtrTraits::default_null_value);
FlatPtr bits = PtrTraits::exchange(m_bits, other_bits);
PtrTraits::exchange(other.m_bits, bits);
}
template<typename U, typename P = RefPtrTraits<U>>
void swap(RefPtr<U, P>& other)
{
// NOTE: swap is not atomic!
FlatPtr other_bits = P::exchange(other.m_bits, P::default_null_value);
FlatPtr bits = PtrTraits::exchange(m_bits, PtrTraits::template convert_from<U, P>(other_bits));
P::exchange(other.m_bits, P::template convert_from<U, P>(bits));
}
ALWAYS_INLINE RefPtr& operator=(RefPtr&& other)
{
RefPtr tmp = move(other);
swap(tmp);
if (this != &other)
assign_raw(other.leak_ref_raw());
return *this;
}
template<typename U>
ALWAYS_INLINE RefPtr& operator=(RefPtr<U, PtrTraits>&& other)
template<typename U, typename P = RefPtrTraits<U>>
ALWAYS_INLINE RefPtr& operator=(RefPtr<U, P>&& other)
{
RefPtr tmp = move(other);
swap(tmp);
assign_raw(PtrTraits::template convert_from<U, P>(other.leak_ref_raw()));
return *this;
}
template<typename U>
ALWAYS_INLINE RefPtr& operator=(NonnullRefPtr<U>&& other)
{
RefPtr tmp = move(other);
swap(tmp);
ASSERT(!PtrTraits::is_null(m_bits));
assign_raw(PtrTraits::as_bits(&other.leak_ref()));
return *this;
}
ALWAYS_INLINE RefPtr& operator=(const NonnullRefPtr<T>& other)
{
RefPtr tmp = other;
swap(tmp);
ASSERT(!PtrTraits::is_null(m_bits));
assign_raw(PtrTraits::as_bits(other.add_ref()));
return *this;
}
template<typename U>
ALWAYS_INLINE RefPtr& operator=(const NonnullRefPtr<U>& other)
{
RefPtr tmp = other;
swap(tmp);
ASSERT(!PtrTraits::is_null(m_bits));
assign_raw(PtrTraits::as_bits(other.add_ref()));
return *this;
}
ALWAYS_INLINE RefPtr& operator=(const RefPtr& other)
{
RefPtr tmp = other;
swap(tmp);
if (this != &other)
assign_raw(other.add_ref_raw());
return *this;
}
template<typename U>
ALWAYS_INLINE RefPtr& operator=(const RefPtr<U>& other)
{
RefPtr tmp = other;
swap(tmp);
assign_raw(other.add_ref_raw());
return *this;
}
ALWAYS_INLINE RefPtr& operator=(const T* ptr)
{
RefPtr tmp = ptr;
swap(tmp);
ref_if_not_null(const_cast<T*>(ptr));
assign_raw(PtrTraits::as_bits(const_cast<T*>(ptr)));
return *this;
}
ALWAYS_INLINE RefPtr& operator=(const T& object)
{
RefPtr tmp = object;
swap(tmp);
const_cast<T&>(object).ref();
assign_raw(PtrTraits::as_bits(const_cast<T*>(&object)));
return *this;
}
@ -226,99 +300,166 @@ public:
return *this;
}
ALWAYS_INLINE void clear()
ALWAYS_INLINE bool assign_if_null(RefPtr&& other)
{
unref_if_not_null(PtrTraits::as_ptr(m_bits));
m_bits = PtrTraits::default_null_value;
if (this == &other)
return is_null();
return PtrTraits::exchange_if_null(m_bits, other.leak_ref_raw());
}
bool operator!() const { return PtrTraits::is_null(m_bits); }
template<typename U, typename P = RefPtrTraits<U>>
ALWAYS_INLINE bool assign_if_null(RefPtr<U, P>&& other)
{
if (this == &other)
return is_null();
return PtrTraits::exchange_if_null(m_bits, PtrTraits::template convert_from<U, P>(other.leak_ref_raw()));
}
ALWAYS_INLINE void clear()
{
assign_raw(PtrTraits::default_null_value);
}
bool operator!() const { return PtrTraits::is_null(m_bits.load(AK::MemoryOrder::memory_order_relaxed)); }
[[nodiscard]] T* leak_ref()
{
FlatPtr bits = exchange(m_bits, PtrTraits::default_null_value);
return !PtrTraits::is_null(bits) ? PtrTraits::as_ptr(bits) : nullptr;
FlatPtr bits = PtrTraits::exchange(m_bits, PtrTraits::default_null_value);
return PtrTraits::as_ptr(bits);
}
NonnullRefPtr<T> release_nonnull()
{
ASSERT(!PtrTraits::is_null(m_bits));
return NonnullRefPtr<T>(NonnullRefPtr<T>::Adopt, *leak_ref());
FlatPtr bits = PtrTraits::exchange(m_bits, PtrTraits::default_null_value);
ASSERT(!PtrTraits::is_null(bits));
return NonnullRefPtr<T>(NonnullRefPtr<T>::Adopt, *PtrTraits::as_ptr(bits));
}
ALWAYS_INLINE T* ptr() { return !PtrTraits::is_null(m_bits) ? PtrTraits::as_ptr(m_bits) : nullptr; }
ALWAYS_INLINE const T* ptr() const { return !PtrTraits::is_null(m_bits) ? PtrTraits::as_ptr(m_bits) : nullptr; }
ALWAYS_INLINE T* ptr() { return as_ptr(); }
ALWAYS_INLINE const T* ptr() const { return as_ptr(); }
ALWAYS_INLINE T* operator->()
{
ASSERT(!PtrTraits::is_null(m_bits));
return PtrTraits::as_ptr(m_bits);
return as_nonnull_ptr();
}
ALWAYS_INLINE const T* operator->() const
{
ASSERT(!PtrTraits::is_null(m_bits));
return PtrTraits::as_ptr(m_bits);
return as_nonnull_ptr();
}
ALWAYS_INLINE T& operator*()
{
ASSERT(!PtrTraits::is_null(m_bits));
return *PtrTraits::as_ptr(m_bits);
return *as_nonnull_ptr();
}
ALWAYS_INLINE const T& operator*() const
{
ASSERT(!PtrTraits::is_null(m_bits));
return *PtrTraits::as_ptr(m_bits);
return *as_nonnull_ptr();
}
ALWAYS_INLINE operator const T*() const { return PtrTraits::as_ptr(m_bits); }
ALWAYS_INLINE operator T*() { return PtrTraits::as_ptr(m_bits); }
ALWAYS_INLINE operator const T*() const { return as_ptr(); }
ALWAYS_INLINE operator T*() { return as_ptr(); }
operator bool() { return !PtrTraits::is_null(m_bits); }
operator bool() { return !is_null(); }
bool operator==(std::nullptr_t) const { return PtrTraits::is_null(m_bits); }
bool operator!=(std::nullptr_t) const { return !PtrTraits::is_null(m_bits); }
bool operator==(std::nullptr_t) const { return is_null(); }
bool operator!=(std::nullptr_t) const { return !is_null(); }
bool operator==(const RefPtr& other) const { return m_bits == other.m_bits; }
bool operator!=(const RefPtr& other) const { return m_bits != other.m_bits; }
bool operator==(const RefPtr& other) const { return as_ptr() == other.as_ptr(); }
bool operator!=(const RefPtr& other) const { return as_ptr() != other.as_ptr(); }
bool operator==(RefPtr& other) { return m_bits == other.m_bits; }
bool operator!=(RefPtr& other) { return m_bits != other.m_bits; }
bool operator==(RefPtr& other) { return as_ptr() == other.as_ptr(); }
bool operator!=(RefPtr& other) { return as_ptr() != other.as_ptr(); }
bool operator==(const T* other) const { return PtrTraits::as_ptr(m_bits) == other; }
bool operator!=(const T* other) const { return PtrTraits::as_ptr(m_bits) != other; }
bool operator==(const T* other) const { return as_ptr() == other; }
bool operator!=(const T* other) const { return as_ptr() != other; }
bool operator==(T* other) { return PtrTraits::as_ptr(m_bits) == other; }
bool operator!=(T* other) { return PtrTraits::as_ptr(m_bits) != other; }
bool operator==(T* other) { return as_ptr() == other; }
bool operator!=(T* other) { return as_ptr() != other; }
bool is_null() const { return PtrTraits::is_null(m_bits.load(AK::MemoryOrder::memory_order_relaxed)); }
bool is_null() const { return PtrTraits::is_null(m_bits); }
template<typename U = T, typename EnableIf<IsSame<U, T>::value && !IsNullPointer<typename PtrTraits::NullType>::value>::Type* = nullptr>
typename PtrTraits::NullType null_value() const
{
// make sure we are holding a null value
ASSERT(PtrTraits::is_null(m_bits));
return PtrTraits::to_null_value(m_bits);
FlatPtr bits = m_bits.load(AK::MemoryOrder::memory_order_relaxed);
ASSERT(PtrTraits::is_null(bits));
return PtrTraits::to_null_value(bits);
}
template<typename U = T, typename EnableIf<IsSame<U, T>::value && !IsNullPointer<typename PtrTraits::NullType>::value>::Type* = nullptr>
void set_null_value(typename PtrTraits::NullType value)
{
// make sure that new null value would be interpreted as a null value
FlatPtr bits = PtrTraits::from_null_value(value);
ASSERT(PtrTraits::is_null(bits));
clear();
m_bits = bits;
// make sure that new null value would be interpreted as a null value
FlatPtr bits = PtrTraits::from_null_value(value);
ASSERT(PtrTraits::is_null(bits));
assign_raw(bits);
}
private:
[[nodiscard]] FlatPtr leak_ref_raw()
template<typename F>
void do_while_locked(F f) const
{
return exchange(m_bits, PtrTraits::default_null_value);
#ifdef KERNEL
// We don't want to be pre-empted while we have the lock bit set
Kernel::ScopedCritical critical;
#endif
FlatPtr bits = PtrTraits::lock(m_bits);
T* ptr = PtrTraits::as_ptr(bits);
f(ptr);
PtrTraits::unlock(m_bits, bits);
}
FlatPtr m_bits { PtrTraits::default_null_value };
[[nodiscard]] ALWAYS_INLINE FlatPtr leak_ref_raw()
{
return PtrTraits::exchange(m_bits, PtrTraits::default_null_value);
}
[[nodiscard]] ALWAYS_INLINE FlatPtr add_ref_raw() const
{
#ifdef KERNEL
// We don't want to be pre-empted while we have the lock bit set
Kernel::ScopedCritical critical;
#endif
// This prevents a race condition between thread A and B:
// 1. Thread A copies RefPtr, e.g. through assignment or copy constructor,
// gets the pointer from source, but is pre-empted before adding
// another reference
// 2. Thread B calls clear, leak_ref, or release_nonnull on source, and
// then drops the last reference, causing the object to be deleted
// 3. Thread A finishes step #1 by attempting to add a reference to
// the object that was already deleted in step #2
FlatPtr bits = PtrTraits::lock(m_bits);
if (T* ptr = PtrTraits::as_ptr(bits))
ptr->ref();
PtrTraits::unlock(m_bits, bits);
return bits;
}
ALWAYS_INLINE void assign_raw(FlatPtr bits)
{
FlatPtr prev_bits = PtrTraits::exchange(m_bits, bits);
unref_if_not_null(PtrTraits::as_ptr(prev_bits));
}
ALWAYS_INLINE T* as_ptr() const
{
return PtrTraits::as_ptr(m_bits.load(AK::MemoryOrder::memory_order_relaxed));
}
ALWAYS_INLINE T* as_nonnull_ptr() const
{
return as_nonnull_ptr(m_bits.load(AK::MemoryOrder::memory_order_relaxed));
}
ALWAYS_INLINE T* as_nonnull_ptr(FlatPtr bits) const
{
ASSERT(!PtrTraits::is_null(bits));
return PtrTraits::as_ptr(bits);
}
mutable Atomic<FlatPtr> m_bits { PtrTraits::default_null_value };
};
template<typename T, typename PtrTraits = RefPtrTraits<T>>
@ -346,6 +487,12 @@ inline RefPtr<T> static_ptr_cast(const RefPtr<U>& ptr)
return RefPtr<T, PtrTraits>(static_cast<const T*>(ptr.ptr()));
}
template<typename T, typename PtrTraitsT, typename U, typename PtrTraitsU>
inline void swap(RefPtr<T, PtrTraitsT>& a, RefPtr<U, PtrTraitsU>& b)
{
a.swap(b);
}
}
using AK::RefPtr;