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AK: Fix a race condition with WeakPtr<T>::strong_ref and destruction

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Since RefPtr<T> decrements the ref counter to 0 and after that starts
destructing the object, there is a window where the ref count is 0
and the weak references have not been revoked.

Also change WeakLink to be able to obtain a strong reference
concurrently and block revoking instead, which should happen a lot
less often.

Fixes a problem observed in #4621
This commit is contained in:
Tom 2020-12-29 13:14:21 -07:00 • committed by Andreas Kling
parent 3e00e3da72
commit 54eeb8ee9a
4 changed files with 88 additions and 24 deletions
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59
AK/Weakable.h
View file

@ -30,6 +30,9 @@
#include "Atomic.h"
#include "RefCounted.h"
#include "RefPtr.h"
#ifdef KERNEL
# include <Kernel/Arch/i386/CPU.h>
#endif
#ifndef WEAKABLE_DEBUG
# define WEAKABLE_DEBUG
@ -56,14 +59,16 @@ public:
{
#ifdef KERNEL
// We don't want to be pre-empted while we have the lock bit set
// We don't want to be pre-empted while we are trying to obtain
// a strong reference
Kernel::ScopedCritical critical;
#endif
FlatPtr bits = RefPtrTraits<void>::lock(m_bits);
T* ptr = static_cast<T*>(RefPtrTraits<void>::as_ptr(bits));
if (ptr)
ref = *ptr;
RefPtrTraits<void>::unlock(m_bits, bits);
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(*ptr);
}
m_consumers.fetch_sub(1u << 1, AK::MemoryOrder::memory_order_release);
}
return ref;
@ -72,26 +77,46 @@ public:
template<typename T>
T* unsafe_ptr() const
{
return static_cast<T*>(RefPtrTraits<void>::as_ptr(m_bits.load(AK::MemoryOrder::memory_order_acquire)));
if (m_consumers.load(AK::MemoryOrder::memory_order_relaxed) & 1u)
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
{
return RefPtrTraits<void>::is_null(m_bits.load(AK::MemoryOrder::memory_order_relaxed));
return !unsafe_ptr<void>();
}
void revoke()
{
RefPtrTraits<void>::exchange(m_bits, RefPtrTraits<void>::default_null_value);
auto current_consumers = m_consumers.fetch_or(1u, AK::MemoryOrder::memory_order_relaxed);
ASSERT(!(current_consumers & 1u));
// We flagged revokation, now wait until everyone trying to obtain
// a strong reference is done
while (current_consumers > 0) {
#ifdef KERNEL
Kernel::Processor::wait_check();
#else
// TODO: yield?
#endif
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:
template<typename T>
explicit WeakLink(T& weakable)
: m_bits(RefPtrTraits<void>::as_bits(&weakable))
: m_ptr(&weakable)
{
}
mutable Atomic<FlatPtr> m_bits;
mutable Atomic<void*> m_ptr;
mutable Atomic<unsigned> m_consumers; // LSB indicates revokation in progress
};
template<typename T>
@ -108,23 +133,19 @@ protected:
~Weakable()
{
#ifdef WEAKABLE_DEBUG
m_being_destroyed = true;
#endif
m_being_destroyed.store(true, AK::MemoryOrder::memory_order_release);
revoke_weak_ptrs();
}
void revoke_weak_ptrs()
{
if (m_link)
m_link->revoke();
if (auto link = move(m_link))
link->revoke();
}
private:
mutable RefPtr<WeakLink> m_link;
#ifdef WEAKABLE_DEBUG
bool m_being_destroyed { false };
#endif
Atomic<bool> m_being_destroyed { false };
};
}