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Kernel: Don't ref/unref the holder thread in Mutex

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There was a whole bunch of ref counting churn coming from Mutex, which
had a RefPtr<Thread> m_holder to (mostly) point at the thread holding
the mutex.

Since we never actually dereference the m_holder value, but only use it
for identity checks against thread pointers, we can store it as an
uintptr_t and skip the ref counting entirely.

Threads can't die while holding a mutex anyway, so there's no risk of
them going missing on us.
This commit is contained in:
Andreas Kling 2023-04-02 21:36:39 +02:00
parent c3915e4058
commit ed1253ab90
2 changed files with 26 additions and 31 deletions
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13
Kernel/Locking/Mutex.h
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@ -57,7 +57,7 @@ public:
VERIFY(m_mode != Mode::Shared); // This method should only be used on exclusively-held locks
if (m_mode == Mode::Unlocked)
return false;
return m_holder == Thread::current();
return m_holder == bit_cast<uintptr_t>(Thread::current());
}
[[nodiscard]] StringView name() const { return m_name; }
@ -96,12 +96,11 @@ private:
// lock it again. When locked in shared mode, any thread can do that.
u32 m_times_locked { 0 };
// One of the threads that hold this lock, or nullptr. When locked in shared
// mode, this is stored on best effort basis: nullptr value does *not* mean
// The address of one of the threads that hold this lock, or 0.
// When locked in shared mode, this is stored on best effort basis: 0 does *not* mean
// the lock is unlocked, it just means we don't know which threads hold it.
// When locked exclusively, this is always the one thread that holds the
// lock.
RefPtr<Thread> m_holder;
// When locked exclusively, this is always the one thread that holds the lock.
uintptr_t m_holder { 0 };
size_t m_shared_holders { 0 };
struct BlockedThreadLists {
@ -124,7 +123,7 @@ private:
mutable Spinlock<LockRank::None> m_lock {};
#if LOCK_SHARED_UPGRADE_DEBUG
HashMap<Thread*, u32> m_shared_holders_map;
HashMap<uintptr_t, u32> m_shared_holders_map;
#endif
};