Kernel: TimerQueue::cancel_timer needs to wait if timer is executing

We need to be able to guarantee that a timer won't be executing after
TimerQueue::cancel_timer returns. In the case of multiple processors
this means that we may need to wait while the timer handler finishes
execution on another core.

This also fixes a problem in Thread::block and Thread::wait_on where
theoretically the timer could execute after the function returned
and the Thread disappeared.

Kernel/Thread.h

@@ -710,6 +710,7 @@ public:
     template<typename T, class... Args>
     [[nodiscard]] BlockResult block(const BlockTimeout& timeout, Args&&... args)
     {
+        ScopedSpinLock scheduler_lock(g_scheduler_lock);
         ScopedSpinLock lock(m_lock);
         // We need to hold m_lock so that nobody can unblock a blocker as soon
         // as it is constructed and registered elsewhere
@@ -718,7 +719,6 @@ public:
         bool did_timeout = false;
         RefPtr<Timer> timer;
         {
-            ScopedSpinLock scheduler_lock(g_scheduler_lock);
             // We should never be blocking a blocked (or otherwise non-active) thread.
             ASSERT(state() == Thread::Running);
             ASSERT(m_blocker == nullptr);
@@ -762,15 +762,16 @@ public:
         }
 
         lock.unlock();
+        scheduler_lock.unlock();
 
         // Yield to the scheduler, and wait for us to resume unblocked.
         yield_without_holding_big_lock();
 
+        scheduler_lock.lock();
         lock.lock();
 
         bool is_stopped = false;
         {
-            ScopedSpinLock scheduler_lock(g_scheduler_lock);
             if (t.was_interrupted_by_signal())
                 dispatch_one_pending_signal();
 
@@ -787,17 +788,21 @@ public:
                 did_timeout = true;
         }
 
+        // Notify the blocker that we are no longer blocking. It may need
+        // to clean up now while we're still holding m_lock
+        auto result = t.end_blocking({}, did_timeout); // calls was_unblocked internally
+
         if (timer && !did_timeout) {
             // Cancel the timer while not holding any locks. This allows
             // the timer function to complete before we remove it
             // (e.g. if it's on another processor)
+            lock.unlock();
+            scheduler_lock.unlock();
             TimerQueue::the().cancel_timer(timer.release_nonnull());
+        } else {
+            scheduler_lock.unlock();
         }
 
-        // Notify the blocker that we are no longer blocking. It may need
-        // to clean up now while we're still holding m_lock
-        auto result = t.end_blocking({}, did_timeout); // calls was_unblocked internally
-
         if (is_stopped) {
             // If we're stopped we need to yield
             yield_without_holding_big_lock();

Kernel/TimerQueue.cpp

@@ -61,7 +61,7 @@ RefPtr<Timer> TimerQueue::add_timer_without_id(const timespec& deadline, Functio
     auto timer = adopt(*new Timer(time_to_ticks(deadline), move(callback)));
 
     ScopedSpinLock lock(g_timerqueue_lock);
-    timer->id = 0; // Don't generate a timer id
+    timer->m_id = 0; // Don't generate a timer id
     add_timer_locked(timer);
     return timer;
 }
@@ -70,31 +70,38 @@ TimerId TimerQueue::add_timer(NonnullRefPtr<Timer>&& timer)
 {
     ScopedSpinLock lock(g_timerqueue_lock);
 
-    timer->id = ++m_timer_id_count;
-    ASSERT(timer->id != 0); // wrapped
+    timer->m_id = ++m_timer_id_count;
+    ASSERT(timer->m_id != 0); // wrapped
     add_timer_locked(move(timer));
     return m_timer_id_count;
 }
 
 void TimerQueue::add_timer_locked(NonnullRefPtr<Timer> timer)
 {
-    u64 timer_expiration = timer->expires;
+    u64 timer_expiration = timer->m_expires;
     ASSERT(timer_expiration >= time_to_ticks(TimeManagement::the().monotonic_time()));
 
+    ASSERT(!timer->is_queued());
+
     if (m_timer_queue.is_empty()) {
-        m_timer_queue.append(move(timer));
+        m_timer_queue.append(&timer.leak_ref());
         m_next_timer_due = timer_expiration;
     } else {
-        auto following_timer = m_timer_queue.find([&timer_expiration](auto& other) { return other->expires > timer_expiration; });
-
-        if (following_timer.is_end()) {
-            m_timer_queue.append(move(timer));
-        } else {
-            auto next_timer_needs_update = following_timer.is_begin();
-            m_timer_queue.insert_before(following_timer, move(timer));
-
+        Timer* following_timer = nullptr;
+        m_timer_queue.for_each([&](Timer& t) {
+            if (t.m_expires > timer_expiration) {
+                following_timer = &t;
+                return IterationDecision::Break;
+            }
+            return IterationDecision::Continue;
+        });
+        if (following_timer) {
+            bool next_timer_needs_update = m_timer_queue.head() == following_timer;
+            m_timer_queue.insert_before(following_timer, &timer.leak_ref());
             if (next_timer_needs_update)
                 m_next_timer_due = timer_expiration;
+        } else {
+            m_timer_queue.append(&timer.leak_ref());
         }
     }
 }
@@ -125,51 +132,106 @@ u64 TimerQueue::time_to_ticks(const timespec& tspec) const
 bool TimerQueue::cancel_timer(TimerId id)
 {
     ScopedSpinLock lock(g_timerqueue_lock);
-    auto it = m_timer_queue.find([id](auto& timer) { return timer->id == id; });
-    if (it.is_end())
+    Timer* found_timer = nullptr;
+    if (m_timer_queue.for_each([&](Timer& timer) {
+            if (timer.m_id == id) {
+                found_timer = &timer;
+                return IterationDecision::Break;
+            }
+            return IterationDecision::Continue;
+        })
+        != IterationDecision::Break) {
+        // The timer may be executing right now, if it is then it should
+        // be in m_timers_executing. If it is then release the lock
+        // briefly to allow it to finish by removing itself
+        // NOTE: This can only happen with multiple processors!
+        while (m_timers_executing.for_each([&](Timer& timer) {
+            if (timer.m_id == id)
+                return IterationDecision::Break;
+            return IterationDecision::Continue;
+        }) == IterationDecision::Break) {
+            // NOTE: This isn't the most efficient way to wait, but
+            // it should only happen when multiple processors are used.
+            // Also, the timers should execute pretty quickly, so it
+            // should not loop here for very long. But we can't yield.
+            lock.unlock();
+            Processor::wait_check();
+            lock.lock();
+        }
+        // We were not able to cancel the timer, but at this point
+        // the handler should have completed if it was running!
         return false;
+    }
 
-    auto was_next_timer = it.is_begin();
-    m_timer_queue.remove(it);
+    ASSERT(found_timer);
+    bool was_next_timer = (m_timer_queue.head() == found_timer);
+
+    m_timer_queue.remove(found_timer);
+    found_timer->set_queued(false);
 
     if (was_next_timer)
         update_next_timer_due();
 
+    lock.unlock();
+    found_timer->unref();
     return true;
 }
 
-bool TimerQueue::cancel_timer(const NonnullRefPtr<Timer>& timer)
+bool TimerQueue::cancel_timer(Timer& timer)
 {
     ScopedSpinLock lock(g_timerqueue_lock);
-    auto it = m_timer_queue.find([timer](auto& t) { return t.ptr() == timer.ptr(); });
-    if (it.is_end())
+    if (!m_timer_queue.contains_slow(&timer)) {
+        // The timer may be executing right now, if it is then it should
+        // be in m_timers_executing. If it is then release the lock
+        // briefly to allow it to finish by removing itself
+        // NOTE: This can only happen with multiple processors!
+        while (m_timers_executing.contains_slow(&timer)) {
+            // NOTE: This isn't the most efficient way to wait, but
+            // it should only happen when multiple processors are used.
+            // Also, the timers should execute pretty quickly, so it
+            // should not loop here for very long. But we can't yield.
+            lock.unlock();
+            Processor::wait_check();
+            lock.lock();
+        }
+        // We were not able to cancel the timer, but at this point
+        // the handler should have completed if it was running!
         return false;
+    }
 
-    auto was_next_timer = it.is_begin();
-    m_timer_queue.remove(it);
+    bool was_next_timer = (m_timer_queue.head() == &timer);
+    m_timer_queue.remove(&timer);
+    timer.set_queued(false);
 
     if (was_next_timer)
         update_next_timer_due();
-
     return true;
 }
 
 void TimerQueue::fire()
 {
     ScopedSpinLock lock(g_timerqueue_lock);
-    if (m_timer_queue.is_empty())
+    auto* timer = m_timer_queue.head();
+    if (!timer)
         return;
 
-    ASSERT(m_next_timer_due == m_timer_queue.first()->expires);
+    ASSERT(m_next_timer_due == timer->m_expires);
 
-    while (!m_timer_queue.is_empty() && TimeManagement::the().monotonic_ticks() > m_timer_queue.first()->expires) {
-        auto timer = m_timer_queue.take_first();
+    while (timer && TimeManagement::the().monotonic_ticks() > timer->m_expires) {
+        m_timer_queue.remove(timer);
+        m_timers_executing.append(timer);
 
         update_next_timer_due();
 
         lock.unlock();
-        timer->callback();
+        timer->m_callback();
         lock.lock();
+
+        m_timers_executing.remove(timer);
+        timer->set_queued(false);
+        timer->unref();
+
+        timer = m_timer_queue.head();
     }
 }
 
@@ -177,10 +239,10 @@ void TimerQueue::update_next_timer_due()
 {
     ASSERT(g_timerqueue_lock.is_locked());
 
-    if (m_timer_queue.is_empty())
-        m_next_timer_due = 0;
+    if (auto* next_timer = m_timer_queue.head())
+        m_next_timer_due = next_timer->m_expires;
     else
-        m_next_timer_due = m_timer_queue.first()->expires;
+        m_next_timer_due = 0;
 }
 
 }

Kernel/TimerQueue.h

@@ -27,41 +27,59 @@
 #pragma once
 
 #include <AK/Function.h>
+#include <AK/InlineLinkedList.h>
 #include <AK/NonnullRefPtr.h>
 #include <AK/OwnPtr.h>
 #include <AK/RefCounted.h>
-#include <AK/SinglyLinkedList.h>
 #include <Kernel/Time/TimeManagement.h>
 
 namespace Kernel {
 
 typedef u64 TimerId;
 
-struct Timer : public RefCounted<Timer> {
-    TimerId id;
-    u64 expires;
-    Function<void()> callback;
+class Timer : public RefCounted<Timer>
+    , public InlineLinkedListNode<Timer> {
+    friend class TimerQueue;
+    friend class InlineLinkedListNode<Timer>;
+
+public:
+    Timer(u64 expires, Function<void()>&& callback)
+        : m_expires(expires)
+        , m_callback(move(callback))
+    {
+    }
+    ~Timer()
+    {
+        ASSERT(!is_queued());
+    }
+
+private:
+    TimerId m_id;
+    u64 m_expires;
+    Function<void()> m_callback;
+    Timer* m_next { nullptr };
+    Timer* m_prev { nullptr };
+    Atomic<bool> m_queued { false };
+
     bool operator<(const Timer& rhs) const
     {
-        return expires < rhs.expires;
+        return m_expires < rhs.m_expires;
     }
     bool operator>(const Timer& rhs) const
     {
-        return expires > rhs.expires;
+        return m_expires > rhs.m_expires;
     }
     bool operator==(const Timer& rhs) const
     {
-        return id == rhs.id;
-    }
-
-    Timer(u64 expires, Function<void()>&& callback)
-        : expires(expires)
-        , callback(move(callback))
-    {
+        return m_id == rhs.m_id;
     }
+    bool is_queued() const { return m_queued.load(AK::MemoryOrder::memory_order_relaxed); }
+    void set_queued(bool queued) { m_queued.store(queued, AK::MemoryOrder::memory_order_relaxed); }
 };
 
 class TimerQueue {
+    friend class Timer;
+
 public:
     TimerQueue();
     static TimerQueue& the();
@@ -70,10 +88,14 @@ public:
     RefPtr<Timer> add_timer_without_id(const timespec& timeout, Function<void()>&& callback);
     TimerId add_timer(timeval& timeout, Function<void()>&& callback);
     bool cancel_timer(TimerId id);
-    bool cancel_timer(const NonnullRefPtr<Timer>&);
+    bool cancel_timer(NonnullRefPtr<Timer>&& timer)
+    {
+        return cancel_timer(timer.leak_ref());
+    }
     void fire();
 
 private:
+    bool cancel_timer(Timer&);
     void update_next_timer_due();
     void add_timer_locked(NonnullRefPtr<Timer>);
 
@@ -83,7 +105,8 @@ private:
     u64 m_next_timer_due { 0 };
     u64 m_timer_id_count { 0 };
     u64 m_ticks_per_second { 0 };
-    SinglyLinkedList<NonnullRefPtr<Timer>> m_timer_queue;
+    InlineLinkedList<Timer> m_timer_queue;
+    InlineLinkedList<Timer> m_timers_executing;
 };
 
 }