diff options
| -rw-r--r-- | src/sim/eventq.cc | 1 | ||||
| -rw-r--r-- | src/sim/eventq.hh | 137 | ||||
| -rw-r--r-- | src/sim/global_event.hh | 9 | ||||
| -rw-r--r-- | src/sim/simulate.cc | 3 |
4 files changed, 149 insertions, 1 deletions
diff --git a/src/sim/eventq.cc b/src/sim/eventq.cc index 0735a011b..b8e45a13e 100644 --- a/src/sim/eventq.cc +++ b/src/sim/eventq.cc @@ -203,6 +203,7 @@ EventQueue::remove(Event *event) Event * EventQueue::serviceOne() { + std::lock_guard<EventQueue> lock(*this); Event *event = head; Event *next = head->nextInBin; event->flags.clear(Event::Scheduled); diff --git a/src/sim/eventq.hh b/src/sim/eventq.hh index 66b324c4f..9b3cd9917 100644 --- a/src/sim/eventq.hh +++ b/src/sim/eventq.hh @@ -398,8 +398,43 @@ operator!=(const Event &l, const Event &r) } #endif -/* +/** * Queue of events sorted in time order + * + * Events are scheduled (inserted into the event queue) using the + * schedule() method. This method either inserts a <i>synchronous</i> + * or <i>asynchronous</i> event. + * + * Synchronous events are scheduled using schedule() method with the + * argument 'global' set to false (default). This should only be done + * from a thread holding the event queue lock + * (EventQueue::service_mutex). The lock is always held when an event + * handler is called, it can therefore always insert events into its + * own event queue unless it voluntarily releases the lock. + * + * Events can be scheduled across thread (and event queue borders) by + * either scheduling asynchronous events or taking the target event + * queue's lock. However, the lock should <i>never</i> be taken + * directly since this is likely to cause deadlocks. Instead, code + * that needs to schedule events in other event queues should + * temporarily release its own queue and lock the new queue. This + * prevents deadlocks since a single thread never owns more than one + * event queue lock. This functionality is provided by the + * ScopedMigration helper class. Note that temporarily migrating + * between event queues can make the simulation non-deterministic, it + * should therefore be limited to cases where that can be tolerated + * (e.g., handling asynchronous IO or fast-forwarding in KVM). + * + * Asynchronous events can also be scheduled using the normal + * schedule() method with the 'global' parameter set to true. Unlike + * the previous queue migration strategy, this strategy is fully + * deterministic. This causes the event to be inserted in a separate + * queue of asynchronous events (async_queue), which is merged main + * event queue at the end of each simulation quantum (by calling the + * handleAsyncInsertions() method). Note that this implies that such + * events must happen at least one simulation quantum into the future, + * otherwise they risk being scheduled in the past by + * handleAsyncInsertions(). */ class EventQueue : public Serializable { @@ -414,6 +449,28 @@ class EventQueue : public Serializable //! List of events added by other threads to this event queue. std::list<Event*> async_queue; + /** + * Lock protecting event handling. + * + * This lock is always taken when servicing events. It is assumed + * that the thread scheduling new events (not asynchronous events + * though) have taken this lock. This is normally done by + * serviceOne() since new events are typically scheduled as a + * response to an earlier event. + * + * This lock is intended to be used to temporarily steal an event + * queue to support inter-thread communication when some + * deterministic timing can be sacrificed for speed. For example, + * the KVM CPU can use this support to access devices running in a + * different thread. + * + * @see EventQueue::ScopedMigration. + * @see EventQueue::ScopedRelease + * @see EventQueue::lock() + * @see EventQueue::unlock() + */ + std::mutex service_mutex; + //! Insert / remove event from the queue. Should only be called //! by thread operating this queue. void insert(Event *event); @@ -427,6 +484,68 @@ class EventQueue : public Serializable EventQueue(const EventQueue &); public: +#ifndef SWIG + /** + * Temporarily migrate execution to a different event queue. + * + * An instance of this class temporarily migrates execution to a + * different event queue by releasing the current queue, locking + * the new queue, and updating curEventQueue(). This can, for + * example, be useful when performing IO across thread event + * queues when timing is not crucial (e.g., during fast + * forwarding). + */ + class ScopedMigration + { + public: + ScopedMigration(EventQueue *_new_eq) + : new_eq(*_new_eq), old_eq(*curEventQueue()) + { + old_eq.unlock(); + new_eq.lock(); + curEventQueue(&new_eq); + } + + ~ScopedMigration() + { + new_eq.unlock(); + old_eq.lock(); + curEventQueue(&old_eq); + } + + private: + EventQueue &new_eq; + EventQueue &old_eq; + }; + + /** + * Temporarily release the event queue service lock. + * + * There are cases where it is desirable to temporarily release + * the event queue lock to prevent deadlocks. For example, when + * waiting on the global barrier, we need to release the lock to + * prevent deadlocks from happening when another thread tries to + * temporarily take over the event queue waiting on the barrier. + */ + class ScopedRelease + { + public: + ScopedRelease(EventQueue *_eq) + : eq(*_eq) + { + eq.unlock(); + } + + ~ScopedRelease() + { + eq.lock(); + } + + private: + EventQueue &eq; + }; +#endif + EventQueue(const std::string &n); virtual const std::string name() const { return objName; } @@ -491,6 +610,22 @@ class EventQueue : public Serializable */ Event* replaceHead(Event* s); + /**@{*/ + /** + * Provide an interface for locking/unlocking the event queue. + * + * @warn Do NOT use these methods directly unless you really know + * what you are doing. Incorrect use can easily lead to simulator + * deadlocks. + * + * @see EventQueue::ScopedMigration. + * @see EventQueue::ScopedRelease + * @see EventQueue + */ + void lock() { service_mutex.lock(); } + void unlock() { service_mutex.unlock(); } + /**@}*/ + #ifndef SWIG virtual void serialize(std::ostream &os); virtual void unserialize(Checkpoint *cp, const std::string §ion); diff --git a/src/sim/global_event.hh b/src/sim/global_event.hh index 7b5fd7485..3a5297e78 100644 --- a/src/sim/global_event.hh +++ b/src/sim/global_event.hh @@ -91,6 +91,15 @@ class BaseGlobalEvent : public EventBase bool globalBarrier() { + // This method will be called from the process() method in + // the local barrier events + // (GlobalSyncEvent::BarrierEvent). The local event + // queues are always locked when servicing events (calling + // the process() method), which means that it will be + // locked when entering this method. We need to unlock it + // while waiting on the barrier to prevent deadlocks if + // another thread wants to lock the event queue. + EventQueue::ScopedRelease release(curEventQueue()); return _globalEvent->barrier->wait(); } diff --git a/src/sim/simulate.cc b/src/sim/simulate.cc index b60b8a783..426c3e662 100644 --- a/src/sim/simulate.cc +++ b/src/sim/simulate.cc @@ -198,6 +198,9 @@ doSimLoop(EventQueue *eventq) } if (async_event && testAndClearAsyncEvent()) { + // Take the event queue lock in case any of the service + // routines want to schedule new events. + std::lock_guard<EventQueue> lock(*eventq); async_event = false; if (async_statdump || async_statreset) { Stats::schedStatEvent(async_statdump, async_statreset); |