Most garbage collection work can proceed concurrently with the client or mutator Java threads. But in certain places, for example while tracing from thread stacks, the garbage collector needs to ensure that Java data processed by the collector is consistent and complete. At these points, the mutators should not hold references to the heap that are invisible to the garbage collector. And they should not be modifying the data that is visible to the collector.
Logically, the collector and mutator share a reader-writer lock on the Java heap and associated data structures. Mutators hold the lock in reader or shared mode while running Java code or touching heap-related data structures. The collector holds the lock in writer or exclusive mode while it needs the heap data structures to be stable. However, this reader-writer lock has a very customized implementation that also provides additional facilities, such as the ability to exclude only a single thread, so that we can specifically examine its heap references.
In order to ensure consistency of the Java data, the compiler inserts “suspend points”, sometimes also called “safe points” into the code. These allow a thread to respond to external requests.
Whenever a thread is runnable, i.e. whenever a thread logically holds the mutator lock in shared mode, it is expected to regularly execute such a suspend point, and check for pending requests. They are currently implemented by setting a flag in the thread structure[^1], which is then explicitly tested by the compiler-generated code.
A thread responds to suspend requests only when it is “runnable”, i.e. logically running Java code. When it runs native code, or is blocked in a kernel call, it logically releases the mutator lock. When the garbage collector needs mutator cooperation, and the thread is not runnable, it is assured that the mutator is not touching Java data, and hence the collector can safely perform the required action itself, on the mutator thread's behalf.
Normally, when a thread makes a JNI call, it is not considered runnable while executing native code. This makes the transitions to and from running native JNI code somewhat expensive (see below). But these transitions are necessary to ensure that such code, which does not execute “suspend points”, and can thus not cooperate with the GC, doesn't delay GC completion. @FastNative and @CriticalNative calls avoid these transitions, instead allowing the thread to remain “runnable”, at the expense of potentially delaying GC operations for the duration of the call.
Although we say that a thread is “suspended” when it is not running Java code, it may in fact still be running native code and touching data structures that are not considered “Java data”. This distinction can be a fine line. For example, a Java thread blocked on a Java monitor will normally be “suspended” and blocked on a mutex contained in the monitor data structure. But it may wake up for reasons beyond ARTs control, which will normally result in touching the mutex. The monitor code must be quite careful to ensure that this does not cause problems, especially if the ART runtime was shut down in the interim and the monitor data structure has been reclaimed.
When a thread changes between running Java and native code, it has to correspondingly change its state between “runnable” and one of several other states, all of which are considered to be “suspended” for our purposes. When a Java thread starts to execute native code, and may thus not respond promptly to suspend requests, it will normally create an object of type ScopedThreadSuspension. ScopedThreadSuspension's constructor changes state to the “suspended” state given as an argument, logically releasing the mutator lock and promising to no longer touch Java data structures. It also handles any pending suspension requests that slid in just before it changed state.
Conversely, ScopedThreadSuspension‘s destructor waits until the GC has finished any actions it is currently performing on the thread’s behalf and effectively released the mutator exclusive lock, and then returns to runnable state, re-acquiring the mutator lock.
Occasionally a thread running native code needs to temporarily again access Java data structures, performing the above transitions in the opposite order. ScopedObjectAccess is a similar RAII object whose constructor and destructor perform those transitions in the reverse order from ScopedThreadSuspension.
The mutator lock is not implemented as a conventional mutex. But it plays by the rules of our normal static thread-safety analysis. Thus a function that is expected to be called in runnable state, with the ability to access Java data, should be annotated with REQUIRES_SHARED(Locks::mutator_lock_).
There is an explicit mutator_lock_ object, of type MutatorMutex. MutatorMutex is seemingly a minor refinement of ReaderWriterMutex, but it is used entirely differently. It is acquired explicitly by clients that need to hold it exclusively, and in a small number of cases, it is acquired in shared mode, e.g. via SharedTryLock(), or by the GC itself. However, more commonly MutatorMutex::TransitionFromSuspendedToRunnable(), is used to logically acquire the mutator mutex, e.g. as part of ScopedObjectAccess construction.
TransitionFromSuspendedToRunnable() does not physically acquire the ReaderWriterMutex in shared mode. Thus any thread acquiring the lock in exclusive mode must, in addition, explicitly arrange for mutator threads to be suspended via the thread suspension mechanism, and then make them runnable again on release.
Logically the mutator lock is held in shared/reader mode if either the underlying reader-writer lock is held in shared mode, or if a mutator is in runnable state.
Suspend point checks enable three kinds of communication with mutator threads:
Checkpoints : Checkpoint requests are used to get a thread to perform an action on our behalf. RequestCheckpoint() asks a specific thread to execute the closure supplied as an argument at its leisure. RequestSynchronousCheckpoint() in addition waits for the thread to complete running the closure, and handles suspended threads by running the closure on their behalf. In addition to these functions provided by Thread, ThreadList provides the RunCheckpoint() function that runs a checkpoint function on behalf of each thread, either by using RequestCheckpoint() to run it inside a running thread, or by ensuring that a suspended thread stays suspended, and then running the function on its behalf. RunCheckpoint() does not wait for completion of the function calls triggered by the resulting RequestCheckpoint() invocations.
Empty checkpoints : ThreadList provides RunEmptyCheckpoint(), which waits until all threads have either passed a suspend point, or have been suspended. This ensures that no thread is still executing Java code inside the same suspend-point-delimited code interval it was executing before the call. For example, a read-barrier started before a RunEmptyCheckpoint() call will have finished before the call returns.
Thread suspension : ThreadList provides a number of SuspendThread...() calls and a SuspendAll() call to suspend one or all threads until they are resumed by Resume() or ResumeAll(). The Suspend... calls guarantee that the target thread(s) are suspended (again, only in the sense of not running Java code) when the call returns.
It is easy to deadlock while attempting to run checkpoints, or suspending threads. In particular, we need to avoid situations in which we cannot suspend a thread because it is blocked, directly, or indirectly, on the GC completing its task. Deadlocks are avoided as follows:
Mutator lock ordering The mutator lock participates in the normal ART lock ordering hierarchy, as though it were a regular lock. See base/locks.h for the hierarchy. In particular, only locks at or below level kPostMutatorTopLockLevel may be acquired after acquiring the mutator lock, e.g. inside the scope of a ScopedObjectAccess. Similarly only locks at level strictly above kMutatatorLockmay be held while acquiring the mutator lock, e.g. either by starting a ScopedObjectAccess, or ending a ScopedThreadSuspension.
This ensures that code that uses purely mutexes and threads state changes cannot deadlock: Since we always wait on a lower-level lock, the holder of the lowest-level lock can always progress. An attempt to initiate a checkpoint or to suspend another thread must also be treated as an acquisition of the mutator lock: A thread that is waiting for a lock before it can respond to the request is itself holding the mutator lock, and can only be blocked on lower-level locks. And acquisition of those can never depend on acquiring the mutator lock.
Checkpoints Running a checkpoint in a thread requires suspending that thread for the duration of the checkpoint, or running the checkpoint on the threads behalf while that thread is blocked from executing Java code. In the former case, the checkpoint code is run from CheckSuspend, which requires the mutator lock, so checkpoint code may only acquire mutexes at or below level kPostMutatorTopLockLevel. But that is not sufficient.
No matter whether the checkpoint is run in the target thread, or on its behalf, the target thread is effectively suspended and prevented from running Java code. However the target may hold arbitrary Java monitors, which it can no longer release. This may also prevent higher level mutexes from getting released. Thus checkpoint code should only acquire mutexes at level kPostMonitorLock or below.
Waiting This becomes much more problematic when we wait for something other than a lock. Waiting for something that may depend on the GC, while holding the mutator lock, can potentially lead to deadlock, since it will prevent the waiting thread from participating in GC checkpoints. Waiting while holding a lower-level lock like thread_list_lock_ is similarly unsafe in general, since a runnable thread may not respond to checkpoints until it acquires thread_list_lock_. In general, waiting for a condition variable while holding an unrelated lock is problematic, and these are specific instances of that general problem.
We do currently provide WaitHoldingLocks, and it is sometimes used with low-level locks held. But such code must somehow ensure that such waits eventually terminate without deadlock.
One common use of WaitHoldingLocks is to wait for weak reference processing. Special rules apply to avoid deadlocks in this case: Such waits must start after weak reference processing is disabled; the GC may not issue further nonempty checkpoints or suspend requests until weak reference processing has been reenabled, and threads have been notified. Thus the waiting thread's inability to respond to nonempty checkpoints and suspend requests cannot directly block the GC. Non-GC checkpoint or suspend requests that target a thread waiting on reference processing will block until reference processing completes.
Consider a case in which thread W1 waits on reference processing, while holding a low-level mutex M. Thread W2 holds the mutator lock and waits on M. We avoid a situation in which the GC needs to suspend or checkpoint W2 by briefly stopping the world to disable weak reference access. During the stop-the-world phase, W1 cannot yet be waiting for weak-reference access. Thus there is no danger of deadlock while entering this phase. After this phase, there is no need for W2 to suspend or execute a nonempty checkpoint. If we replaced the stop-the-world phase by a checkpoint, W2 could receive the checkpoint request too late, and be unable to respond.
Empty checkpoints can continue to occur during reference processing. Reference processing wait loops explicitly handle empty checkpoints, and an empty checkpoint request notifies the condition variable used to wait for reference processing, after acquiring reference_processor_lock_. This means that empty checkpoints do not preclude client threads from being in the middle of an operation that involves a weak reference access, while nonempty checkpoints do.
[^1]: Some comments in the code refer to a not-yet-really-implemented scheme in which the compiler-generated code would load through the address at tlsPtr_.suspend_trigger. A thread suspension is requested by setting this to null, triggering a SIGSEGV, causing that thread to check for GC cooperation requests. The real mechanism instead sets an appropriate ThreadFlag entry to request suspension or a checkpoint. Note that the actual checkpoint function value is set, along with the flag, while holding suspend_count_lock_. If the target thread notices that a checkpoint is requested, it then acquires the suspend_count_lock_ to read the checkpoint function.