diff options
| -rw-r--r-- | rts/STM.c | 24 |
1 files changed, 23 insertions, 1 deletions
@@ -332,7 +332,29 @@ static void unpark_tso(Capability *cap, StgTSO *tso) { // queues: it's up to the thread itself to remove it from the wait queues // if it decides to do so when it is scheduled. - tryWakeupThread(cap,tso); + // Only the capability that owns this TSO may unblock it. We can + // call tryWakeupThread() which will either unblock it directly if + // it belongs to this cap, or send a message to the owning cap + // otherwise. + + // But we don't really want to send multiple messages if we write + // to the same TVar multiple times, and the owning cap hasn't yet + // woken up the thread and removed it from the TVar's watch list. + // So, we use the tso->block_info as a flag to indicate whether + // we've already done tryWakeupThread() for this thread. + + // Safety Note: we hold the TVar lock at this point, so we know + // that this thread is definitely still blocked, since the first + // thing a thread will do when it runs is remove itself from the + // TVar watch queues, and to do that it would need to lock the + // TVar. + + if (tso->block_info.closure != &stg_STM_AWOKEN_closure) { + // safe to do a non-atomic test-and-set here, because it's + // fine if we do multiple tryWakeupThread()s. + tso->block_info.closure = &stg_STM_AWOKEN_closure; + tryWakeupThread(cap,tso); + } } static void unpark_waiters_on(Capability *cap, StgTVar *s) { |