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Diffstat (limited to 'docs/replay.txt')
| -rw-r--r-- | docs/replay.txt | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/docs/replay.txt b/docs/replay.txt index c52407fe23..959633e7ea 100644 --- a/docs/replay.txt +++ b/docs/replay.txt @@ -49,6 +49,28 @@ Modifications of qemu include: * recording/replaying user input (mouse and keyboard) * adding internal checkpoints for cpu and io synchronization +Locking and thread synchronisation +---------------------------------- + +Previously the synchronisation of the main thread and the vCPU thread +was ensured by the holding of the BQL. However the trend has been to +reduce the time the BQL was held across the system including under TCG +system emulation. As it is important that batches of events are kept +in sequence (e.g. expiring timers and checkpoints in the main thread +while instruction checkpoints are written by the vCPU thread) we need +another lock to keep things in lock-step. This role is now handled by +the replay_mutex_lock. It used to be held only for each event being +written but now it is held for a whole execution period. This results +in a deterministic ping-pong between the two main threads. + +As the BQL is now a finer grained lock than the replay_lock it is almost +certainly a bug, and a source of deadlocks, to take the +replay_mutex_lock while the BQL is held. This is enforced by an assert. +While the unlocks are usually in the reverse order, this is not +necessary; you can drop the replay_lock while holding the BQL, without +doing a more complicated unlock_iothread/replay_unlock/lock_iothread +sequence. + Non-deterministic events ------------------------ |