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replay: push replay_mutex_lock up the call tree

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Now instead of using the replay_lock to guard the output of the log we
now use it to protect the whole execution section. This replaces what
the BQL used to do when it was held during TCG execution.

We also introduce some rules for locking order - mainly that you
cannot take the replay_mutex while holding the BQL. This leads to some
slight sophistry during start-up and extending the
replay_mutex_destroy function to unlock the mutex without checking
for the BQL condition so it can be cleanly dropped in the non-replay
case.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Tested-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Message-Id: <20180227095248.1060.40374.stgit@pasha-VirtualBox>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
This commit is contained in:
Alex Bennée 2018-02-27 12:52:48 +03:00 • committed by Paolo Bonzini
parent 1a423896fa
commit d759c951f3
10 changed files with 109 additions and 76 deletions
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22
docs/replay.txt
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@ -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
------------------------