libvirt/src/qemu/THREADS.txt
Daniel P. Berrange f9c56cce79 Release driver and domain lock when running monitor commands
QEMU monitor commands may sleep for a prolonged period of time.
If the virDomainObjPtr or qemu driver lock is held this will
needlessly block execution of many other API calls. it also
prevents asynchronous monitor events from being dispatched
while a monitor command is executing, because deadlock will
ensure.

To resolve this, it is neccessary to release all locks while
executing a monitor command. This change introduces a flag
indicating that a monitor job is active, and a condition
variable to synchronize access to this flag. This ensures that
only a single thread can be making a state change or executing
a monitor command at a time, while still allowing other API
calls to be completed without blocking

* src/qemu/qemu_driver.c: Release driver and domain lock when
  running monitor commands. Re-add locking to disk passphrase
  callback
* src/qemu/THREADS.txt: Document threading rules
2009-11-10 13:27:18 +00:00

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QEMU Driver Threading: The Rules
=================================
This document describes how thread safety is ensured throughout
the QEMU driver. The criteria for this model are:
- Objects must never be exclusively locked for any pro-longed time
- Code which sleeps must be able to time out after suitable period
- Must be safe against dispatch asynchronous events from monitor
Basic locking primitives
------------------------
There are a number of locks on various objects
* struct qemud_driver: RWLock
This is the top level lock on the entire driver. Every API call in
the QEMU driver is blocked while this is held, though some internal
callbacks may still run asynchronously. This lock must never be held
for anything which sleeps/waits (ie monitor commands)
When obtaining the driver lock, under *NO* circumstances must
any lock be held on a virDomainObjPtr. This *WILL* result in
deadlock.
* virDomainObjPtr: Mutex
Will be locked after calling any of the virDomainFindBy{ID,Name,UUID}
methods.
Lock must be held when changing/reading any variable in the virDomainObjPtr
Once the lock is held, you must *NOT* try to lock the driver. You must
release all virDomainObjPtr locks before locking the driver, or deadlock
*WILL* occurr.
If the lock needs to be dropped & then re-acquired for a short period of
time, the reference count must be incremented first using virDomainObjRef().
If the reference count is incremented in this way, it is not neccessary
to have the driver locked when re-acquiring the dropped locked, since the
reference count prevents it being freed by another thread.
This lock must not be held for anything which sleeps/waits (ie monitor
commands).
* qemuMonitorPrivatePtr: Job condition
Since virDomainObjPtr lock must not be held during sleeps, the job condition
provides additional protection for code making updates.
Immediately after acquiring the virDomainObjPtr lock, any method which intends
to update state, must acquire the job condition. The virDomainObjPtr lock
is released while blocking on this condition variable. Once the job condition
is acquired a method can safely release the virDomainObjPtr lock whenever it
hits a piece of code which may sleep/wait, and re-acquire it after the sleep/
wait.
* qemuMonitorPtr: Mutex
Lock to be used when invoking any monitor command to ensure safety
wrt any asynchronous events that may be dispatched from the monitor.
It should be acquired before running a command.
The job condition *MUST* be held before acquiring the monitor lock
The virDomainObjPtr lock *MUST* be held before acquiring the monitor
lock.
The virDomainObjPtr lock *MUST* then be released when invoking the
monitor command.
The driver lock *MUST* be released when invoking the monitor commands.
This ensures that the virDomainObjPtr & driver are both unlocked while
sleeping/waiting for the monitor response.
Helper methods
--------------
To lock the driver
qemuDriverLock()
- Acquires the driver lock
qemuDriverUnlock()
- Releases the driver lock
To lock the virDomainObjPtr
virDomainObjLock()
- Acquires the virDomainObjPtr lock
virDomainObjUnlock()
- Releases the virDomainObjPtr lock
To acquire the job mutex
qemuDomainObjBeginJob() (if driver is unlocked)
- Increments ref count on virDomainObjPtr
- Wait qemuDomainObjPrivate condition 'jobActive != 0' using virDomainObjPtr mutex
- Sets jobActive to 1
qemuDomainObjBeginJobWithDriver() (if driver needs to be locked)
- Unlocks driver
- Increments ref count on virDomainObjPtr
- Wait qemuDomainObjPrivate condition 'jobActive != 0' using virDomainObjPtr mutex
- Sets jobActive to 1
- Unlocks virDomainObjPtr
- Locks driver
- Locks virDomainObjPtr
NB: this variant is required in order to comply with lock ordering rules
for virDomainObjPtr vs driver
qemuDomainObjEndJob()
- Set jobActive to 0
- Signal on qemuDomainObjPrivate condition
- Decrements ref count on virDomainObjPtr
To acquire the QEMU monitor lock
qemuDomainObjEnterMonitor()
- Acquires the qemuMonitorObjPtr lock
- Releases the virDomainObjPtr lock
qemuDomainObjExitMonitor()
- Acquires the virDomainObjPtr lock
- Releases the qemuMonitorObjPtr lock
NB: caller must take care to drop the driver lock if neccessary
To acquire the QEMU monitor lock with the driver lock held
qemuDomainObjEnterMonitorWithDriver()
- Acquires the qemuMonitorObjPtr lock
- Releases the virDomainObjPtr lock
- Releases the driver lock
qemuDomainObjExitMonitorWithDriver()
- Acquires the driver lock
- Acquires the virDomainObjPtr lock
- Releases the qemuMonitorObjPtr lock
NB: caller must take care to drop the driver lock if neccessary
Design patterns
---------------
* Accessing or updating something with just the driver
qemuDriverLock(driver);
...do work...
qemuDriverUnlock(driver);
* Accessing something directly todo with a virDomainObjPtr
virDomainObjPtr obj;
qemuDriverLock(driver);
obj = virDomainFindByUUID(driver->domains, dom->uuid);
qemuDriverUnlock(driver);
...do work...
virDomainObjUnlock(obj);
* Accessing something directly todo with a virDomainObjPtr and driver
virDomainObjPtr obj;
qemuDriverLock(driver);
obj = virDomainFindByUUID(driver->domains, dom->uuid);
...do work...
virDomainObjUnlock(obj);
qemuDriverUnlock(driver);
* Updating something directly todo with a virDomainObjPtr
virDomainObjPtr obj;
qemuDriverLockRO(driver);
obj = virDomainFindByUUID(driver->domains, dom->uuid);
qemuDriverUnlock(driver);
qemuDomainObjBeginJob(obj);
...do work...
qemuDomainObjEndJob(obj);
virDomainObjUnlock(obj);
* Invoking a monitor command on a virDomainObjPtr
virDomainObjPtr obj;
qemuDomainObjPrivatePtr priv;
qemuDriverLockRO(driver);
obj = virDomainFindByUUID(driver->domains, dom->uuid);
qemuDriverUnlock(driver);
qemuDomainObjBeginJob(obj);
...do prep work...
qemuDomainObjEnterMonitor(obj);
qemuMonitorXXXX(priv->mon);
qemuDomainObjExitMonitor(obj);
...do final work...
qemuDomainObjEndJob(obj);
virDomainObjUnlock(obj);
* Invoking a monitor command on a virDomainObjPtr with driver locked too
virDomainObjPtr obj;
qemuDomainObjPrivatePtr priv;
qemuDriverLock(driver);
obj = virDomainFindByUUID(driver->domains, dom->uuid);
qemuDomainObjBeginJobWithDriver(obj);
...do prep work...
qemuDomainObjEnterMonitorWithDriver(driver, obj);
qemuMonitorXXXX(priv->mon);
qemuDomainObjExitMonitorWithDriver(driver, obj);
...do final work...
qemuDomainObjEndJob(obj);
virDomainObjUnlock(obj);
qemuDriverUnlock(driver);
Summary
-------
* Respect lock ordering rules: never lock driver if anything else is
already locked
* Don't hold locks in code which sleeps: unlock driver & virDomainObjPtr
when using monitor