The current QEMU driver makes use of 2 locks
- The driver lock
- The virDomainObjPtr lock
The idea is the driver lock is not held for long periods of time.
Unfortunately we don't always deal with this very well - some
code needs todo quite alot with the driver - particularly starting
and stopping of guests.
The bigger problem is that the virDomainObjPtr lock is often held
for long periods, specifically whenever we invoke a monitor command.
Some of these commands can take a very long time (even infinite if
someone has send SIGSTOP to QEMU). This very quickly blocks the
whole driver.
I've realized that even with the series of monitor patches I sent
out, changing the driver mutex to a RWLock, and adding a separate
lock on the qemuMonitorPtr object iself, there's still a major
concurrency problem: the virDomainObjPtr lock is held for too
long. I propose to drop the RWLock patch, and do something totally
different instead....
We fundamentally need to drop the virDomainObjPtr lock whenever
we invoke a monitor command. Unfortunately, merely dropping the
virDomainObjPtr and acquiring the qemuMonitorPtr is not safe.
An API call which changes the VM state typically has 3 phases
1. Check what state/config the VM is in
2. Invoke the monitor command
3. Update the state/config of the VM
If we release the virDomainObjPtr, and acquire qemuMonitorPtr
at step 2, then other APIs calls will be able to complete
their own step 1 checks, and get blocked at step 2. This is
not safe, because when the original call moves onto step 3
and changes the state, this will have invalidated the checks
the other sleeping API calls made in step 1.
We need to prevent any API call starting step 1, for as long
as there is a monitor command being run, even if the lock on
virDomainObjPtr is not held.
The only way I see todo this, is to introduce a condition
variable indicating that a state change is to be made. Any
API call which intends to make a state change must acquire
this condition prior to step 1. They can thus safely do their
checks, and move onto step 2, releasing the virDomainObj lock
whle the monitor command is running, and reacquiring it after.
All other API calls making changes get safely queued up at
step 1, but API calls which simply wish to query information
can run without being blocked at all. This fixes the major
concurrency problem with running monitor commands. The use
of a condition variable at the start of step 1, also allows
us to time out API calls, if some other thread get stuck in
the monitor for too long. I think this also makes the use of
a RWLock on the QEMU driver unneccessary, since no code will
ever be holding a mutex in any place that sleeps/wait. Only
the condition variable will be held during sleeps/waits.
Since we'll now effectively have 3 locks, and 1 condition
variable this is getting kind of complex. So the rest of this
mail is a file I propose to put in src/qemu/THREADS.txt
describing what is going on, and showing the recommended
design patterns to use.
Daniel
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 condition variable (int jobActive)
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
Design patterns
---------------
All driver methods must follow one of these design patterns to
ensure thread safety and lock correctness.
* 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;
qemuDriverLock(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...
qemuDomainObjEnterMonitor(obj);
qemuDriverUnlock(driver);
qemuMonitorXXXX(priv->mon);
qemuDriverLock(driver);
qemuDomainObjExitMonitor(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
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