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 prolonged time - Code which sleeps must be able to time out after suitable period - Must be safe against dispatch of 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 (i.e. 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* occur. 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 necessary 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 (i.e. monitor commands). * qemuMonitorPrivatePtr: Job conditions Since virDomainObjPtr lock must not be held during sleeps, the job conditions provide additional protection for code making updates. Qemu driver uses two kinds of job conditions: asynchronous and normal. Asynchronous job condition is used for long running jobs (such as migration) that consist of several monitor commands and it is desirable to allow calling a limited set of other monitor commands while such job is running. This allows clients to, e.g., query statistical data, cancel the job, or change parameters of the job. Normal job condition is used by all other jobs to get exclusive access to the monitor and also by every monitor command issued by an asynchronous job. When acquiring normal job condition, the job must specify what kind of action it is about to take and this is checked against the allowed set of jobs in case an asynchronous job is running. If the job is incompatible with current asynchronous job, it needs to wait until the asynchronous job ends and try to acquire the job again. Immediately after acquiring the virDomainObjPtr lock, any method which intends to update state must acquire either asynchronous or normal job condition. The virDomainObjPtr lock is released while blocking on these condition variables. 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. Whenever an asynchronous job wants to talk to the monitor, it needs to acquire nested job (a special kind of normal job) to obtain exclusive access to the monitor. Since the virDomainObjPtr lock was dropped while waiting for the job condition, it is possible that the domain is no longer active when the condition is finally obtained. The monitor lock is only safe to grab after verifying that the domain is still active. * 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 normal job condition qemuDomainObjBeginJob() (if driver is unlocked) - Increments ref count on virDomainObjPtr - Waits until the job is compatible with current async job or no async job is running - Waits for job.cond condition 'job.active != 0' using virDomainObjPtr mutex - Rechecks if the job is still compatible and repeats waiting if it isn't - Sets job.active to the job type qemuDomainObjBeginJobWithDriver() (if driver needs to be locked) - Increments ref count on virDomainObjPtr - Unlocks driver - Waits until the job is compatible with current async job or no async job is running - Waits for job.cond condition 'job.active != 0' using virDomainObjPtr mutex - Rechecks if the job is still compatible and repeats waiting if it isn't - Sets job.active to the job type - Unlocks virDomainObjPtr - Locks driver - Locks virDomainObjPtr NB: this variant is required in order to comply with lock ordering rules for virDomainObjPtr vs. driver qemuDomainObjEndJob() - Sets job.active to 0 - Signals on job.cond condition - Decrements ref count on virDomainObjPtr To acquire the asynchronous job condition qemuDomainObjBeginAsyncJob() (if driver is unlocked) - Increments ref count on virDomainObjPtr - Waits until no async job is running - Waits for job.cond condition 'job.active != 0' using virDomainObjPtr mutex - Rechecks if any async job was started while waiting on job.cond and repeats waiting in that case - Sets job.asyncJob to the asynchronous job type qemuDomainObjBeginAsyncJobWithDriver() (if driver needs to be locked) - Increments ref count on virDomainObjPtr - Unlocks driver - Waits until no async job is running - Waits for job.cond condition 'job.active != 0' using virDomainObjPtr mutex - Rechecks if any async job was started while waiting on job.cond and repeats waiting in that case - Sets job.asyncJob to the asynchronous job type - Unlocks virDomainObjPtr - Locks driver - Locks virDomainObjPtr NB: this variant is required in order to comply with lock ordering rules for virDomainObjPtr vs driver qemuDomainObjEndAsyncJob() - Sets job.asyncJob to 0 - Broadcasts on job.asyncCond condition - Decrements ref count on virDomainObjPtr To acquire the QEMU monitor lock qemuDomainObjEnterMonitor() - Acquires the qemuMonitorObjPtr lock - Releases the virDomainObjPtr lock qemuDomainObjExitMonitor() - Releases the qemuMonitorObjPtr lock - Acquires the virDomainObjPtr lock NB: caller must take care to drop the driver lock if necessary These functions must not be used by an asynchronous job. 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() - Releases the qemuMonitorObjPtr lock - Acquires the driver lock - Acquires the virDomainObjPtr lock NB: caller must take care to drop the driver lock if necessary These functions must not be used inside an asynchronous job. To acquire the QEMU monitor lock with the driver lock held and as part of an asynchronous job qemuDomainObjEnterMonitorAsync() - Validates that the right async job is still running - Acquires the qemuMonitorObjPtr lock - Releases the virDomainObjPtr lock - Releases the driver lock - Validates that the VM is still active qemuDomainObjExitMonitorWithDriver() - Releases the qemuMonitorObjPtr lock - Acquires the driver lock - Acquires the virDomainObjPtr lock NB: caller must take care to drop the driver lock if necessary These functions are for use inside an asynchronous job; the caller must check for a return of -1 (VM not running, so nothing to exit). Helper functions may also call this with QEMU_ASYNC_JOB_NONE when used from a sync job (such as when first starting a domain). To keep a domain alive while waiting on a remote command, starting with the driver lock held qemuDomainObjEnterRemoterWithDriver() - Increments ref count on virDomainObjPtr - Releases the virDomainObjPtr lock - Releases the driver lock qemuDomainObjExitRemoteWithDriver() - Acquires the driver lock - Acquires the virDomainObjPtr lock - Decrements ref count on virDomainObjPtr Design patterns --------------- * Accessing or updating something with just the driver qemuDriverLock(driver); ...do work... qemuDriverUnlock(driver); * Accessing something directly to do with a virDomainObjPtr virDomainObjPtr obj; qemuDriverLock(driver); obj = virDomainFindByUUID(driver->domains, dom->uuid); qemuDriverUnlock(driver); ...do work... virDomainObjUnlock(obj); * Accessing something directly to do with a virDomainObjPtr and driver virDomainObjPtr obj; qemuDriverLock(driver); obj = virDomainFindByUUID(driver->domains, dom->uuid); ...do work... virDomainObjUnlock(obj); qemuDriverUnlock(driver); * Updating something directly to do with a virDomainObjPtr virDomainObjPtr obj; qemuDriverLock(driver); obj = virDomainFindByUUID(driver->domains, dom->uuid); qemuDriverUnlock(driver); qemuDomainObjBeginJob(obj, QEMU_JOB_TYPE); ...do work... qemuDomainObjEndJob(obj); virDomainObjUnlock(obj); * Invoking a monitor command on a virDomainObjPtr virDomainObjPtr obj; qemuDomainObjPrivatePtr priv; qemuDriverLock(driver); obj = virDomainFindByUUID(driver->domains, dom->uuid); qemuDriverUnlock(driver); qemuDomainObjBeginJob(obj, QEMU_JOB_TYPE); ...do prep work... if (virDomainObjIsActive(vm)) { 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, QEMU_JOB_TYPE); ...do prep work... if (virDomainObjIsActive(vm)) { qemuDomainObjEnterMonitorWithDriver(driver, obj); qemuMonitorXXXX(priv->mon); qemuDomainObjExitMonitorWithDriver(driver, obj); } ...do final work... qemuDomainObjEndJob(obj); virDomainObjUnlock(obj); qemuDriverUnlock(driver); * Running asynchronous job with driver lock held virDomainObjPtr obj; qemuDomainObjPrivatePtr priv; qemuDriverLock(driver); obj = virDomainFindByUUID(driver->domains, dom->uuid); qemuDomainObjBeginAsyncJobWithDriver(obj, QEMU_ASYNC_JOB_TYPE); qemuDomainObjSetAsyncJobMask(obj, allowedJobs); ...do prep work... if (qemuDomainObjEnterMonitorAsync(driver, obj, QEMU_ASYNC_JOB_TYPE) < 0) { /* domain died in the meantime */ goto error; } ...start qemu job... qemuDomainObjExitMonitorWithDriver(driver, obj); while (!finished) { if (qemuDomainObjEnterMonitorAsync(driver, obj, QEMU_ASYNC_JOB_TYPE) < 0) { /* domain died in the meantime */ goto error; } ...monitor job progress... qemuDomainObjExitMonitorWithDriver(driver, obj); virDomainObjUnlock(obj); sleep(aWhile); virDomainObjLock(obj); } ...do final work... qemuDomainObjEndAsyncJob(obj); virDomainObjUnlock(obj); qemuDriverUnlock(driver); * Coordinating with a remote server for migration virDomainObjPtr obj; qemuDomainObjPrivatePtr priv; qemuDriverLock(driver); obj = virDomainFindByUUID(driver->domains, dom->uuid); qemuDomainObjBeginAsyncJobWithDriver(obj, QEMU_ASYNC_JOB_TYPE); ...do prep work... if (virDomainObjIsActive(vm)) { qemuDomainObjEnterRemoteWithDriver(driver, obj); ...communicate with remote... qemuDomainObjExitRemoteWithDriver(driver, obj); /* domain may have been stopped while we were talking to remote */ if (!virDomainObjIsActive(vm)) { qemuReportError(VIR_ERR_INTERNAL_ERROR, "%s", _("guest unexpectedly quit")); } } ...do final work... qemuDomainObjEndAsyncJob(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