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63 lines
2.5 KiB
Plaintext
63 lines
2.5 KiB
Plaintext
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Threading: the RULES.
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====================
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If you don't understand this, don't touch the code. Ask for
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further advice / explanation on the mailing list first.
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- the top level lock is on 'struct qemud_server'. This must be
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held before acquiring any other lock
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- Each 'struct qemud_client' object has a lock. The server lock
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must be held before acquiring it. Once the client lock is acquired
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the server lock can (optionally) be dropped.
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- The event loop has its own self-contained lock. You can ignore
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this as a caller of virEvent APIs.
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The server lock is only needed / used once the daemon has entered
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its main loop, which is the qemudRunLoop() . The initial thread
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acquires the lock upon entering this method.
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It immediatelty spawns 'n' worker threads, whose main loop is
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the qemudWorker() method. The workers will immediately try to
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acquire the server lock, and thus block since its held by the
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initial thread.
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When the initial thread enters the poll() call, it drops the
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server lock. The worker locks now each wakeup, acquire the
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server lock and go into a condition wait on the 'job' condition
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variable. The workers are now all 'primed' for incoming RPC
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calls.
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A file descriptor event now occurrs, causing the initial thread
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to exit poll(). It invokes the registered callback associated
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with the file descriptors on which the event occurrs. The callbacks
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are required to immediately acquire the server lock.
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If the callback is dealing with a client event, it will then
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acquire the client lock, and drop the server lock.
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The callback will now handle the I/O event, reading or writing
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a RPC message. Once a complete RPC message has been read the
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client is marked as being in state QEMUD_MODE_WAIT_DISPATCH,
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and the 'job' condition variable is signaled. The callback
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now drops the client lock and goes back into the poll() loop
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waiting for more I/O events.
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Meanwhile one of the worker threads wakes up from its condition
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variable sleep, holding the server lock. It now searches for a
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client in state QEMUD_MODE_WAIT_DISPATCH. If it doesn't find
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one, it goes back to sleep. If it does find one, then it calls
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into the remoteDispatchClientRequest() method de-serialize the
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incoming message into an XDR object and invoke the helper method
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for the associated RPC call.
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While the helper method is executing, no locks are held on either
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the client or server, but the ref count on the 'struct qemud_client'
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object is incremented to ensure its not deleted. The helper can
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now safely invoke the neccessary libvirt API call.
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