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138 lines
5.6 KiB
138 lines
5.6 KiB
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CEssNamespace Locks
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===================
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There are two namespace locks : level1 and level2.
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Level1 is supposed to be a lightweight lock and guards members like
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the current state of the namespace and members that must be accessed when
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signaling events ( such as the deferred event queue ). Because of the last
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point, very little should be done while holding this lock.
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Level2 is the heavy handed lock and guards all of the changes to the
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provider cache and to the subscription objects ( e.g. binding.h ).
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Level2 must always be aquired before level1 if both are needed.
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Level2 or Level1 can never be held when making calls to the providers.
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This is problematic because level2 is aquired at the top level and the calls
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to the providers occur down deep in the provider cache. To handle this, all
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calls to providers are scheduled on a Postponed list associatated with the
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threads. After the level2 is released, then the provider calls can be made
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and the postponed operations are executed. Note that these calls must occur
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on the same control path as the one that scheduled them so they cannot be
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asynchronously executed.
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Level2 can never be held when signaling an event. This is because some
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subscriptions can be synchronous and the action taken on notification could
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be to call back into ess ( say to cancel a subscription or something ). The
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other reason is that it is possible to aquire the level2 when holding a
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filter proxy lock and we must avoid the reverse scenario to avoid a deadlock.
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ESS Sink Lock
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==================
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This is a shared lock whose only purpose is to facilitate shutdown of ESS.
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Since all public access to ESS is performed through the esssink, this is
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where the ess shutdown check is.
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Each entry point except shutdown() will ...
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1 ) enter the esssink lock with shared access,
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2 ) check to see if shutdown has been performed, if so then goto (4)
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3 ) perform the op
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4 ) then release it.
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Shutdown will ..
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1 ) aquires the lock for exclusive access
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2 ) set shutdown
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3 ) release lock
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Since the shared lock handles writer starvation, the shutdown op waits for
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all current ops to finish, but does not allow any new ones to procede until
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it has executed.
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Filter Proxy Lock
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==================
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PURPOSE : To synchronize the signaling of an event through the proxy with
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disconnecting the proxy. When disconneting the proxy from the stub, we want
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to ensure that all calls currently executing through that proxy are complete.
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( We could have used CoDisconnectObject on the stub for the same functionality,
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but this would only work when the proxy was in a separate process/apartment
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from the stub which is not always the case ).
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TYPE : This is a CWbemCriticalSection ( but should be a shared lock so that
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the signaling threads requests shared access and the Disconnect() thread was
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exclusive access. )
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RULES :
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Must be aquired before Namespace Level2 Lock. Reason is that the lock MUST
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be held across the signaling of an event, for reasons described above. Since
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we support synchronous delivery, there is nothing stopping a consumer from
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turning around and issuing a request that will grab the level2 lock in the
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same namespace. Because of this, we must always ensure that the proxy lock
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is aquired BEFORE acquiring the level2 namespace lock.
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Provider Exec Line
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=========================
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PURPOSE :
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This is a different sort of sync mechanism. Its really a queue more than
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a lock. It allows the user to place requests in a queue and then to execute
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them later. The major difference between this and a normal queue is in the
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way that requests are fetched from the queue and executed. The exec line
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allows multiple threads to fetch requests from the queue and execute them
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while still preserving the logical ordering of the requests in the queue.
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For example, lets say that there are the following requests placed in the
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queue ...
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A, B, C <-- rear
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Lets say that T1 placed A and B in the queue and T2 placed C in the queue.
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Then both threads try to service their requests. This structure would ensure
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that A and B completed before C could execute.
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The reason for such a sync structure is that we do not make calls to a provider
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while holding the namespace lock. So we 'postpone' the requests to the
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provider. Later, after releasing the namespace lock, we execute the
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'postponed' operations. This structure ensures that execution of those
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postponed operations occurs in the same logical order as the namespace
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operations.
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e.g. If Namespace Op N1 causes Postponed Operation P1. And N2
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causes Postponed Operation P2. Then P1 will be executed before P2 even if
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the thread handling N2 tries to execute its postponed operations first.
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This is the following protocol used with this sync mechanism
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1 ) Get In Line - this reserves a place in the line, called a Turn. A turn
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is associated with a postponed request. The turn is returned from this
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step.
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2 ) Wait For Turn - once obtained, the request can be executed.
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3 ) End Turn - after the request is executed, the turn is ended thereby
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allowing the next turn to execute.
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Each provider record has an associated exec line.
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RULES : It is illegal to obtain a proxy lock when holding one or more turns
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in any exec line. The reason is that is possible that when holding the
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proxy lock that we could wait for a turn. ( Just as it is possible when
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holding a proxy lock to obtain the namespace lock ). For this reason, if
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we allowed waiting for the proxy lock while holding a turn then we'd have
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a deadlock issue.
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