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c-user: Add SMP internal locking

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Sebastian Huber 2017-02-03 14:26:03 +01:00
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c-user/symmetric_multiprocessing_services.rst
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@ -600,6 +600,44 @@ processors.
:width: 400
:align: center
Internal Locking
----------------
In SMP configurations, the operating system uses non-recursive SMP locks for
low-level mutual exclusion. The locking domains are roughly
* a particular data structure,
* the thread queue operations,
* the thread state changes, and
* the scheduler operations.
For a good average-case performance it is vital that every high-level
synchronization object, e.g. mutex, has its own SMP lock. In the average-case,
only this SMP lock should be involved to carry out a specific operation, e.g.
obtain/release a mutex. In general, the high-level synchronization objects
have a thread queue embedded and use its SMP lock.
In case a thread must block on a thread queue, then things get complicated.
The executing thread first acquires the SMP lock of the thread queue and then
figures out that it needs to block. The procedure to block the thread on this
particular thread queue involves state changes of the thread itself and for
this thread-specific SMP locks must be used.
In order to determine if a thread is blocked on a thread queue or not
thread-specific SMP locks must be used. A thread priority change must
propagate this to the thread queue (possibly recursively). Care must be taken
to not have a lock order reversal between thread queue and thread-specific SMP
locks.
Each scheduler instance has its own SMP lock. For the scheduler helping
protocol multiple scheduler instances may be in charge of a thread. It is not
possible to acquire two scheduler instance SMP locks at the same time,
otherwise deadlocks would happen. A thread-specific SMP lock is used to
synchronize the thread data shared by different scheduler instances.
The thread state SMP lock protects various things, e.g. the thread state, join
operations, signals, post-switch actions, the home scheduler instance, etc.
Profiling
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