[RTEMS Project] #2555: Eliminate the Giant lock

[RTEMS trac]

#2555: Eliminate the Giant lock
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 Reporter:  sebastian.huber  |      Owner:  sebastian.huber
     Type:  enhancement      |     Status:  new
 Priority:  normal           |  Milestone:  4.12
Component:  cpukit           |    Version:  4.12
 Severity:  normal           |   Keywords:
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 = Background =

 The standard approach to turn a uni-processor operating system into an
 SMP-capable operating system is to encapsulate virtually the complete
 operating system state and protect it by one global recursive lock - the
 Giant lock.  Thus, at most one processor can modify the operating system
 state at a time.  Under Giant lock protection interrupt disable/enable
 critical sections still work.  This approach is easy to realize and leads
 to something that runs on SMP with a minimal effort.  Unfortunately, such
 an operating system does not scale with the processor count and offers
 very poor performance.  It is quite useless for real applications.

 The first steps to get rid of the Giant lock are tackled with the
 introduction of fine grained locking for the scheduler, watchdog handler,
 timestamps, thread queues, events, semaphores and message queues.  The
 Giant lock is still used in a couple of places, e.g. all other objects
 using thread queues, thread life cycle changes (termination, restart) and
 scheduler changes.  It is a straight forward task to eliminate it
 entirely, but it is also somewhat labour intensive since a lot of code
 must be changed.

 = Implementation =

 Eliminate all remaining code areas that use
 * `_ISR_Disable()`,
 * `_ISR_Enable()`, and
 * `_ISR_Flash()`.
 Direct users of these macros are
 * `_Chain_Append()`,
 * `_Chain_Extract()`,
 * `_Chain_Get()`,
 * `_Chain_Insert()`, and
 * `_Chain_Prepend()`.
 Each spot must be dealt with individually.  Once this is done, delete
 these macros since they are now superfluous.  Rename
 `_ISR_Disable_without_giant()` into `_ISR_Locale_disable()`.  Rename
 `_ISR_Enable_without_giant()` into `_ISR_Locale_enable()`.

 Eliminate all remaining code areas that use
 * `_Thread_Disable_dispatch()` and
 * `_Thread_Enable_dispatch()`.
 A prominent user of this functions is `_Objects_Get()`.  The following
 components are affected by these functions
 * Classic barrier handler,
 * Classic dual-ported memory handler,
 * Classic message queue handler,
 * Classic partition handler,
 * Classic rate-monotonic handler,
 * Classic region handler,
 * Classic semaphore handler,
 * Classic timer handler,
 * extension handler,
 * IO manager,
 * multi-processing management,
 * objects management,
 * POSIX barrier handler,
 * POSIX condition handler,
 * POSIX key handler,
 * POSIX message queue handler,
 * POSIX mutex handler,
 * POSIX rwlock handler,
 * POSIX semaphore handler,
 * POSIX spinlock handler,
 * POSIX timer handler,
 * signals,
 * thread cancellation,
 * thread life-cycle changes, and
 * thread scheduler changes.
 Once this is done, delete `_Thread_Disable_dispatch()` and
 `_Thread_Enable_dispatch()`.  As a side-effect the Giant lock will be
 removed.

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Ticket URL: <http://devel.rtems.org/ticket/2555>
RTEMS Project <http://www.rtems.org/>
RTEMS Project