+++ /dev/null
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->Scheduler Control</TITLE
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-><H1
-><A
-NAME="KERNEL-SCHEDCONTROL">Scheduler Control</H1
-><DIV
-CLASS="REFNAMEDIV"
-><A
-NAME="AEN1784"
-></A
-><H2
->Name</H2
->cyg_scheduler_start, cyg_scheduler_lock, cyg_scheduler_unlock, cyg_scheduler_safe_lock, cyg_scheduler_read_lock -- Control the state of the scheduler</DIV
-><DIV
-CLASS="REFSYNOPSISDIV"
-><A
-NAME="AEN1791"><H2
->Synopsis</H2
-><DIV
-CLASS="FUNCSYNOPSIS"
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-><TD
-><PRE
-CLASS="FUNCSYNOPSISINFO"
->#include <cyg/kernel/kapi.h>
- </PRE
-></TD
-></TR
-></TABLE
-><P
-><CODE
-><CODE
-CLASS="FUNCDEF"
->void cyg_scheduler_start</CODE
->(void);</CODE
-></P
-><P
-><CODE
-><CODE
-CLASS="FUNCDEF"
->void cyg_scheduler_lock</CODE
->(void);</CODE
-></P
-><P
-><CODE
-><CODE
-CLASS="FUNCDEF"
->void cyg_scheduler_unlock</CODE
->(void);</CODE
-></P
-><P
-><CODE
-><CODE
-CLASS="FUNCDEF"
->cyg_ucount32 cyg_scheduler_read_lock</CODE
->(void);</CODE
-></P
-><P
-></P
-></DIV
-></DIV
-><DIV
-CLASS="REFSECT1"
-><A
-NAME="KERNEL-SCHEDCONTROL-DESCRIPTION"
-></A
-><H2
->Description</H2
-><P
-><TT
-CLASS="FUNCTION"
->cyg_scheduler_start</TT
-> should only be called once,
-to mark the end of system initialization. In typical configurations it
-is called automatically by the system startup, but some applications
-may bypass the standard startup in which case
-<TT
-CLASS="FUNCTION"
->cyg_scheduler_start</TT
-> will have to be called
-explicitly. The call will enable system interrupts, allowing I/O
-operations to commence. Then the scheduler will be invoked and control
-will be transferred to the highest priority runnable thread. The call
-will never return.
- </P
-><P
->The various data structures inside the eCos kernel must be protected
-against concurrent updates. Consider a call to
-<TT
-CLASS="FUNCTION"
->cyg_semaphore_post</TT
-> which causes a thread to be
-woken up: the semaphore data structure must be updated to remove the
-thread from its queue; the scheduler data structure must also be
-updated to mark the thread as runnable; it is possible that the newly
-runnable thread has a higher priority than the current one, in which
-case preemption is required. If in the middle of the semaphore post
-call an interrupt occurred and the interrupt handler tried to
-manipulate the same data structures, for example by making another
-thread runnable, then it is likely that the structures will be left in
-an inconsistent state and the system will fail.
- </P
-><P
->To prevent such problems the kernel contains a special lock known as
-the scheduler lock. A typical kernel function such as
-<TT
-CLASS="FUNCTION"
->cyg_semaphore_post</TT
-> will claim the scheduler lock,
-do all its manipulation of kernel data structures, and then release
-the scheduler lock. The current thread cannot be preempted while it
-holds the scheduler lock. If an interrupt occurs and a DSR is supposed
-to run to signal that some event has occurred, that DSR is postponed
-until the scheduler unlock operation. This prevents concurrent updates
-of kernel data structures.
- </P
-><P
->The kernel exports three routines for manipulating the scheduler lock.
-<TT
-CLASS="FUNCTION"
->cyg_scheduler_lock</TT
-> can be called to claim the
-lock. On return it is guaranteed that the current thread will not be
-preempted, and that no other code is manipulating any kernel data
-structures. <TT
-CLASS="FUNCTION"
->cyg_scheduler_unlock</TT
-> can be used to
-release the lock, which may cause the current thread to be preempted.
-<TT
-CLASS="FUNCTION"
->cyg_scheduler_read_lock</TT
-> can be used to query the
-current state of the scheduler lock. This function should never be
-needed because well-written code should always know whether or not the
-scheduler is currently locked, but may prove useful during debugging.
- </P
-><P
->The implementation of the scheduler lock involves a simple counter.
-Code can call <TT
-CLASS="FUNCTION"
->cyg_scheduler_lock</TT
-> multiple times,
-causing the counter to be incremented each time, as long as
-<TT
-CLASS="FUNCTION"
->cyg_scheduler_unlock</TT
-> is called the same number of
-times. This behaviour is different from mutexes where an attempt by a
-thread to lock a mutex multiple times will result in deadlock or an
-assertion failure.
- </P
-><P
->Typical application code should not use the scheduler lock. Instead
-other synchronization primitives such as mutexes and semaphores should
-be used. While the scheduler is locked the current thread cannot be
-preempted, so any higher priority threads will not be able to run.
-Also no DSRs can run, so device drivers may not be able to service
-I/O requests. However there is one situation where locking the
-scheduler is appropriate: if some data structure needs to be shared
-between an application thread and a DSR associated with some interrupt
-source, the thread can use the scheduler lock to prevent concurrent
-invocations of the DSR and then safely manipulate the structure. It is
-desirable that the scheduler lock is held for only a short period of
-time, typically some tens of instructions. In exceptional cases there
-may also be some performance-critical code where it is more
-appropriate to use the scheduler lock rather than a mutex, because the
-former is more efficient.
- </P
-></DIV
-><DIV
-CLASS="REFSECT1"
-><A
-NAME="KERNEL-SCHEDCONTROL-CONTEXT"
-></A
-><H2
->Valid contexts</H2
-><P
-><TT
-CLASS="FUNCTION"
->cyg_scheduler_start</TT
-> can only be called during
-system initialization, since it marks the end of that phase. The
-remaining functions may be called from thread or DSR context. Locking
-the scheduler from inside the DSR has no practical effect because the
-lock is claimed automatically by the interrupt subsystem before
-running DSRs, but allows functions to be shared between normal thread
-code and DSRs.
- </P
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