Transcript Synchronization - Vanderbilt University

Synchronization
Akos Ledeczi
EECE 354, Fall 2012
Vanderbilt University
Semaphores
• They work for synchronization too
• ISR to Task or
• Task to Task
Unilateral Rendezvous
• Task calling Pend() does not need to know about the details: the
Pend() call will return when the event has occurred
• Maximizes use of CPU; no busy waiting
• Event (i.e., Post()) will cause the waiting task to run immediately
(assuming it has the highest priority)
Credit Tracking
• Semaphore accumulates the number of times Post()
has been called /without a corresponding Pend()/
Broadcasting
•
•
•
•
Multiple tasks waiting on a semaphore
OS_OPT_POST_ALL
All waiting tasks become ready to run
Care must be taken because some of the tasks may not have called Pend() yet
Task Semaphores
•
•
•
•
In µC/OS-III each task has a built-in semaphore
More efficient
OSTaskSemPend(); OSTaskSemPost(); etc.
Post()-er (ISR or Task) needs to know which task to notify
Bilateral Rendezvous
void Task1(void *p_arg)
{
…
OSTaskSemPost(&Task2_TCB,
OS_OPT_POST_1, &err);
OSTaskSemPend(0,OS_OPT_PEND_BLOCKING,&ts,&err);
…
}
void Task2(void *p_arg)
{
…
OSTaskSemPost(&Task1_TCB,
OS_OPT_POST_1, &err);
OSTaskSemPend(0,OS_OPT_PEND_BLOCKING,&ts,&err);
…
}
• Only between tasks, as
an ISR cannot Pend()
Event Flags
•
•
•
•
•
When tasks need to synchronize on multiple events
When any of the events is enough: disjunctive synchronization (OR)
When all events are needed: conjunctive synchronization (AND)
OSFlag???();
OSFlagPendGetFlagsRdy(): which flags caused task to become ready
EventFlags Example
#define TEMP_LOW
#define BATT_LOW
#define SW_PRESSED
(OS_FLAGS)0x01
(OS_FLASG)0x02
(OS_FLAGS)0x04
OS_FLAG_GRP MyEventFlagGrp;
void main()
{
…
OSFlagCreate(&MyEventFlagGrp, “My flags”,0,&err);
…
}
void Task1(void *p_arg)
{
…
for( ; ; )
{
OSFlagPend(&MyEventFlagGrp,
TEMP_LOW + BATT_LOW,
0
OS_OPT_PEND_FLAG_SET_ANY,
ts,&err);
…
}
}
void Task2(void *p_arg)
{
…
for ( ; ; )
{
OSFlagPost(&MyEventFlagGrp,
BATT_LOW,
OS_OPT_POST_FLAG_SET,
&err);
}
…
}
Status and Transient Events
• Typically:
– Status info monitored by
one task, handled by
another task using a
non-blocking call
– A transient event is
generated by an ISR or
task and handled and
consumed by a task
using a blocking call
Event Flag Internals
struct os_flag_grp {
OS_OBJ_TYPE
CPU_CHAR
OS_PEND_LIST
OS_FLAGS
CPU_TS
};
Type
*NamePtr
PendList;
Flags
TS
OSFlagPost(&flagGrp,
(OS_FLAGS)0x0C,
OS_OPT_FLAG_SET,
&err);
• Pend list is not in priority
order as all tasks need to
be looked at anyways
• Look at code on the left:
• If OS_OPT_FLAG_SET
and flags contain 0x03,
result will be 0x0F
• If OS_OPT_FLAG_CLR
and flags contain 0x0F,
result will be 0x03
Synchronizing Multiple Tasks