3. semaphore tn

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Semaphores

Prepared by:Lâm Ngọc ẨnTăng Trường Tuyển

4/19/12 11Semaphores



4/19/12 Semaphores

Contents

Definition1.

Operators2.

Typical Use3.

Implementation in T-kernel4.




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1. Definition

v A semaphore (sometimes called asemaphore token) is a kernelobject that one or more threads of

execution can acquire or releasefor the purposes of synchronization or mutual

exclusion




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1. Definition




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1. Definition

v 3 groups:§ Binary Semaphores§ Counting Semaphores§ Mutual Exclusion Semaphores




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1.1. Binary Semaphores

v Binary semaphores are treated asglobal resources



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1.2. Counting Semaphores

v global resourcesv bounded count vs unbounded count




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1.3. Mutual Exclusion Semaphores

v A mutual exclusion (mutex)semaphore is a special binarysemaphore




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v Ownershipv Recursive accessv Task deletion safetyv Protocols for avoiding problems

inherent to mutual exclusion




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v Owner is the first task acquiring itv Only owner can release it (contrast

with binary semaphores)




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v Recursive locking (optional): owner can acquire multiple times in thelocked state

v Avoid causing the deadlock innested attempts to acquire theresource




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v Task Deletion Safety (optional):using task deletion locks toensure that the owner task cannot

be deleted




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v Priority Inversion Avoidance:




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v Priority Inversion Avoidance§ Common protocols:

• priority inheritance• ceiling priority




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2. Operator

v creating and deleting semaphoresv acquiring and releasing semaphoresv clearing a semaphore’s task-waiting

listv getting semaphore information.




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3. Typical Use

v

Wait-and-Signal Synchronization




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3. Typical Use

v

Wait-and-Signal SynchronizationtWaitTask ( ) {

:

Acquire binary semaphore token

:

}tSignalTask ( ) {

:

Release binary semaphore token

:}




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3. Typical Use

v

Multiple-Task Wait-and-SignalSynchronization




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3. Typical Use

v

Multiple-Task Wait-and-SignalSynchronization tWaitTask () {

:

Do some processing specific to task Acquire binary semaphoretoken

:

}

tSignalTask () {

:Do some processing Flush binary semaphore's task-waiting list

:

}




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3. Typical Use

v

Credit-Tracking Synchronization




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3. Typical Use

v

Credit-Tracking Synchronization tWaitTask () {

:

Acquire counting semaphore token

:}

tSignalTask () {

:

Release counting semaphore token

:}




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3. Typical Use

v

Single Shared-Resource-AccessSynchronization




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3. Typical Use

v

Single Shared-Resource-AccessSynchronizationtAccessTask () {

:

Acquire binary semaphore token Read or write to sharedresource Release binary semaphore token

:

}




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3. Typical Use

v

Multiple Shared-Resource-AccessSynchronization




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3. Typical Use

v

Multiple Shared-Resource-AccessSynchronizationtAccessTask () {

:

Acquire a counting semaphore tokenRead or Write to shared resource

Release a counting semaphore token

:

}




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3. Typical Use

v

Multiple Shared-Resource-AccessSynchronization

tAccessTask () {

:Acquire first mutex in non-blocking way

If not successful then acquire 2nd mutex in a blocking way

Read or Write to shared resource

Release the acquired mutex

:}




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3. Typical Use

v

Recursive Shared-Resource-Access Synchronization




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3. Typical Use

v

Recursive Shared-Resource-Access SynchronizationtAccessTask () {

Acquire mutex

Access shared resourceCall Routine A

Release mutex

}

Routine A () {

Acquire mutex Access shared resource

Call Routine B

Release mutex

}4/19/12 2929Semaphores

Routine B () {:

Acquire mutex Access shared resourceRelease mutex:

}



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4. Implementation in T-Kernel

v

The semaphores functions:ID semid = tk_cre_sem ( T_CSEM *pk_csem );

ER ercd = tk_del_sem ( ID semid ) ;

ER ercd = tk_sig_sem ( ID semid, INT cnt ) ; ER ercd = tk_wai_sem ( ID semid, INT cnt,

TMO tmout ) ;

ER ercd = tk_ref_sem ( ID semid, T_RSEM*pk_rsem ) ;




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4. Implementation in T-Kernel

v

The mutex functions:ID mtxid = tk_cre_mtx ( T_CMTX *pk_cmtx ) ;

ER ercd = tk_del_mtx ( ID mtxid ) ;

ER ercd = tk_loc_mtx ( ID mtxid, TMO tmout ) ;ER ercd = tk_unl_mtx ( ID mtxid ) ;

ER ercd = tk_ref_mtx ( ID mtxid, T_RMTX

*pk_rmtx ) ;




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4. Reference

v

T-kernel Specification – T-engine Forumv Real-Time Concepts for Embedded Systems -

Qing Li with Caroline Yao




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Thank

You !

4/19/12 3333S h

3. semaphore tn

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