Uploaded byAnil Kumar Pugalia
Synchronization
This document discusses thread synchronization and concurrency in multithreaded programs. It covers various synchronization mechanisms like mutexes, condition variables, read/write locks, spin locks, barriers and semaphores. It also discusses issues like priority inversion, deadlocks, and their solutions. Finally, it talks about inter-thread communication techniques and provides examples to demonstrate the use of these synchronization primitives and communication methods.
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Synchronization
- 1. © 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Synchronization
- 2. 2© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. What to Expect? Thread Synchronization Mechanisms Mutex Conditional Variables Read/Write Locks Spin Locks Barriers Semaphores Priority Inversion & its Solutions Understanding a Deadlock Inter Thread Communication
- 3. 3© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Recall Thread Management Creation, Termination, Joining, Cleanup Thread-specific Data What about the other data? Shared: By virtue of existence Communication: Inherent No ITC mechanisms required Where is the catch? Concurrency Issues & Race Conditions
- 4. 4© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Solutions to Race Conditions Concurrency issues exist within a process Do not need kernel resources to solve them Just some internal synchronization & access protection mechanisms Implemented at pthreads library level Mutex → Critical Sections Condition Variables → Atomic Checks & Actions Read/Write Locks → Readers & Writers Resources Spin Locks → Critical Sections but without yielding Barriers → Serializing Execution Sempahores → Resource Usage
- 5. 5© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Mutual Exclusion Type: pthread_mutex_t APIs int pthread_mutex_init(&mutex, &attr); int pthread_mutex_destroy(&mutex); int pthread_mutex_lock(&mutex); int pthread_mutex_unlock(&mutex); int pthread_mutex_trylock(&mutex); Macro PTHREAD_MUTEX_INITIALIZER
- 6. 6© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Priority Inversion Mutex Task H Task M Task L Priority Inversion Time
- 7. 7© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Priority Inheritance Mutex Priority H Priority L Time Priority M
- 8. 8© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Mutual Exclusion Attributes Type: pthread_mutexattr_t APIs pthread_mutexattr_init(&attr); pthread_mutexattr_destroy(&attr); pthread_mutexattr_settype(&attr, type); PTHREAD_MUTEX_RECURSIVE / NORMAL / ERRORCHECK pthread_mutexattr_setpshared(&attr, pshared); PTHREAD_PROCESS_SHARED / PRIVATE pthread_mutexattr_setprotocol(&attr, protocol); (Only for RT) PTHREAD_PRIO_NONE, PTHREAD_PRIO_INHERIT, PTHREAD_PRIO_PROTECT pthread_mutexattr_setprioceiling(&attr, proioceiling); (Only for RT) Try out: thread_mutex_attr.c, thread_mutex_error.c
- 9. 9© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Deadlock Set of entities waiting for each other Four necessary & sufficient conditions Mutual Exclusion Hold & Wait Non-Preemptive Circular Wait Have a look @ thread_deadlock.c
- 10. 10© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Dealing with Deadlock The Ostrich Approach Deadlock Detection & Recovery Deadlock Algorithm: Current Resource Availability → Possible Allocation Sequences Iterative Recovery Algorithm: O(n2) Deadlock Avoidance Banker's Algorithm Deadlock Prevention Eliminate one of the four conditions
- 11. 11© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Condition Variables Type: pthread_cond_t APIs int pthread_cond_init(&cond, &attr); int pthread_cond_destroy(&cond); int pthread_cond_wait(&cond, &mutex); int pthread_cond_signal(&cond); int pthread_cond_broadcast(&cond); Macro PTHREAD_COND_INITIALIZER
- 12. 12© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Condition Variable Attributes Type: pthread_condattr_t APIs pthread_condattr_init(&attr); pthread_condattr_destroy(&attr); pthread_condattr_setpshared(&attr, pshared); PTHREAD_PROCESS_SHARED / PRIVATE Try out: thread_cond.c
- 13. 13© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Read / Write Locks Type: pthread_rwlock_t APIs int pthread_rwlock_init(&rwlock, &attr); int pthread_rwlock_destroy(&rwlock); int pthread_rwlock_rdlock(&rwlock); int pthread_rwlock_tryrdlock(&rwlock); int pthread_rwlock_wrlock(&rwlock); int pthread_rwlock_trywrlock(&rwlock); int pthread_rwlock_unlock(&rwlock); Macro PTHREAD_RWLOCK_INITIALIZER
- 14. 14© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Read / Write Lock Attributes Type: pthread_rwlockattr_t APIs pthread_rwlockattr_init(&attr); pthread_rwlockattr_destroy(&attr); pthread_rwlockattr_setpshared(&attr, pshared); PTHREAD_PROCESS_SHARED / PRIVATE pthread_rwlockattr_setkind_np(&attr, kind); PTHREAD_RWLOCK_PREFER_READER_NP / WRITER_NP / WRITER_NONRECURSIVE_NP Try out: thread_rwlock.c
- 15. 15© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Spin Locks Type: pthread_spinlock_t APIs int pthread_spin_init(&spin, pshared); int pthread_spin_destroy(&spin); int pthread_spin_lock(&spin); int pthread_spin_trylock(&spin); int pthread_spin_unlock(&spin); Try out: thread_spinlock.c
- 16. 16© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Barriers & its Attributes Type: pthread_barrier_t APIs int pthread_barrier_init(&barrier, &attr, cnt); int pthread_barrier_destroy(&barrier); int pthread_barrier_wait(&barrier); Attribute Type: pthread_barrierattr_t Attribute APIs pthread_barrierattr_init(&attr); pthread_barrierattr_destroy(&attr); pthread_barrierattr_setpshared(&attr, pshared); PTHREAD_PROCESS_SHARED / PRIVATE Try out: thread_barrier.c
- 17. 17© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Semaphores Type: sem_t APIs int sem_init(sem_t *sem, int pshared, unsigned int value); int sem_destroy(sem_t *sem); int sem_post(sem_t *sem); int sem_wait(sem_t *sem); int sem_trywait(sem_t *sem); Try out: thread_sem.c
- 18. 18© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Exchange Offer Thread synchronizing mechanisms Can be used for same ancestor processes by setting there pshared attribute to non-zero Example: Semaphores for Processes Similarly, IPC mechanisms Can be used by threads as well Let's see some examples
- 19. 19© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Inter Thread Communication Signals in Threads Pipe for Threads Memory Map Sharing for Threads Examples thread_kill.c thread_ipc.c thread_ipc2.c
- 20. 20© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. What all have we learnt? Thread Synchronization Mechanisms Mutex Conditional Variables Read/Write Locks Spin Locks Barriers Semaphores Priority Inversion & its Solutions Understanding a Deadlock Inter Thread Communication
- 21. 21© 2010-17 SysPlayWorkshops <workshop@sysplay.in> All Rights Reserved. Any Queries?