NAME¶

pmemobj_mutex_zero(), pmemobj_mutex_lock(), pmemobj_mutex_timedlock(), pmemobj_mutex_trylock(), pmemobj_mutex_unlock(),

pmemobj_rwlock_zero(), pmemobj_rwlock_rdlock(), pmemobj_rwlock_wrlock(), pmemobj_rwlock_timedrdlock(), pmemobj_rwlock_timedwrlock(), pmemobj_rwlock_tryrdlock(), pmemobj_rwlock_trywrlock(), pmemobj_rwlock_unlock(),

pmemobj_cond_zero(), pmemobj_cond_broadcast(), pmemobj_cond_signal(), pmemobj_cond_timedwait(), pmemobj_cond_wait() - pmemobj synchronization primitives

SYNOPSIS¶

#include <libpmemobj.h>
void pmemobj_mutex_zero(PMEMobjpool *pop, PMEMmutex *mutexp);
int pmemobj_mutex_lock(PMEMobjpool *pop, PMEMmutex *mutexp);
int pmemobj_mutex_timedlock(PMEMobjpool *pop, PMEMmutex *restrict mutexp,
    const struct timespec *restrict abs_timeout);
int pmemobj_mutex_trylock(PMEMobjpool *pop, PMEMmutex *mutexp);
int pmemobj_mutex_unlock(PMEMobjpool *pop, PMEMmutex *mutexp);
void pmemobj_rwlock_zero(PMEMobjpool *pop, PMEMrwlock *rwlockp);
int pmemobj_rwlock_rdlock(PMEMobjpool *pop, PMEMrwlock *rwlockp);
int pmemobj_rwlock_wrlock(PMEMobjpool *pop, PMEMrwlock *rwlockp);
int pmemobj_rwlock_timedrdlock(PMEMobjpool *pop, PMEMrwlock *restrict rwlockp,
    const struct timespec *restrict abs_timeout);
int pmemobj_rwlock_timedwrlock(PMEMobjpool *pop, PMEMrwlock *restrict rwlockp,
    const struct timespec *restrict abs_timeout);
int pmemobj_rwlock_tryrdlock(PMEMobjpool *pop, PMEMrwlock *rwlockp);
int pmemobj_rwlock_trywrlock(PMEMobjpool *pop, PMEMrwlock *rwlockp);
int pmemobj_rwlock_unlock(PMEMobjpool *pop, PMEMrwlock *rwlockp);
void pmemobj_cond_zero(PMEMobjpool *pop, PMEMcond *condp);
int pmemobj_cond_broadcast(PMEMobjpool *pop, PMEMcond *condp);
int pmemobj_cond_signal(PMEMobjpool *pop, PMEMcond *condp);
int pmemobj_cond_timedwait(PMEMobjpool *pop, PMEMcond *restrict condp,
    PMEMmutex *restrict mutexp, const struct timespec *restrict abs_timeout);
int pmemobj_cond_wait(PMEMobjpool *pop, PMEMcond *restrict condp,
    PMEMmutex *restrict mutexp);

DESCRIPTION¶

libpmemobj(7) provides several types of synchronization primitives designed to be used with persistent memory. The pmem-aware lock implementation is based on the standard POSIX Threads Library, as described in pthread_mutex_init(3), pthread_rwlock_init(3) and pthread_cond_init(3). Pmem-aware locks provide semantics similar to standard pthread locks, except that they are embedded in pmem-resident objects and are considered initialized by zeroing them. Therefore, locks allocated with pmemobj_zalloc(3) or pmemobj_tx_zalloc(3) do not require another initialization step. For performance reasons, they are also padded up to 64 bytes (cache line size).

On FreeBSD, since all pthread locks are dynamically allocated, while the lock object is still padded up to 64 bytes for consistency with Linux, only the pointer to the lock is embedded in the pmem-resident object. libpmemobj(7) transparently manages freeing of the locks when the pool is closed.

The fundamental property of pmem-aware locks is their automatic reinitialization every time the persistent object store pool is opened. Thus, all the pmem-aware locks may be considered initialized (unlocked) immediately after the pool is opened, regardless of their state at the time the pool was closed for the last time.

Pmem-aware mutexes, read/write locks and condition variables must be declared with the PMEMmutex, PMEMrwlock, or PMEMcond type, respectively.

The pmemobj_mutex_zero() function explicitly initializes the pmem-aware mutex mutexp by zeroing it. Initialization is not necessary if the object containing the mutex has been allocated using pmemobj_zalloc(3) or pmemobj_tx_zalloc(3).

The pmemobj_mutex_lock() function locks the pmem-aware mutex mutexp. If the mutex is already locked, the calling thread will block until the mutex becomes available. If this is the first use of the mutex since the opening of the pool pop, the mutex is automatically reinitialized and then locked.

pmemobj_mutex_timedlock() performs the same action as pmemobj_mutex_lock(), but will not wait beyond abs_timeout to obtain the lock before returning.

The pmemobj_mutex_trylock() function locks pmem-aware mutex mutexp. If the mutex is already locked, pthread_mutex_trylock() will not block waiting for the mutex, but will return an error. If this is the first use of the mutex since the opening of the pool pop, the mutex is automatically reinitialized and then locked.

The pmemobj_mutex_unlock() function unlocks the pmem-aware mutex mutexp. Undefined behavior follows if a thread tries to unlock a mutex that has not been locked by it, or if a thread tries to release a mutex that is already unlocked or has not been initialized.

The pmemobj_rwlock_zero() function is used to explicitly initialize the pmem-aware read/write lock rwlockp by zeroing it. Initialization is not necessary if the object containing the lock has been allocated using pmemobj_zalloc(3) or pmemobj_tx_zalloc(3).

The pmemobj_rwlock_rdlock() function acquires a read lock on rwlockp, provided that the lock is not presently held for writing and no writer threads are presently blocked on the lock. If the read lock cannot be acquired immediately, the calling thread blocks until it can acquire the lock. If this is the first use of the lock since the opening of the pool pop, the lock is automatically reinitialized and then acquired.

pmemobj_rwlock_timedrdlock() performs the same action as pmemobj_rwlock_rdlock(), but will not wait beyond abs_timeout to obtain the lock before returning. A thread may hold multiple concurrent read locks. If so, pmemobj_rwlock_unlock() must be called once for each lock obtained. The results of acquiring a read lock while the calling thread holds a write lock are undefined.

The pmemobj_rwlock_wrlock() function blocks until a write lock can be acquired against read/write lock rwlockp. If this is the first use of the lock since the opening of the pool pop, the lock is automatically reinitialized and then acquired.

pmemobj_rwlock_timedwrlock() performs the same action, but will not wait beyond abs_timeout to obtain the lock before returning.

The pmemobj_rwlock_tryrdlock() function performs the same action as pmemobj_rwlock_rdlock(), but does not block if the lock cannot be immediately obtained. The results are undefined if the calling thread already holds the lock at the time the call is made.

The pmemobj_rwlock_trywrlock() function performs the same action as pmemobj_rwlock_wrlock(), but does not block if the lock cannot be immediately obtained. The results are undefined if the calling thread already holds the lock at the time the call is made.

The pmemobj_rwlock_unlock() function is used to release the read/write lock previously obtained by pmemobj_rwlock_rdlock(), pmemobj_rwlock_wrlock(), pthread_rwlock_tryrdlock(), or pmemobj_rwlock_trywrlock().

The pmemobj_cond_zero() function explicitly initializes the pmem-aware condition variable condp by zeroing it. Initialization is not necessary if the object containing the condition variable has been allocated using pmemobj_zalloc(3) or pmemobj_tx_zalloc(3).

The difference between pmemobj_cond_broadcast() and pmemobj_cond_signal() is that the former unblocks all threads waiting for the condition variable, whereas the latter blocks only one waiting thread. If no threads are waiting on condp, neither function has any effect. If more than one thread is blocked on a condition variable, the used scheduling policy determines the order in which threads are unblocked. The same mutex used for waiting must be held while calling either function. Although neither function strictly enforces this requirement, undefined behavior may follow if the mutex is not held.

The pmemobj_cond_timedwait() and pmemobj_cond_wait() functions block on a condition variable. They must be called with mutex mutexp locked by the calling thread, or undefined behavior results. These functions atomically release mutex mutexp and cause the calling thread to block on the condition variable condp; atomically here means “atomically with respect to access by another thread to the mutex and then the condition variable”. That is, if another thread is able to acquire the mutex after the about-to-block thread has released it, then a subsequent call to pmemobj_cond_broadcast() or pmemobj_cond_signal() in that thread will behave as if it were issued after the about-to-block thread has blocked. Upon successful return, the mutex will be locked and owned by the calling thread.

RETURN VALUE¶

The pmemobj_mutex_zero(), pmemobj_rwlock_zero() and pmemobj_cond_zero() functions return no value.

Other locking functions return 0 on success. Otherwise, an error number will be returned to indicate the error.

SEE ALSO¶

pmemobj_tx_zalloc(3), pmemobj_zalloc(3), pthread_cond_init(3), pthread_mutex_init(3), pthread_rwlock_init(3), libpmem(7), libpmemobj(7) and <https://pmem.io>