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Support condition variables.

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Condition variables provide a flexible way to sleep until a
cooperating process causes an arbitrary condition to become true.  In
simple cases, this can be accomplished with a WaitLatch/ResetLatch
loop; the cooperating process can call SetLatch after performing work
that might cause the condition to be satisfied, and the waiting
process can recheck the condition each time.  However, if the process
performing the work doesn't have an easy way to identify which
processes might be waiting, this doesn't work, because it can't
identify which latches to set.  Condition variables solve that problem
by internally maintaining a list of waiters; a process that may have
caused some waiter's condition to be satisfied must "signal" or
"broadcast" on the condition variable.

Robert Haas and Thomas Munro
This commit is contained in:
Robert Haas
2016-11-22 14:26:40 -05:00
parent 1c7861e81b
commit e8ac886c24
12 changed files with 364 additions and 2 deletions
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59
src/include/storage/condition_variable.h Normal file
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/*-------------------------------------------------------------------------
*
* condition_variable.h
* Condition variables
*
* A condition variable is a method of waiting until a certain condition
* becomes true. Conventionally, a condition variable supports three
* operations: (1) sleep; (2) signal, which wakes up one process sleeping
* on the condition variable; and (3) broadcast, which wakes up every
* process sleeping on the condition variable. In our implementation,
* condition variables put a process into an interruptible sleep (so it
* can be cancelled prior to the fulfillment of the condition) and do not
* use pointers internally (so that they are safe to use within DSMs).
*
* Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/include/storage/condition_variable.h
*
*-------------------------------------------------------------------------
*/
#ifndef CONDITION_VARIABLE_H
#define CONDITION_VARIABLE_H
#include "storage/s_lock.h"
#include "storage/proclist_types.h"
typedef struct
{
slock_t mutex;
proclist_head wakeup;
} ConditionVariable;
/* Initialize a condition variable. */
extern void ConditionVariableInit(ConditionVariable *);
/*
* To sleep on a condition variable, a process should use a loop which first
* checks the condition, exiting the loop if it is met, and then calls
* ConditionVariableSleep. Spurious wakeups are possible, but should be
* infrequent. After exiting the loop, ConditionVariableCancelSleep should
* be called to ensure that the process is no longer in the wait list for
* the condition variable.
*/
extern void ConditionVariableSleep(ConditionVariable *, uint32 wait_event_info);
extern void ConditionVariableCancelSleep(void);
/*
* The use of this function is optional and not necessary for correctness;
* for efficiency, it should be called prior entering the loop described above
* if it is thought that the condition is unlikely to hold immediately.
*/
extern void ConditionVariablePrepareToSleep(ConditionVariable *);
/* Wake up a single waiter (via signal) or all waiters (via broadcast). */
extern bool ConditionVariableSignal(ConditionVariable *);
extern int ConditionVariableBroadcast(ConditionVariable *);
#endif /* CONDITION_VARIABLE_H */