NAME
lock, canlock, unlock, qlock, canqlock, qunlock, rlock, canrlock, runlock, wlock, canwlock, wunlock, rsleep, rwakeup, rwakeupall, incref, decref – spin locks, queueing rendezvous locks, reader–writer locks, rendezvous points, and reference counts

SYNOPSIS
#include <u.h>
#include <libc.h>

void lock(Lock *l)
int    canlock(Lock *l)
void unlock(Lock *l)

void qlock(QLock *l)
int    canqlock(QLock *l)
void qunlock(QLock *l)

void rlock(RWLock *l)
int    canrlock(RWLock *l)
void runlock(RWLock *l)

void wlock(RWLock *l)
int    canwlock(RWLock *l)
void wunlock(RWLock *l)

typedef struct Rendez {
QLock *l;
...
} Rendez;

void rsleep(Rendez *r)
int    rwakeup(Rendez *r)
int    rwakeupall(Rendez *r)

#include <thread.h>

typedef struct Ref {
long ref;
} Ref;

void incref(Ref*)
long decref(Ref*)

DESCRIPTION
These routines are used to synchronize processes sharing memory.

Locks are spin locks, QLocks and RWLocks are different types of queueing rendezvous locks, and Rendezes are rendezvous points.

Locks and rendezvous points work in regular programs as well as programs that use the thread library (see thread(2)). The thread library replaces the rendezvous(2) system call with its own implementation, threadrendezvous, so that threads as well as processes may be synchronized by locking calls in threaded programs.

Used carelessly, spin locks can be expensive and can easily generate deadlocks. Their use is discouraged, especially in programs that use the thread library because they prevent context switches between threads.

Lock blocks until the lock has been obtained. Canlock is non–blocking. It tries to obtain a lock and returns a non–zero value if it was successful, 0 otherwise. Unlock releases a lock.

QLocks have the same interface but are not spin locks; instead if the lock is taken qlock will suspend execution of the calling task until it is released.

Although Locks are the more primitive lock, they have limitations; for example, they cannot synchronize between tasks in the same proc. Use QLocks instead.

RWLocks manage access to a data structure that has distinct readers and writers. Rlock grants read access; runlock releases it. Wlock grants write access; wunlock releases it. Canrlock and canwlock are the non–blocking versions. There may be any number of simultaneous readers, but only one writer. Moreover, if write access is granted no one may have read access until write access is released.

All types of lock should be initialized to all zeros before use; this puts them in the unlocked state.

Rendezes are rendezvous points. Each Rendez r is protected by a QLock r–>l, which must be held by the callers of rsleep, rwakeup, and rwakeupall. Rsleep atomically releases r–>l and suspends execution of the calling task. After resuming execution, rsleep will reacquire r–>l before returning. If any processes are sleeping on r, rwakeup wakes one of them. it returns 1 if a process was awakened, 0 if not. Rwakeupall wakes all processes sleeping on r, returning the number of processes awakened. Rwakeup and rwakeupall do not release r–>l and do not suspend execution of the current task.

Before use, Rendezes should be initialized to all zeros except for r–>l pointer, which should point at the QLock that will guard r. It is important that this QLock is the same one that protects the rendezvous condition; see the example.

A Ref contains a long that can be incremented and decremented atomically: Incref increments the Ref in one atomic operation. Decref atomically decrements the Ref and returns zero if the resulting value is zero, non–zero otherwise.

EXAMPLE
Implement a buffered single–element channel using rsleep and rwakeup:
typedef struct Chan
{
QLock l;
Rendez full, empty;
int val, haveval;
} Chan;
Chan*
mkchan(void)
{
Chan *c;
c = mallocz(sizeof *c, 1);
c–>full.l = &c–>l;
c–>empty.l = &c–>l;
return c;
}
void
send(Chan *c, int val)
{
qlock(&c–>l);
while(c–>haveval)
rsleep(&c–>full);
c–>haveval = 1;
c–>val = val;
rwakeup(&c–>empty);    /* no longer empty */
qunlock(&c–>l);
}
int
recv(Chan *c)
{
int v;
qlock(&c–>l);
while(!c–>haveval)
rsleep(&c–>empty);
c–>haveval = 0;
v = c–>val;
rwakeup(&c–>full);    /* no longer full */
qunlock(&c–>l);
return v;
}

Note that the QLock protecting the Chan is the same QLock used for the Rendez; this ensures that wakeups are not missed.

SOURCE
/sys/src/libc/port/lock.c
/sys/src/libc/9sys/qlock.c
/sys/src/libthread/ref.c

SEE ALSO
rfork in fork(2)

BUGS
Locks are not strictly spin locks. After each unsuccessful attempt, lock calls sleep(0) to yield the CPU; this handles the common case where some other process holds the lock. After a thousand unsuccessful attempts, lock sleeps for 100ms between attempts. After another thousand unsuccessful attempts, lock sleeps for a full second between attempts. Locks are not intended to be held for long periods of time. The 100ms and full second sleeps are only heuristics to avoid tying up the CPU when a process deadlocks. As discussed above, if a lock is to be held for much more than a few instructions, the queueing lock types should be almost always be used.

It is an error for a program to fork when it holds a lock in shared memory, since this will result in two processes holding the same lock at the same time, which should not happen.

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