lock(3) - Plan 9 from User Space

| LOCK(3) | LOCK(3)
| 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 locks, and `Rendezes` are rendezvous points.
Locks and rendezvous points have trivial implementations in programs not using the thread library (see thread(3)), since such programs have no concurrency.
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 thread 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.
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.
SOURCE
| `/usr/lib/plan9/src/lib9/qlock.c /usr/lib/plan9/src/libthread`
BUGS
| `Locks` are not always spin locks. Instead they are usually implemented using the pthreads library’s `pthread_mutex_t`, whose implementation method is not defined.
On pthreads-based systems, the implementation of `Lock` never calls pthread_mutex_destroy to free the `pthread_mutex_t`’s. This leads to resource leaks on FreeBSD 5 (though not on Linux 2.6, where pthread_mutex_destroy is a no-op).
On systems that do not have a usable pthreads implementation, the `Lock` implementation provided by libthread is still not exactly a spin lock. 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. Another 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.