LOCK(3)                                                   LOCK(3)

          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

          #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*)

          These routines are used  to synchronize processes sharing

          Locks are spin locks, QLocks and RWLocks are different types

     LOCK(3)                                                   LOCK(3)

          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 limi-
          tations; 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

     LOCK(3)                                                   LOCK(3)

          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 other-


          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 implementa-
          tion, 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 com-
          mon 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 dis-
          cussed 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.