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NAME
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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
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SYNOPSIS
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#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 {
} Rendez;
void rsleep(Rendez *r)
int rwakeup(Rendez *r)
int rwakeupall(Rendez *r)
#include <thread.h>
typedef struct Ref {
} Ref;
void incref(Ref*)
long decref(Ref*)
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DESCRIPTION
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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.
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SOURCE
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/usr/local/plan9/src/lib9/qlock.c
/usr/local/plan9/src/libthread
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BUGS
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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.
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