ref: 2459e34d77e2e21ef829c0dfaafd99433899494f
dir: /man/10/lock/
.TH LOCK 10.2 .SH NAME lock, canlock, ilock, iunlock, unlock \- spin locks .SH SYNOPSIS .ta \w'\fLvoid 'u .B void lock(Lock *l) .PP .B int canlock(Lock *l) .PP .B void unlock(Lock *l) .PP .B void ilock(Lock *l) .PP .B void iunlock(Lock *l) .SH DESCRIPTION These primitives control access to shared resources using spin locks. They in turn are used to build higher-level synchronisation mechanisms such as those described in .IR sleep (10.2), .IR qlock (10.2) and .IR qio (10.2). They should be used only to protect short critical sections that update shared data structures. .PP .I Lock loops repeatedly attempting acquire the spin lock .I l until it succeeds. .I Lock should not be used to lock a structure shared with an interrupt handler unless interrupts are disabled by .IR splhi (10.2) before attempting the lock; it is better to use .IR ilock , below. .PP .I Canlock is non-blocking. Only one attempt is made for the lock. It returns non-zero if the lock was successfully acquired; 0 otherwise. .PP .I Unlock releases the lock .IR l . A lock must be unlocked only by the locking process. .PP When called by a process, the functions above temporarily boost its priority to the highest priority, .BR PriLock ; its original priority is restored at the end of the critical section by .IR unlock . On a uniprocessor, if .I l is unavailable, .I lock can reschedule unless interrupts are disabled before entering .I lock or there is no current process (eg, when executing the scheduler). .PP .I Ilock disables interrupts before attempting to acquire the lock. It should be used to lock a resource shared between a process and an interrupt handler. On a uniprocessor, disabling interrupts is sufficient to exclude an interrupt handler from the critical section, and on a multiprocessor the spin lock excludes an interrupt handler running on another processor. .I Ilock never reschedules the caller, nor must a caller allow itself to be rescheduled (eg, by calling .IR sleep (10.2)) before releasing the lock. .PP .I Iunlock releases a lock previously got by .IR ilock . .SH DIAGNOSTICS The lock functions guard against the possibility of never acquiring the lock by capping the number of lock attempts. If the limit is reached, a message of the following form is written on the console: .IP .EX lock loop on \fIlock-address\fP key \fIkey-value\fP pc \fIcaller-pc\fP held by pc \fIlock-pc\fP .EE .PP Most lock loops represent deadlocks caused by failing to unlock a resource, attempting to lock (eg, by recursive call) a resource already held by the process, inconsistent locking and unlocking of nested resources, using a spin-lock to guard code that reschedules, using .I lock not .I ilock to interlock with an interrupt routine, and similar blunders. .SH SOURCE .B /os/port/taslock.c .br .B /os/*/l.s .br .B /emu/port/lock.c .br .B /emu/*/os.c .SH SEE ALSO .IR qlock (10.2)