LOCK(10.2) LOCK(10.2)
NAME
lock, canlock, ilock, iunlock, unlock - spin locks
SYNOPSIS
void lock(Lock *l)
int canlock(Lock *l)
void unlock(Lock *l)
void ilock(Lock *l)
void iunlock(Lock *l)
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
sleep(10.2), qlock(10.2) and qio(10.2). They should be used
only to protect short critical sections that update shared
data structures.
Lock loops repeatedly attempting acquire the spin lock l
until it succeeds. Lock should not be used to lock a struc-
ture shared with an interrupt handler unless interrupts are
disabled by splhi(10.2) before attempting the lock; it is
better to use ilock, below.
Canlock is non-blocking. Only one attempt is made for the
lock. It returns non-zero if the lock was successfully
acquired; 0 otherwise.
Unlock releases the lock l. A lock must be unlocked only by
the locking process.
When called by a process, the functions above temporarily
boost its priority to the highest priority, PriLock; its
original priority is restored at the end of the critical
section by unlock. On a uniprocessor, if l is unavailable,
lock can reschedule unless interrupts are disabled before
entering lock or there is no current process (eg, when exe-
cuting the scheduler).
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, dis-
abling interrupts is sufficient to exclude an interrupt han-
dler from the critical section, and on a multiprocessor the
spin lock excludes an interrupt handler running on another
processor. Ilock never reschedules the caller, nor must a
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LOCK(10.2) LOCK(10.2)
caller allow itself to be rescheduled (eg, by calling
sleep(10.2)) before releasing the lock.
Iunlock releases a lock previously got by ilock.
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:
lock loop on lock-address key key-value pc caller-pc held by pc lock-pc
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 lock not ilock to
interlock with an interrupt routine, and similar blunders.
SOURCE
/os/port/taslock.c
/os/*/l.s
/emu/port/lock.c
/emu/*/os.c
SEE ALSO
qlock(10.2)
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