lock(9)

NAME

lockinit, lockdestroy, lockcount, lockmgr, lockstatus,

lockmgr_printinfo
- lockmgr family of functions

SYNOPSIS

#include <sys/types.h>
#include <sys/lockmgr.h>
void
lockinit(struct  lock *lkp, int prio, const char *wmesg, int
timo,
        int flags);
void
lockdestroy(struct lock *lkp);
int
lockcount(struct lock *lkp);
int
lockmgr(struct lock *lkp, u_int flags, struct mtx *interlkp,
        struct thread *td);
int
lockstatus(struct lock *lkp, struct thread *td);
void
lockmgr_printinfo(struct lock *lkp);

DESCRIPTION

The lockinit() function is used to initialize a lock. It

must be called
before any operation can be performed on a lock. Its argu

ments are:

lkp A pointer to the lock to initialize.

prio The priority passed to msleep(9).

wmesg The lock message. This is used for both debug

ging output and

msleep(9).

timo The timeout value passed to msleep(9).

flags The flags the lock is to be initialized with.

LK_NOWAIT Do not sleep while acquiring

the lock.

LK_SLEEPFAIL Fail after a sleep.

LK_CANRECURSE Allow recursive exclusive

locks.

LK_REENABLE Re-enable the lock after a

drain.

LK_NOPAUSE Disable the spinlock while ac

quiring the

lock.

LK_TIMELOCK Use timo during a sleep; oth

erwise, 0 is

used.

The lockdestroy() function is used to destroy a lock, and

while it is
called in a number of places in the kernel, it currently

does nothing.

The lockcount() function returns a count of the number of

exclusive locks
and shared locks held against the lock lkp.

The lockmgr() function handles general locking functionality

within the
kernel, including support for shared and exclusive locks,

and recursion.
lockmgr() is also able to upgrade and downgrade locks.

Its arguments are:

lkp A pointer to the lock to manipulate.

flags Flags indicating what action is to be taken.

LK_SHARED Acquire a shared lock. If

an exclusive

lock is currently held, it

will be
downgraded.

LK_EXCLUSIVE Acquire an exclusive lock.

If an

exclusive lock is already

held, and
LK_CANRECURSE is not set,

the system
will panic(9).

LK_DOWNGRADE Downgrade exclusive lock

to a shared

lock. Downgrading a

shared lock is not
permitted. If an exclu

sive lock has
been recursed, all refer

ences will be
downgraded.

LK_EXCLUPGRADE Upgrade a shared lock to

an exclusive

lock. Fails with EBUSY if

there is
someone ahead of you in

line waiting
for an upgrade. If this

call fails,
the shared lock is lost.

Attempts to
upgrade an exclusive lock

will cause a
panic(9).

LK_UPGRADE Upgrade a shared lock to

an exclusive

lock. If this call fails,

the shared
lock is lost. Attempts to

upgrade an
exclusive lock will cause

a panic(9).

LK_RELEASE Release the lock. Releas

ing a lock

that is not held can cause

a panic(9).

LK_DRAIN Wait for all activity on

the lock to

end, then mark it decom

missioned. This
is used before freeing a

lock that is
part of a piece of memory

that is about
to be freed. (As docu

mented in

LK_SLEEPFAIL Fail if operation has

slept.

LK_NOWAIT Do not allow the call to

sleep. This

can be used to test the

lock.

LK_CANRECURSE Allow recursion on an ex

clusive lock.

For every lock there must

be a release.

LK_INTERLOCK Unlock the interlock

(which should be

locked already).

interlkp An interlock mutex for controlling group ac

cess to the

lock. If LK_INTERLOCK is specified, lockmgr()

assumes
interlkp is currently owned and not recursed,

and will
return it unlocked. See mtx_assert(9).

td A thread responsible for this call. NULL be

comes

LK_KERNPROC.

The lockstatus() function returns the status of the lock in

relation to
the thread passed to it. Note that if td is NULL and an ex

clusive lock
is held, LK_EXCLUSIVE will be returned.

The lockmgr_printinfo() function prints debugging informa

tion about the
lock. It is used primarily by VOP_PRINT(9) functions.

RETURN VALUES

The lockcount() function returns an integer greater than or

equal to
zero.

The lockmgr() function returns 0 on success and non-zero on

failure.

The lockstatus() function returns:

LK_EXCLUSIVE An exclusive lock is held by the thread

td.

LK_EXCLOTHER An exclusive lock is held by someone other

than the

thread td.

LK_SHARED A shared lock is held.

0 The lock is not held by anyone.

ERRORS

lockmgr() fails if:

[EBUSY] LK_FORCEUPGRADE was requested and another

thread had

already requested a lock upgrade.

[EBUSY] LK_NOWAIT was set, and a sleep would have

been

required.

[ENOLCK] LK_SLEEPFAIL was set and lockmgr() did

sleep.

[EINTR] PCATCH was set in the lock priority, and

a signal was

delivered during a sleep. Note the ER

ESTART error
below.

[ERESTART] PCATCH was set in the lock priority, a

signal was

delivered during a sleep, and the system

call is to be
restarted.

[EWOULDBLOCK] a non-zero timeout was given, and the

timeout expired.

LOCKS

If LK_INTERLOCK is passed in the flags argument to

lockmgr(), the
interlkp must be held prior to calling lockmgr(), and will

be returned
unlocked.

Upgrade attempts that fail result in the loss of the lock

that is currently held. Also, it is invalid to upgrade an exclusive

lock, and a
panic(9) will be the result of trying.

SEE ALSO

msleep(9), mtx_assert(9), panic(9), VOP_PRINT(9)

AUTHORS

This manual page was written by Chad David <davidc@ac

ns.ab.ca>.

BSD July 9, 2001

BSD