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lockmgr(9)
LOCK(9) BSD Kernel Developer's Manual LOCK(9)
NAME
lockinit, lockdestroy, lockmgr, lockmgr_args, lockmgr_args_rw,
lockmgr_disown, lockmgr_printinfo, lockmgr_recursed, lockmgr_rw,
lockstatus, lockmgr_assert — lockmgr family of functions
SYNOPSIS
#include <sys/types.h>
#include <sys/lock.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
lockmgr(struct lock *lkp, u_int flags, struct mtx *ilk);
int
lockmgr_args(struct lock *lkp, u_int flags, struct mtx *ilk,
const char *wmesg, int prio, int timo);
int
lockmgr_args_rw(struct lock *lkp, u_int flags, struct rwlock *ilk,
const char *wmesg, int prio, int timo);
void
lockmgr_disown(struct lock *lkp);
void
lockmgr_printinfo(const struct lock *lkp);
int
lockmgr_recursed(const struct lock *lkp);
int
lockmgr_rw(struct lock *lkp, u_int flags, struct rwlock *ilk);
int
lockstatus(const struct lock *lkp);
options INVARIANTS
options INVARIANT_SUPPORT
void
lockmgr_assert(const struct lock *lkp, int what);
DESCRIPTION
The lockinit() function is used to initialize a lock. It must be called
before any operation can be performed on a lock. Its arguments are:
lkp A pointer to the lock to initialize.
prio The priority passed to sleep(9).
wmesg The lock message. This is used for both debugging output and
sleep(9).
timo The timeout value passed to sleep(9).
flags The flags the lock is to be initialized with:
LK_CANRECURSE Allow recursive exclusive locks.
LK_NOPROFILE Disable lock profiling for this lock.
LK_NOSHARE Allow exclusive locks only.
LK_NOWITNESS Instruct witness(4) to ignore this lock.
LK_NODUP witness(4) should log messages about duplicate
locks being acquired.
LK_QUIET Disable ktr(4) logging for this lock.
LK_TIMELOCK Use timo during a sleep; otherwise, 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 lockmgr() and lockmgr_rw() functions handle general locking function‐
ality within the kernel, including support for shared and exclusive
locks, and recursion. lockmgr() and lockmgr_rw() are also able to
upgrade and downgrade locks.
Their 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, EDEADLK will be returned.
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. Down‐
grading a shared lock is not permitted. If an
exclusive lock has been recursed, the system will
panic(9).
LK_UPGRADE Upgrade a shared lock to an exclusive lock. If
this call fails, the shared lock is lost, even if
the LK_NOWAIT flag is specified. During the
upgrade, the shared lock could be temporarily
dropped. Attempts to upgrade an exclusive lock
will cause a panic(9).
LK_TRYUPGRADE Try to upgrade a shared lock to an exclusive lock.
The failure to upgrade does not result in the
dropping of the shared lock ownership.
LK_RELEASE Release the lock. Releasing 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 decommissioned. This is used before free‐
ing a lock that is part of a piece of memory that
is about to be freed. (As documented in
<sys/lockmgr.h>.)
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_NOWITNESS Skip the witness(4) checks for this instance.
LK_CANRECURSE Allow recursion on an exclusive lock. For every
lock there must be a release.
LK_INTERLOCK Unlock the interlock (which should be locked
already).
LK_NODDLKTREAT Normally, lockmgr() postpones serving further
shared requests for shared-locked lock if there is
exclusive waiter, to avoid exclusive lock starva‐
tion. But, if the thread requesting the shared
lock already owns a shared lockmgr lock, the
request is granted even in presence of the paral‐
lel exclusive lock request, which is done to avoid
deadlocks with recursive shared acquisition.
The LK_NODDLKTREAT flag can only be used by code
which requests shared non-recursive lock. The
flag allows exclusive requests to preempt the cur‐
rent shared request even if the current thread
owns shared locks. This is safe since shared lock
is guaranteed to not recurse, and is used when
thread is known to held unrelated shared locks, to
not cause unnecessary starvation. An example is
vp locking in VFS lookup(9), when dvp is already
locked.
ilk An interlock mutex for controlling group access to the lock. If
LK_INTERLOCK is specified, lockmgr() and lockmgr_rw() assume ilk
is currently owned and not recursed, and will return it unlocked.
See mtx_assert(9).
The lockmgr_args() and lockmgr_args_rw() function work like lockmgr() and
lockmgr_rw() but accepting a wmesg, timo and prio on a per-instance
basis. The specified values will override the default ones, but this can
still be used passing, respectively, LK_WMESG_DEFAULT, LK_PRIO_DEFAULT
and LK_TIMO_DEFAULT.
The lockmgr_disown() function switches the owner from the current thread
to be LK_KERNPROC, if the lock is already held.
The lockmgr_printinfo() function prints debugging information about the
lock. It is used primarily by VOP_PRINT(9) functions.
The lockmgr_recursed() function returns true if the lock is recursed, 0
otherwise.
The lockstatus() function returns the status of the lock in relation to
the current thread.
When compiled with options INVARIANTS and options INVARIANT_SUPPORT, the
lockmgr_assert() function tests lkp for the assertions specified in what,
and panics if they are not met. One of the following assertions must be
specified:
KA_LOCKED Assert that the current thread has either a shared or an
exclusive lock on the lkp lock pointed to by the first argu‐
ment.
KA_SLOCKED Assert that the current thread has a shared lock on the lkp
lock pointed to by the first argument.
KA_XLOCKED Assert that the current thread has an exclusive lock on the
lkp lock pointed to by the first argument.
KA_UNLOCKED Assert that the current thread has no lock on the lkp lock
pointed to by the first argument.
In addition, one of the following optional assertions can be used with
either an KA_LOCKED, KA_SLOCKED, or KA_XLOCKED assertion:
KA_RECURSED Assert that the current thread has a recursed lock on
lkp.
KA_NOTRECURSED Assert that the current thread does not have a recursed
lock on lkp.
RETURN VALUES
The lockmgr() and lockmgr_rw() functions return 0 on success and non-zero
on failure.
The lockstatus() function returns:
LK_EXCLUSIVE An exclusive lock is held by the current thread.
LK_EXCLOTHER An exclusive lock is held by someone other than the current
thread.
LK_SHARED A shared lock is held.
0 The lock is not held by anyone.
ERRORS
lockmgr() and lockmgr_rw() fail 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, or LK_TRYUPGRADE operation was not able to
upgrade the lock.
[ENOLCK] LK_SLEEPFAIL was set and lockmgr() or lockmgr_rw() did
sleep.
[EINTR] PCATCH was set in the lock priority, and a signal was
delivered during a sleep. Note the ERESTART 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() or
lockmgr_rw(), the ilk must be held prior to calling lockmgr() or
lockmgr_rw(), and will be returned unlocked.
Upgrade attempts that fail result in the loss of the lock that is cur‐
rently held. Also, it is invalid to upgrade an exclusive lock, and a
panic(9) will be the result of trying.
SEE ALSO
condvar(9), locking(9), mtx_assert(9), mutex(9), panic(9), rwlock(9),
sleep(9), sx(9), VOP_PRINT(9)
AUTHORS
This manual page was written by Chad David <davidc@acns.ab.ca>.
BSD November 17, 2017 BSD