xref: /freebsd/share/man/man9/lock.9 (revision af176d7306a52d525d415405875a7f9b7fe48637)

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.Dd February 13, 2008
.Dt LOCK 9
.Os
.Sh NAME
.Nm lockinit ,
.Nm lockdestroy ,
.Nm lockmgr ,
.Nm lockstatus ,
.Nm lockmgr_disown ,
.Nm lockmgr_printinfo ,
.Nm lockmgr_recursed ,
.Nm lockmgr_assert
.Nd "lockmgr family of functions"
.Sh SYNOPSIS
.In sys/types.h
.In sys/lockmgr.h
.Ft void
.Fn lockinit "struct lock *lkp" "int prio" "const char *wmesg" "int timo" "int flags"
.Ft void
.Fn lockdestroy "struct lock *lkp"
.Ft int
.Fn lockmgr "struct lock *lkp" "u_int flags" "struct mtx *interlkp"
.Ft int
.Fn lockstatus "struct lock *lkp" "struct thread *td"
.Ft void
.Fn lockmgr_disown "struct lock *lkp"
.Ft void
.Fn lockmgr_printinfo "struct lock *lkp"
.Ft int
.Fn lockmgr_recursed "struct lock *lkp"
.Pp
.Cd "options INVARIANTS"
.Cd "options INVARIANT_SUPPORT"
.Ft void
.Fn lockmgr_assert "struct lock *lkp" "int what"
.Sh DESCRIPTION
The
.Fn lockinit
function is used to initialize a lock.
It must be called before any operation can be performed on a lock.
Its arguments are:
.Bl -tag -width ".Fa wmesg"
.It Fa lkp
A pointer to the lock to initialize.
.It Fa prio
The priority passed to
.Xr sleep 9 .
.It Fa wmesg
The lock message.
This is used for both debugging output and
.Xr sleep 9 .
.It Fa timo
The timeout value passed to
.Xr sleep 9 .
.It Fa flags
The flags the lock is to be initialized with.
.Bl -tag -width ".Dv LK_CANRECURSE"
.It Dv LK_NOWAIT
Do not sleep while acquiring the lock.
.It Dv LK_SLEEPFAIL
Fail after a sleep.
.It Dv LK_CANRECURSE
Allow recursive exclusive locks.
.It Dv LK_NOSHARE
Allow exclusive locks only.
.It Dv LK_NOWITNESS
Instruct
.Xr witness 4
to ignore this lock.
.It Dv LK_NODUP
.Xr witness 4
should log messages about duplicate locks being acquired.
.It Dv LK_TIMELOCK
Use
.Fa timo
during a sleep; otherwise, 0 is used.
.El
.El
.Pp
The
.Fn 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.
.Pp
The
.Fn lockmgr
function handles general locking functionality within the kernel, including
support for shared and exclusive locks, and recursion.
.Fn lockmgr
is also able to upgrade and downgrade locks.
.Pp
Its arguments are:
.Bl -tag -width ".Fa interlkp"
.It Fa lkp
A pointer to the lock to manipulate.
.It Fa flags
Flags indicating what action is to be taken.
.Bl -tag -width ".Dv LK_CANRECURSE"
.It Dv LK_SHARED
Acquire a shared lock.
If an exclusive lock is currently held, it will be downgraded.
.It Dv LK_EXCLUSIVE
Acquire an exclusive lock.
If an exclusive lock is already held, and
.Dv LK_CANRECURSE
is not set, the system will
.Xr panic 9 .
.It Dv LK_DOWNGRADE
Downgrade exclusive lock to a shared lock.
Downgrading a shared lock is not permitted.
If an exclusive lock has been recursed, all references will be downgraded.
.It Dv LK_UPGRADE
Upgrade a shared lock to an exclusive lock.
If this call fails, the shared lock is lost.
During the upgrade, the shared lock could
be temporarily dropped.
Attempts to upgrade an exclusive lock will cause a
.Xr panic 9 .
.It Dv LK_RELEASE
Release the lock.
Releasing a lock that is not held can cause a
.Xr panic 9 .
.It Dv LK_DRAIN
Wait for all activity on the lock to end, then mark it decommissioned.
This is used before freeing a lock that is part of a piece of memory that is
about to be freed.
(As documented in
.In sys/lockmgr.h . )
.It Dv LK_SLEEPFAIL
Fail if operation has slept.
.It Dv LK_NOWAIT
Do not allow the call to sleep.
This can be used to test the lock.
.It Dv LK_CANRECURSE
Allow recursion on an exclusive lock.
For every lock there must be a release.
.It Dv LK_INTERLOCK
Unlock the interlock (which should be locked already).
.El
.It Fa interlkp
An interlock mutex for controlling group access to the lock.
If
.Dv LK_INTERLOCK
is specified,
.Fn lockmgr
assumes
.Fa interlkp
is currently owned and not recursed, and will return it unlocked.
See
.Xr mtx_assert 9 .
.El
.Pp
The
.Fn lockstatus
function returns the status of the lock in relation to the
.Vt thread
passed to it.
Note that if
.Fa td
is
.Dv NULL
and an exclusive lock is held,
.Dv LK_EXCLUSIVE
will be returned.
.Pp
The
.Fn lockmgr_disown
function switches the owner from curthread to be
.Dv LK_KERNPROC,
if the lock is already held.
.Pp
The
.Fn lockmgr_printinfo
function prints debugging information about the lock.
It is used primarily by
.Xr VOP_PRINT 9
functions.
.Pp
The
.Fn lockmgr_recursed
function returns true if the lock is recursed, 0
otherwise.
.Pp
When compiled with
.Cd "options INVARIANTS"
and
.Cd "options INVARIANT_SUPPORT" ,
the
.Fn lockmgr_assert
function tests
.Fa lkp
for the assertions specified in
.Fa what ,
and panics if they are not met.
One of the following assertions must be specified:
.Bl -tag -width ".Dv KA_UNLOCKED"
.It Dv KA_LOCKED
Assert that the current thread has either a shared or an exclusive lock on the
.Vt lkp
lock pointed to by the first argument.
.It Dv KA_SLOCKED
Assert that the current thread has a shared lock on the
.Vt lkp
lock pointed to by the first argument.
.It Dv KA_XLOCKED
Assert that the current thread has an exclusive lock on the
.Vt lkp
lock pointed to by the first argument.
.It Dv KA_UNLOCKED
Assert that the current thread has no lock on the
.Vt lkp
lock pointed to by the first argument.
.It Dv KA_HELD
Assert that an unspecified thread has a lock on the
.Vt lkp
lock pointed to by the first argument.
.It Dv KA_UNHELD
Assert that no thread has a lock on the
.Vt lkp
lock pointed to by the first argument.
.El
.Pp
In addition, one of the following optional assertions can be used with
either an
.Dv KA_LOCKED ,
.Dv KA_SLOCKED ,
or
.Dv KA_XLOCKED
assertion:
.Bl -tag -width ".Dv KA_NOTRECURSED"
.It Dv KA_RECURSED
Assert that the current thread has a recursed lock on
.Fa lkp .
.It Dv KA_NOTRECURSED
Assert that the current thread does not have a recursed lock on
.Fa lkp .
.El
.Pp
Note that
.Dv KA_HELD
and
.Dv KA_UNHELD
usage is highly discouraged.
They are intended to cater a bad behaviour
introduced by buffer cache lock handling.
They will hopefully be
made useless by revisiting such locks.
.Sh RETURN VALUES
The
.Fn lockmgr
function returns 0 on success and non-zero on failure.
.Pp
The
.Fn lockstatus
function returns:
.Bl -tag -width ".Dv LK_EXCLUSIVE"
.It Dv LK_EXCLUSIVE
An exclusive lock is held by the thread
.Fa td .
.It Dv LK_EXCLOTHER
An exclusive lock is held by someone other than the thread
.Fa td .
.It Dv LK_SHARED
A shared lock is held.
.It Li 0
The lock is not held by anyone.
.El
.Sh ERRORS
.Fn lockmgr
fails if:
.Bl -tag -width Er
.It Bq Er EBUSY
.Dv LK_FORCEUPGRADE
was requested and another thread had already requested a lock upgrade.
.It Bq Er EBUSY
.Dv LK_NOWAIT
was set, and a sleep would have been required.
.It Bq Er ENOLCK
.Dv LK_SLEEPFAIL
was set and
.Fn lockmgr
did sleep.
.It Bq Er EINTR
.Dv PCATCH
was set in the lock priority, and a signal was delivered during a sleep.
Note the
.Er ERESTART
error below.
.It Bq Er ERESTART
.Dv PCATCH
was set in the lock priority, a signal was delivered during a sleep,
and the system call is to be restarted.
.It Bq Er EWOULDBLOCK
a non-zero timeout was given, and the timeout expired.
.El
.Sh LOCKS
If
.Dv LK_INTERLOCK
is passed in the
.Fa flags
argument to
.Fn lockmgr ,
the
.Fa interlkp
must be held prior to calling
.Fn lockmgr ,
and will be returned unlocked.
.Pp
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
.Xr panic 9
will be the result of trying.
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr locking 9 ,
.Xr mutex 9 ,
.Xr rwlock 9 ,
.Xr sleep 9 ,
.Xr sx 9 ,
.Xr mtx_assert 9 ,
.Xr panic 9 ,
.Xr VOP_PRINT 9
.Sh AUTHORS
This manual page was written by
.An Chad David Aq davidc@acns.ab.ca .