man/man9/lock.9

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.\" $FreeBSD: src/share/man/man9/lock.9,v 1.11 2003/09/08 19:57:21 ru Exp $
.\"
.Dd January 25, 2024
.Dt LOCK 9
.Os
.Sh NAME
.Nm lockinit ,
.Nm lockmgr ,
.Nm lockmgr_try ,
.Nm lockstatus ,
.Nm lockmgr_printinfo ,
.Nm lockowned
.Nd "lockmgr family of functions"
.Sh SYNOPSIS
.In sys/types.h
.In sys/lock.h
.Ft void
.Fn lockinit "struct lock *lkp" "const char *wmesg" "int timo" "int flags"
.Ft void
.Fn lockuninit "struct lock *lkp"
.Ft int
.Fn lockmgr "struct lock *lkp" "u_int flags"
.Ft boolean_t
.Fn lockmgr_try "struct lock *lkp" "u_int flags"
.Ft int
.Fn lockstatus "struct lock *lkp" "struct thread *td"
.Ft void
.Fn lockmgr_printinfo "struct lock *lkp"
.Ft int
.Fn lockowned "struct lock *lkp"
.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 wmesg
The lock message.
This is used for both debugging output and
.Xr tsleep 9 .
.It Fa timo
The timeout value passed to
.Xr tsleep 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_TIMELOCK
Use
.Fa timo
during a sleep; otherwise, 0 is used.
.El
.El
.Pp
The
.Fn lockuninit
function destroys a lock that was previously initialized using
.Fn lockinit .
.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 flags"
.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_EXCLUPGRADE"
.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_EXCLUPGRADE
Upgrade a shared lock to an exclusive lock.
Fails with
.Er EBUSY
if there is someone ahead of you in line waiting for an upgrade.
If this call fails for any reason, the shared lock is lost.
Attempts to upgrade an exclusive lock not already
owned by the caller will cause a
.Xr panic 9 ,
but otherwise will always succeed.
NOTE! When this operation succeeds, it guarantees that no other
exclusive lock was able to acquire the lock ahead of you, but
as indicated above, if it fails your current shared lock is lost.
.It Dv LK_UPGRADE
Upgrade a shared lock to an exclusive lock.
If this call fails for any reason, the shared lock is lost.
Attempts to upgrade an exclusive lock not already
owned by the caller will cause a
.Xr panic 9 ,
but otherwise will always succeed.
WARNING!  This operation can block with the current lock temporarily
released, and other exclusive or shared lock holders can inject before
the lock is acquired on your behalf.
.It Dv LK_RELEASE
Release the lock.
Releasing a lock that is not held can cause a
.Xr panic 9 .
.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.
.El
.El
.Pp
The
.Fn lockmgr_try
function is similar to
.Fn lockmgr
but it will not sleep and returns
.Dv TRUE
if the lock was successfully obtained and
.Dv FALSE
if it was not.
.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_printinfo
function prints debugging information about the lock.
It is used primarily by
.Xr VOP_PRINT 9
functions.
.Pp
The
.Fn lockowned
function is used to determine whether the calling thread owns a lock.
.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" -offset indent
.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
.Pp
The
.Fn lockowned
function returns a non-zero value if the caller owns the lock shared or
exclusive.
.Sh FILES
.Bl -tag -width /sys/kern/kern_lock.c -compact
.It Pa /sys/kern/kern_lock.c
the lock manager implementation
.It Pa /sys/sys/lock.h
data structures and function prototypes for the lock manager
.El
.Sh ERRORS
.Fn lockmgr
fails if:
.Bl -tag -width Er
.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
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 locking 9 ,
.Xr panic 9 ,
.Xr tsleep 9 ,
.Xr VOP_PRINT 9
.Sh HISTORY
The lock manager appeared in
.Dx 1.0 .
.Pp
The lock manager API first appeared in
.Bx 4.4 lite2 .
.Sh AUTHORS
This man page was written by
.An Chad David Aq Mt davidc@acns.ab.ca .