man/man9/lock.9

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.Dd June 21, 2024
.Dt LOCK 9
.Os
.Sh NAME
.Nm lockinit ,
.Nm lockdestroy ,
.Nm lockmgr ,
.Nm lockmgr_args ,
.Nm lockmgr_args_rw ,
.Nm lockmgr_disown ,
.Nm lockmgr_disowned ,
.Nm lockmgr_lock_flags ,
.Nm lockmgr_printinfo ,
.Nm lockmgr_recursed ,
.Nm lockmgr_rw ,
.Nm lockmgr_slock ,
.Nm lockmgr_unlock ,
.Nm lockmgr_xlock ,
.Nm lockstatus ,
.Nm lockmgr_assert
.Nd "lockmgr family of functions"
.Sh SYNOPSIS
.In sys/types.h
.In sys/lock.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 *ilk"
.Ft int
.Fn lockmgr_args "struct lock *lkp" "u_int flags" "struct mtx *ilk" "const char *wmesg" "int prio" "int timo"
.Ft int
.Fn lockmgr_args_rw "struct lock *lkp" "u_int flags" "struct rwlock *ilk" "const char *wmesg" "int prio" "int timo"
.Ft void
.Fn lockmgr_disown "struct lock *lkp"
.Ft int
.Fn lockmgr_disowned "const struct lock *lkp"
.Ft int
.Fn lockmgr_lock_flags "struct lock *lkp" "u_int flags" "struct lock_object *ilk" "const char *file" "int line"
.Ft void
.Fn lockmgr_printinfo "const struct lock *lkp"
.Ft int
.Fn lockmgr_recursed "const struct lock *lkp"
.Ft int
.Fn lockmgr_rw "struct lock *lkp" "u_int flags" "struct rwlock *ilk"
.Ft int
.Fn lockmgr_slock "struct lock *lkp" "u_int flags" "const char *file" "int line"
.Ft int
.Fn lockmgr_unlock "struct lock *lkp"
.Ft int
.Fn lockmgr_xlock "struct lock *lkp" "u_int flags" "const char *file" "int line"
.Ft int
.Fn lockstatus "const struct lock *lkp"
.Pp
.Cd "options INVARIANTS"
.Cd "options INVARIANT_SUPPORT"
.Ft void
.Fn lockmgr_assert "const 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_CANRECURSE
Allow recursive exclusive locks.
.It Dv LK_NOPROFILE
Disable lock profiling for this lock.
.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_QUIET
Disable
.Xr ktr 4
logging for this lock.
.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
and
.Fn lockmgr_rw
functions handle general locking functionality within the kernel, including
support for shared and exclusive locks, and recursion.
.Fn lockmgr
and
.Fn lockmgr_rw
are also able to upgrade and downgrade locks.
.Pp
Their 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_NODDLKTREAT"
.It Dv LK_SHARED
Acquire a shared lock.
If an exclusive lock is currently held,
.Dv EDEADLK
will be returned.
.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, the system will
.Xr panic 9 .
.It Dv LK_UPGRADE
Upgrade a shared lock to an exclusive lock.
If this call fails, the shared lock is lost, even if the
.Dv LK_NOWAIT
flag is specified.
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_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.
.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_TIMELOCK
Use
.Fa timo
during a sleep; otherwise, 0 is used.
.It Dv LK_NOWITNESS
Skip the
.Xr witness 4
checks for this instance.
.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).
.It Dv LK_NODDLKTREAT
Normally,
.Fn lockmgr
postpones serving further shared requests for shared-locked lock if there is
exclusive waiter, to avoid exclusive lock starvation.
But, if the thread requesting the shared lock already owns a shared lockmgr
lock, the request is granted even in presence of the parallel exclusive lock
request, which is done to avoid deadlocks with recursive shared acquisition.
.Pp
The
.Dv LK_NODDLKTREAT
flag can only be used by code which requests shared non-recursive lock.
The flag allows exclusive requests to preempt the current 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
.Dv vp
locking in VFS
.Xr lookup 9 ,
when
.Dv dvp
is already locked.
.El
.It Fa ilk
An interlock mutex for controlling group access to the lock.
If
.Dv LK_INTERLOCK
is specified,
.Fn lockmgr
and
.Fn lockmgr_rw
assume
.Fa ilk
is currently owned and not recursed, and will return it unlocked.
See
.Xr mtx_assert 9 .
.El
.Pp
The
.Fn lockmgr_args
and
.Fn lockmgr_args_rw
function work like
.Fn lockmgr
and
.Fn lockmgr_rw
but accepting a
.Fa wmesg ,
.Fa timo
and
.Fa prio
on a per-instance basis.
The specified values will override the default
ones, but this can still be used passing, respectively,
.Dv LK_WMESG_DEFAULT ,
.Dv LK_PRIO_DEFAULT
and
.Dv LK_TIMO_DEFAULT .
.Pp
The
.Fn lockmgr_lock_flags
function works like
.Fn lockmgr
but accepts explicit
.Fa file
and
.Fa line
arguments for lock tracing.
.Pp
The
.Fn lockmgr_slock ,
.Fn lockmgr_xlock ,
and
.Fn lockmgr_unlock
functions are lightweight entry points that function like
.Fn lockmgr
for the
.Dv LK_SHARED ,
.Dv LK_EXCLUSIVE ,
and
.Dv LK_RELEASE
operations respectively.
They provide functionality similar to
.Xr sx 9
locks in that none of the additional
.Xr lockmgr 9
features are supported.
Specifically, these functions do not support unlocking interlocks, the
.Dv LK_SLEEPFAIL
flag, or locks with shared locking disabled via
.Dv LK_NOSHARE .
They also accept explicit
.Fa file
and
.Fa line
arguments for lock tracing.
.Pp
The
.Fn lockmgr_disown
function switches the owner from the current thread to be
.Dv LK_KERNPROC ,
if the lock is already held.
.Pp
The
.Fn lockmgr_disowned
function returns true or false according to whether the lock is held by
.Dv LK_KERNPROC .
.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
The
.Fn lockstatus
function returns the status of the lock in relation to the current thread.
.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.
.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
.Sh RETURN VALUES
The
.Fn lockmgr
and
.Fn lockmgr_rw
functions return 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 current thread.
.It Dv LK_EXCLOTHER
An exclusive lock is held by someone other than the current thread.
.It Dv LK_SHARED
A shared lock is held.
.It Li 0
The lock is not held by anyone.
.El
.Sh ERRORS
.Fn lockmgr
and
.Fn lockmgr_rw
fail 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, or
.Dv LK_TRYUPGRADE
operation was not able to upgrade the lock.
.It Bq Er EDEADLK
A shared lock was attempted while the thread already held the exclusive lock.
.It Bq Er ENOLCK
.Dv LK_SLEEPFAIL
was set and
.Fn lockmgr
or
.Fn lockmgr_rw
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
or
.Fn lockmgr_rw ,
the
.Fa ilk
must be held prior to calling
.Fn lockmgr
or
.Fn lockmgr_rw ,
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 witness 4 ,
.Xr condvar 9 ,
.Xr locking 9 ,
.Xr mtx_assert 9 ,
.Xr mutex 9 ,
.Xr panic 9 ,
.Xr rwlock 9 ,
.Xr sleep 9 ,
.Xr sx 9 ,
.Xr VOP_PRINT 9
.Sh AUTHORS
This manual page was written by
.An Chad David Aq Mt davidc@acns.ab.ca .