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LOCK(9) |
FreeBSD Kernel Developer's Manual |
LOCK(9) |
lockinit , lockdestroy ,
lockmgr , lockmgr_args ,
lockmgr_args_rw ,
lockmgr_disown ,
lockmgr_printinfo ,
lockmgr_recursed , lockmgr_rw ,
lockstatus , lockmgr_assert
—
lockmgr family of functions
#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);
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
functionality 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. Downgrading 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 freeing 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 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.
The 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 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 argument.
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.
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.
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.
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
currently held. Also, it is invalid to upgrade an exclusive lock, and a
panic(9)
will be the result of trying.
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