NetBSD/share/man/man9/lock.9

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.Dd June 23, 2000
.Dt LOCK 9
.Os
.Sh NAME
.Nm lock ,
.Nm simple_lock_init ,
.Nm simple_lock ,
.Nm simple_lock_try ,
.Nm simple_unlock ,
.Nm simple_lock_freecheck ,
.Nm simple_lock_dump ,
.Nm lockinit ,
.Nm lockmgr ,
.Nm lockstatus ,
.Nm lockmgr_printinfo ,
.Nm spinlockinit ,
.Nm spinlockmgr
.Nd kernel lock functions
.Sh SYNOPSIS
.Fd #include <sys/lock.h>
.Ft void
.Fn simple_lock_init "struct simplelock *slock"
.Ft void
.Fn simple_lock "struct simplelock *slock"
.Ft int
.Fn simple_lock_try "struct simplelock *slock"
.Ft void
.Fn simple_lock_unlock "struct simplelock *slock"
.Ft void
.Fn simple_lock_freecheck "void *start" "void *end"
.Ft void
.Fn simple_lock_dump "void"
.Ft void
.Fn lockinit "struct lock *lock" "int prio" "const char *wmesg" \
"int timo" "int flags"
.Ft int
.Fn lockmgr "struct lock *lock" "u_int flags" "struct simplelock *slock"
.Ft int
.Fn lockstatus "struct lock *lock"
.Ft void
.Fn lockmgr_printinfo "struct lock *lock"
.Ft void
.Fn spinlockinit "struct lock *lock" "const char *wmesg" "int flags"
.Ft int
.Fn spinlockmgr "struct lock *lock" "u_int flags" "struct simplelock *slock"
.Sh DESCRIPTION
.Pp
The
.Nm
functions provide synchronisation capabilities in the kernel by preventing
multiple threads from simultaneously executing critical section of code which
access shared data.  A number of different locks are available:
.Pp
.Bl -tag -width compact
.It struct simplelock
Provides a simple spinning mutex.  The simplelock operations are implemented
with machine-dependent locking primitives.
.It struct lock
A general lock structure for multiple shared locks, upgrading from
shared to exclusive, and sleeping/spinning until the lock can be
gained.
.El
.Pp
If the kernel option LOCKDEBUG is enabled, additional facilities
are provided to record additional lock information.  These facilities are
provided to assist in determining deadlock occurrences.
.Sh FUNCTIONS
The functions which operate on simplelocks are:
.Pp
.Bl -tag -width compact
.It Fn simple_lock_init "slock"
The simplelock
.Fa slock
is initialised to the unlocked state.  A statically allocated simplelock
also can be initialised with the macro SIMPLELOCK_INITIALIZER.  The
effect is the same as the dynamic initialisation by a call to
simple_lock_init.  For example,
.Pp
struct simplelock slock = SIMPLELOCK_INITIALIZER;
.It Fn simple_lock "slock"
The simplelock
.Fa slock
is locked.  If the simplelock is held then execution will
spin until the simplelock is acquired.  Care must be taken that the
calling thread does not already hold the simplelock.  In this case, the
simplelock can never be acquired.  If kernel option LOCKDEBUG is enabled,
a "locking against myself" panic will occur.
.It Fn simple_lock_try "slock"
Try to acquire the simplelock
.Fa slock
without spinning.  If the simplelock is held by another thread
then the return value is 0.  If the simplelock was acquired
successfully then the return value is 1.
.It Fn simple_lock_unlock "slock"
The simplelock
.Fa slock
is unlocked.  The simplelock must be locked and the calling thread must
be the one that last acquired the simplelock.  If the calling
thread does not hold the simplelock, the simplelock will be released
but the kernel behaviour is undefined.
.It Fn simple_lock_freecheck "start" "end"
Check that all simplelocks in the address range
.Fa start
to
.Fa end
are not held.  If a simplelock within the range is found, the kernel
enters the debugger.  This function is available only with kernel
option LOCKDEBUG.  It provides a mechanism for basic simplelock
consistency checks.
.It Fn simple_lock_dump "void"
Dump the state of all simplelocks in the kernel.  This function is
available only with kernel option LOCKDEBUG.
.El
.Pp
The functions which operate on locks are:
.Pp
.Bl -tag -width compact
.It Fn lockinit "lock" "prio" "wmesg" "timo" "flags"
The lock
.Fa lock
is initialised according to the parameters provided.  Arguments are as
follows:
.Bl -tag -width compact
.It Fa lock
The lock.
.It Fa prio
The priority at which to sleep.
.It Fa wmesg
This is the sleep message for sleep locks or a simple name for spin locks.
.It Fa timo
The maximum sleep time.  Used by tsleep(9).
.It Fa flags
Flags to specify the lock type.  Valid lock request types are:
.Bl -tag -width compact
.It LK_SHARED
Get one of many possible shared locks. If a thread holding an
exclusive lock requests a shared lock, the exclusive lock(s) will be
downgraded to shared locks.
.It LK_EXCLUSIVE
Stop further shared locks, when they are cleared, grant a pending
upgrade if it exists, then grant an exclusive lock. Only one exclusive
lock may exist at a time, except that a thread holding an exclusive
lock may get additional exclusive locks if it explicitly sets the
LK_CANRECURSE flag in the lock request, or if the LK_CANRECUSE flag
was set when the lock was initialized.
.It LK_UPGRADE
The thread must hold a shared lock that it wants to have upgraded to
an exclusive lock. Other threads may get exclusive access to the
resource between the time that the upgrade is requested and the time
that it is granted.
.It LK_EXCLUPGRADE
The thread must hold a shared lock that it wants to have upgraded to
an exclusive lock. If the request succeeds, no other threads will
have gotten exclusive access to the resource between the time that the
upgrade is requested and the time that it is granted. However, if
another thread has already requested an upgrade, the request will
fail (see error returns below).
.It LK_DOWNGRADE
The thread must hold an exclusive lock that it wants to have
downgraded to a shared lock.  If the thread holds multiple (recursive)
exclusive locks, they will all be downgraded to shared locks.
.It LK_RELEASE
Release one instance of a lock.
.It LK_DRAIN
Wait for all activity on the lock to end, then mark it
decommissioned. This feature is used before freeing a lock that is
part of a piece of memory that is about to be freed.
.El
.Pp
Additional flags which may be used:
.Bl -tag -width compact
.It LK_NOWAIT
Do not sleep to await lock.
.It LK_SLEEPFAIL
Sleep, then return failure.
.It LK_CANRECURSE
This may be recursive lock attempt.
.It LK_REENABLE
Lock is to be reenabled after drain.  The LK_REENABLE flag may be set
only at the release of a lock obtained by drain.
.It LK_SETRECURSE
Other locks while we have it OK.
.It LK_RECURSEFAIL
Attempt at recursive lock fails.
.El
.El
.It Fn lockmgr "lock" "flags" "slock"
Set, change or release a lock according to the parameters provided.
Arguments are as follows:
.Bl -tag -width compact
.It Fa lock
The lock.
.It Fa flags
Flags to specify the lock type.  Lock request types are the same as outlined
above.
.It Fa slock
Simplelock interlock.  The simplelock
.Fa slock
is set by the caller.  When the lock
.Fa lock
is acquired, the simplelock is released.
.El
.It Fn lockstatus "lock"
Determine the status of lock
.Fa lock .
Returns LK_EXCLUSIVE or LK_SHARED for exclusive and shared locks
respectively.
.It Fn lockmgr_printinfo "lock"
Print out information about state of lock
.Fa lock .
.It Fn spinlockinit "lock" "wmesg" "flags"
The lock
.Fa lock
is initialised as a spinlock according to the parameters provided.
Arguments are as follows:
.Bl -tag -width compact
.It Fa lock
The lock.
.It Fa wmesg
This is a simple name for lock.
.It Fa flags
Flags to specify the lock type.  Lock request types are the same as outlined
above.
.El
.It Fn spinlockmgr "lock" "flags" "slock"
Set, change or release a lock according to the parameters provided.
Arguments are as follows:
.Bl -tag -width compact
.It Fa lock
The spin lock.
.It Fa flags
Flags to specify the lock type.  Lock request types are the same as outlined
above.
.It Fa slock
Simplelock interlock.  The simplelock
.Fa slock
is set by the caller.  When the lock
.Fa lock
is acquired, the simplelock is released.
.El
.El
.Sh RETURN VALUES
Successfully obtained locks return 0.  Locks will always succeed
unless one of the following is true:
.Bl -tag -width compact
.It
LK_FORCEUPGRADE is requested and some other process has already
requested a lock upgrade (returns EBUSY).
.It
LK_WAIT is set and a sleep would be required (returns EBUSY).
.It
LK_SLEEPFAIL is set and a sleep was done (returns ENOLCK).
.It
PCATCH is set in lock priority and a signal arrives (returns
either EINTR or ERESTART if system calls is to be restarted).
.It
Non-null lock timeout and timeout expires (returns EWOULDBLOCK).
.It
A failed lock attempt always returns a non-zero error value.  No lock
is held after an error return (in particular, a failed LK_UPGRADE
or LK_FORCEUPGRADE will have released its shared access lock).
.El