freebsd/share/man/man9/mutex.9

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.\"	from BSDI $Id: mutex.4,v 1.1.2.3 1998/04/27 22:53:13 ewv Exp $
.\" $FreeBSD$
.\"
.Dd April 20, 1998
.Dt MUTEX 9
.Sh NAME
.Nm mutex,
.Nm mtx_enter,
.Nm mtx_exit
.Nd kernel synchronization primitives
.Sh SYNOPSIS
.Ft void
.Fn mtx_enter "mtx_t *mutex" "int flags"
.Ft void
.Fn mtx_exit "mtx_t *mutex" "int flags"
.Ft int
.Fn mtx_owned "mtx_t *mutex"
.Sh DESCRIPTION
The
.Fn mtx_enter
function acquires a mutual exclusion lock
on behalf of the currently running kernel thread.
If another kernel thread is holding the mutex,
the caller will be disconnected from the CPU
until the mutex is available
(i.e. it will sleep),
spin wait for the mutex,
or possibly a combination of both.
.Pp
It is possible for the same thread to recursively acquire a mutex
with no ill effects;
if recursion on a given mutex can be avoided,
faster and smaller code will usually be generated.
.Pp
The
.Fn mtx_exit
function releases a mutual exclusion lock;
if a higher priority thread is waiting for the mutex,
the releasing thread may be disconnected
to allow the higher priority thread to acquire the mutex and run.
.Pp
The type of a mutex is not an attribute of the mutex,
but instead a function of the
.Fa flags
argument passed to
.Fn mtx_enter
and
.Fn mtx_exit ;
this allows code to be generated for the specific mutex type
at compile time
and avoids wasting run time on the determination of lock features.
This does place on the programmer,
the burden of using matching forms of the
.Fn mtx_enter
and
.Fn mtx_exit
functions for a given mutex.
It is an error to acquire a mutex in one mode (e.g. spin)
and release it in another (e.g. default).
It is also an error to get the lock in one mode
and allow another thread to attempt to get the lock in another mode.
A good general rule is to always use a given mutex in one mode only.
.Pp
The
.Fn mtx_owned
function returns a non-zero value
if the mutex pointed to is already held by the current thread.
.Ss The default Mutex Type
Most kernel code should use the default lock type;
the default lock type will allow the thread
to be disconnected from the CPU
if it cannot get the lock.
The machine dependent implementation
may treat the lock as a short term spin lock
under some circumstances.
However, it is always safe to use these forms of locks
in an interrupt thread
without fear of deadlock
against an interrupted thread on the same CPU.
.Ss The spin Mutex Type
A spin mutex will not relinquish the CPU
when it cannot immediately get the requested lock,
but will loop, waiting for the mutex to be released by another CPU.
This could result in deadlock
if a thread interrupted the thread which held a mutex
and then tried to acquire the mutex;
for this reason spin locks will disable all interrupts
(on the local CPU only)
by default.
.Pp
Spin locks are fairly specialized locks
that are intended to be held for very short periods of time;
their primary purpose is to protect portions of the code
that implement default (i.e. sleep) locks.
.Ss Flags
The flags passed to the
.Fn mtx_enter
and
.Fn mtx_exit
functions determine what type of mutex is being used
and also provide various options
used to generate more efficient code under certain circumstances.
.Pp
Both lock types (default and spin)
can be acquired recursively by the same thread.
This behavior can be changed with flags.
.Pp
The type of the mutex must always be specified:
.Bl -tag -width MTX_NORECURSE
.It Dv MTX_DEF
Default lock type;
will always allow the current thread to be suspended
to avoid deadlock conditions against interrupt threads.
The machine dependent implementation of this lock type
may spin for a while before suspending the current thread.
Most locks should be of this type.
.It Dv MTX_SPIN
Spin lock;
will never relinquish the CPU.
By default all interrupts are disabled on the local CPU
while any spin lock is held.
.El
.Pp
Options that modify mutex behavior:
.Bl -tag -width MTX_NORECURSE
.It Dv MTX_NORECURSE
If it is known, absolutely,
that the mutex will not be recursively acquired at this invocation
then this flag should be specified.
.Pp
If the lock is already held by the current thread,
then a kernel with
.Dv SMP_DEBUG
defined will panic;
without debugging enabled,
the thread may deadlock against itself
or leave the mutex in a corrupted state.
.Pp
This flag prevents generation of additional inline code
to deal with recursive lock acquisitions
and should be specified whenever possible
in the interests of efficiency.
Not specifying this flag will only cause the generated code
to be a little larger than necessary;
it will still operate correctly.
.It Dv MTX_RLIKELY
This provides a hint that it is likely that this mutex
will be held recursively at this invocation.
The actual optimization used is machine dependent;
generally, this will inline code to handle recursion
where a function call would otherwise be needed.
.Pp
This is a hint only;
leaving it out or specifying it inappropriately
will not cause any great harm other than
possibly generating less efficient code.
.It Dv MTX_TOPHALF
This option applies to spin locks only.
It indicates that the mutex is never acquired
from an interrupt thread,
so it is safe to leave interrupts enabled while holding the lock.
Since an interrupt may occur while holding the lock,
this may be detrimental to other processors
spin waiting for the lock.
Do not forget to include this option when the lock is released.
.Pp
This option should not be used in new code;
it is documented here for completeness only.
.It Dv MTX_FIRST
This option applies to spin locks only.
It indicates this is the first spin lock acquired by the thread.
No other spin locks may be held,
and the requested lock also may not be currently held.
Do not forget to include this option when the lock is released.
.It Dv MTX_NOSWITCH
When releasing a mutex,
this flag prevents a thread switch that might occur
if another higher priority thread was waiting for the mutex.
This may cause priority inversion and should be used carefully.
.Pp
This flag is used internally by the lock code.
It should not be used in general kernel code
and is documented here for completeness only.
.It Dv MTX_NOSPIN
For default locks,
this hint will prevent spinning before relinquishing the CPU.
This should be specified when it is known
that the lock will usually remain unavailable for some time
when it is not immediately available
(i.e.: coarse grained locks protecting large subsystems).
.El
.Sh HISTORY
These
functions appeared in BSD/OS 4.1 and
.Fx 5.0 .