mtx_lock_spin (9)

Leading comments

Copyright (c) 1998 Berkeley Software Design, Inc. All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
  ...

(The comments found at the beginning of the groff file "man9/mtx_lock_spin.9freebsd".)

NAME

mutex mtx_init mtx_destroy mtx_lock mtx_lock_spin mtx_lock_flags mtx_lock_spin_flags mtx_trylock mtx_trylock_flags mtx_unlock mtx_unlock_spin mtx_unlock_flags mtx_unlock_spin_flags mtx_sleep mtx_initialized mtx_owned mtx_recursed mtx_assert MTX_SYSINIT - kernel synchronization primitives

SYNOPSIS

In sys/param.h In sys/lock.h In sys/mutex.h Ft void Fn mtx_init struct mtx *mutex const char *name const char *type int opts Ft void Fn mtx_destroy struct mtx *mutex Ft void Fn mtx_lock struct mtx *mutex Ft void Fn mtx_lock_spin struct mtx *mutex Ft void Fn mtx_lock_flags struct mtx *mutex int flags Ft void Fn mtx_lock_spin_flags struct mtx *mutex int flags Ft int Fn mtx_trylock struct mtx *mutex Ft int Fn mtx_trylock_flags struct mtx *mutex int flags Ft void Fn mtx_unlock struct mtx *mutex Ft void Fn mtx_unlock_spin struct mtx *mutex Ft void Fn mtx_unlock_flags struct mtx *mutex int flags Ft void Fn mtx_unlock_spin_flags struct mtx *mutex int flags Ft int Fn mtx_sleep void *chan struct mtx *mtx int priority const char *wmesg int timo Ft int Fn mtx_initialized const struct mtx *mutex Ft int Fn mtx_owned const struct mtx *mutex Ft int Fn mtx_recursed const struct mtx *mutex

options INVARIANTS options INVARIANT_SUPPORT Ft void Fn mtx_assert const struct mtx *mutex int what In sys/kernel.h Fn MTX_SYSINIT name struct mtx *mtx const char *description int opts

DESCRIPTION

Mutexes are the most basic and primary method of thread synchronization. The major design considerations for mutexes are:

1. Acquiring and releasing uncontested mutexes should be as cheap as possible.
2. They must have the information and storage space to support priority propagation.
3. A thread must be able to recursively acquire a mutex, provided that the mutex is initialized to support recursion.

There are currently two flavors of mutexes, those that context switch when they block and those that do not.

By default, MTX_DEF mutexes will context switch when they are already held. As an optimization, they may spin for some amount of time before context switching. It is important to remember that since a thread may be preempted at any time, the possible context switch introduced by acquiring a mutex is guaranteed to not break anything that is not already broken.

Mutexes which do not context switch are MTX_SPIN mutexes. These should only be used to protect data shared with primary interrupt code. This includes interrupt filters and low level scheduling code. In all architectures both acquiring and releasing of a uncontested spin mutex is more expensive than the same operation on a non-spin mutex. In order to protect an interrupt service routine from blocking against itself all interrupts are either blocked or deferred on a processor while holding a spin lock. It is permissible to hold multiple spin mutexes.

Once a spin mutex has been acquired it is not permissible to acquire a blocking mutex.

The storage needed to implement a mutex is provided by a Vt struct mtx . In general this should be treated as an opaque object and referenced only with the mutex primitives.

The Fn mtx_init function must be used to initialize a mutex before it can be passed to any of the other mutex functions. The Fa name option is used to identify the lock in debugging output etc. The Fa type option is used by the witness code to classify a mutex when doing checks of lock ordering. If Fa type is NULL Fa name is used in its place. The pointer passed in as Fa name and Fa type is saved rather than the data it points to. The data pointed to must remain stable until the mutex is destroyed. The Fa opts argument is used to set the type of mutex. It may contain either MTX_DEF or MTX_SPIN but not both. See below for additional initialization options. It is not permissible to pass the same Fa mutex to Fn mtx_init multiple times without intervening calls to Fn mtx_destroy .

The Fn mtx_lock function acquires a MTX_DEF 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 block).

The Fn mtx_lock_spin function acquires a MTX_SPIN mutual exclusion lock on behalf of the currently running kernel thread. If another kernel thread is holding the mutex, the caller will spin until the mutex becomes available. Interrupts are disabled during the spin and remain disabled following the acquiring of the lock.

It is possible for the same thread to recursively acquire a mutex with no ill effects, provided that the MTX_RECURSE bit was passed to Fn mtx_init during the initialization of the mutex.

The Fn mtx_lock_flags and Fn mtx_lock_spin_flags functions acquire a MTX_DEF or MTX_SPIN lock, respectively, and also accept a Fa flags argument. In both cases, the only flags presently available for lock acquires are MTX_QUIET and MTX_RECURSE If the MTX_QUIET bit is turned on in the Fa flags argument, then if KTR_LOCK tracing is being done, it will be silenced during the lock acquire. If the MTX_RECURSE bit is turned on in the Fa flags argument, then the mutex can be acquired recursively.

The Fn mtx_trylock attempts to acquire the MTX_DEF mutex pointed to by Fa mutex . If the mutex cannot be immediately acquired Fn mtx_trylock will return 0, otherwise the mutex will be acquired and a non-zero value will be returned.

The Fn mtx_trylock_flags function has the same behavior as Fn mtx_trylock but should be used when the caller desires to pass in a Fa flags value. Presently, the only valid value in the Fn mtx_trylock case is MTX_QUIET and its effects are identical to those described for Fn mtx_lock above.

The Fn mtx_unlock function releases a MTX_DEF mutual exclusion lock. The current thread may be preempted if a higher priority thread is waiting for the mutex.

The Fn mtx_unlock_spin function releases a MTX_SPIN mutual exclusion lock.

The Fn mtx_unlock_flags and Fn mtx_unlock_spin_flags functions behave in exactly the same way as do the standard mutex unlock routines above, while also allowing a Fa flags argument which may specify MTX_QUIET The behavior of MTX_QUIET is identical to its behavior in the mutex lock routines.

The Fn mtx_destroy function is used to destroy Fa mutex so the data associated with it may be freed or otherwise overwritten. Any mutex which is destroyed must previously have been initialized with Fn mtx_init . It is permissible to have a single hold count on a mutex when it is destroyed. It is not permissible to hold the mutex recursively, or have another thread blocked on the mutex when it is destroyed.

The Fn mtx_sleep function is used to atomically release Fa mtx while waiting for an event. For more details on the parameters to this function, see sleep(9).

The Fn mtx_initialized function returns non-zero if Fa mutex has been initialized and zero otherwise.

The Fn mtx_owned function returns non-zero if the current thread holds Fa mutex . If the current thread does not hold Fa mutex zero is returned.

The Fn mtx_recursed function returns non-zero if the Fa mutex is recursed. This check should only be made if the running thread already owns Fa mutex .

The Fn mtx_assert function allows assertions specified in Fa what to be made about Fa mutex . If the assertions are not true and the kernel is compiled with options INVARIANTS and options INVARIANT_SUPPORT the kernel will panic. Currently the following assertions are supported:

MA_OWNED

Assert that the current thread holds the mutex pointed to by the first argument.

MA_NOTOWNED

Assert that the current thread does not hold the mutex pointed to by the first argument.

MA_RECURSED

Assert that the current thread has recursed on the mutex pointed to by the first argument. This assertion is only valid in conjunction with MA_OWNED

MA_NOTRECURSED

Assert that the current thread has not recursed on the mutex pointed to by the first argument. This assertion is only valid in conjunction with MA_OWNED

The Fn MTX_SYSINIT macro is used to generate a call to the Fn mtx_sysinit routine at system startup in order to initialize a given mutex lock. The parameters are the same as Fn mtx_init but with an additional argument, Fa name , that is used in generating unique variable names for the related structures associated with the lock and the sysinit routine.

The Default Mutex Type

Most kernel code should use the default lock type, MTX_DEF The default lock type will allow the thread to be disconnected from the CPU if the lock is already held by another thread. The 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.

The Spin Mutex Type

A MTX_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 another thread interrupted the thread which held a mutex and then tried to acquire the mutex. For this reason spin locks disable all interrupts on the local CPU.

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 other synchronization primitives such as default mutexes, thread scheduling, and interrupt threads.

Initialization Options

The options passed in the Fa opts argument of Fn mtx_init specify the mutex type. One of the MTX_DEF or MTX_SPIN options is required and only one of those two options may be specified. The possibilities are:

MTX_DEF

Default mutexes will always allow the current thread to be suspended to avoid deadlock conditions against interrupt threads. The implementation of this lock type may spin for a while before suspending the current thread.

MTX_SPIN

Spin mutexes will never relinquish the CPU. All interrupts are disabled on the local CPU while any spin lock is held.

MTX_RECURSE

Specifies that the initialized mutex is allowed to recurse. This bit must be present if the mutex is permitted to recurse.

MTX_QUIET

Do not log any mutex operations for this lock.

MTX_NOWITNESS

Instruct witness(4) to ignore this lock.

MTX_DUPOK

Witness should not log messages about duplicate locks being acquired.

MTX_NOPROFILE

Do not profile this lock.

Lock and Unlock Flags

The flags passed to the Fn mtx_lock_flags , Fn mtx_lock_spin_flags , Fn mtx_unlock_flags , and Fn mtx_unlock_spin_flags functions provide some basic options to the caller, and are often used only under special circumstances to modify lock or unlock behavior. Standard locking and unlocking should be performed with the Fn mtx_lock , Fn mtx_lock_spin , Fn mtx_unlock , and Fn mtx_unlock_spin functions. Only if a flag is required should the corresponding flags-accepting routines be used.

Options that modify mutex behavior:

MTX_QUIET

This option is used to quiet logging messages during individual mutex operations. This can be used to trim superfluous logging messages for debugging purposes.

Giant

If Giant must be acquired, it must be acquired prior to acquiring other mutexes. Put another way: it is impossible to acquire Giant non-recursively while holding another mutex. It is possible to acquire other mutexes while holding Giant and it is possible to acquire Giant recursively while holding other mutexes.

Sleeping

Sleeping while holding a mutex (except for Giant is never safe and should be avoided. There are numerous assertions which will fail if this is attempted.

Functions Which Access Memory in Userspace

No mutexes should be held (except for Giant across functions which access memory in userspace, such as copyin(9), copyout(9), uiomove(9), fuword(9), etc. No locks are needed when calling these functions.

SEE ALSO

condvar(9), LOCK_PROFILING9, locking(9), mtx_pool9, panic(9), rwlock(9), sema(9), sleep(9), sx(9)

HISTORY

These functions appeared in Bs x 4.1 and Fx 5.0 .