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.Dd February 14, 2019
.Dt MUTEX 9
.Os
.Sh NAME
.Nm mutex ,
.Nm mtx_init ,
.Nm mtx_uninit ,
.Nm mtx_lock_sh ,
.\".Nm mtx_lock_sh_link ,
.Nm mtx_lock_sh_quick ,
.Nm mtx_lock_ex ,
.\".Nm mtx_lock_ex_link ,
.Nm mtx_lock_ex_quick ,
.Nm mtx_lock ,
.Nm mtx_spinlock ,
.Nm mtx_lock_ex_try ,
.Nm mtx_lock_sh_try ,
.Nm mtx_spinlock_try ,
.Nm mtx_downgrade ,
.Nm mtx_upgrade_try ,
.Nm mtx_unlock ,
.Nm mtx_unlock_ex ,
.Nm mtx_unlock_sh ,
.Nm mtx_spinunlock ,
.Nm mtx_islocked ,
.Nm mtx_islocked_ex ,
.Nm mtx_notlocked ,
.Nm mtx_notlocked_ex ,
.Nm mtx_owned ,
.Nm mtx_notowned ,
.Nm mtx_lockrefs
.Nd general blocking/spinnable mutex functions
.Sh SYNOPSIS
.In sys/globaldata.h
.In sys/mutex2.h
.Ft void
.Fn mtx_init "mtx_t *mtx" "const char *ident"
.Ft void
.Fn mtx_uninit "mtx_t *mtx"
.Ft int
.Fn mtx_lock_sh "mtx_t *mtx" "int flags" "int to"
.\".Ft int
.\".Fn mtx_lock_sh_link "mtx_t *mtx" "mtx_link_t *link" "int flags" "int to"
.Ft int
.Fn mtx_lock_sh_quick "mtx_t *mtx"
.Ft int
.Fn mtx_lock_ex "mtx_t *mtx" "int flags" "int to"
.\".Ft int
.\".Fn mtx_lock_ex_link "mtx_t *mtx" "mtx_link_t *link" "int flags" "int to"
.Ft int
.Fn mtx_lock_ex_quick "mtx_t *mtx"
.Ft void
.Fn mtx_lock "mtx_t *mtx"
.Ft void
.Fn mtx_spinlock "mtx_t *mtx"
.Ft int
.Fn mtx_lock_ex_try "mtx_t *mtx"
.Ft int
.Fn mtx_lock_sh_try "mtx_t *mtx"
.Ft int
.Fn mtx_spinlock_try "mtx_t *mtx"
.Ft void
.Fn mtx_downgrade "mtx_t *mtx"
.Ft int
.Fn mtx_upgrade_try "mtx_t *mtx"
.Ft void
.Fn mtx_unlock "mtx_t *mtx"
.Ft void
.Fn mtx_unlock_ex "mtx_t *mtx"
.Ft void
.Fn mtx_unlock_sh "mtx_t *mtx"
.Ft void
.Fn mtx_spinunlock "mtx_t *mtx"
.Ft int
.Fn mtx_islocked "mtx_t *mtx"
.Ft int
.Fn mtx_islocked_ex "mtx_t *mtx"
.Ft int
.Fn mtx_notlocked "mtx_t *mtx"
.Ft int
.Fn mtx_notlocked_ex "mtx_t *mtx"
.Ft int
.Fn mtx_owned "mtx_t *mtx"
.Ft int
.Fn mtx_notowned "mtx_t *mtx"
.Ft int
.Fn mtx_lockrefs "mtx_t *mtx"
.Sh DESCRIPTION
Mutexes are used to implement mutual exclusion between threads.
Mutexes can be locked in shared or exclusive mode; they can also block
or spin the current thread when there is contention.
.Pp
Mutexes also have an associated reference count, independent of the lock.
.Pp
System-wide mutex contention statistics can be found in the
.Va kern.mtx_contention_count ,
.Va kern.mtx_collision_count ,
and
.Va kern.mtx_wakeup_count
variables.
.Va kern.mtx_contention_count
is incremented each time an attempt to acquire a mutex fails due to contention.
.Va kern.mtx_wakeup_count
is incremented each time an exclusive lock is converted to either a shared or
unlocked state an waiters for the shared state are woken.
.Pp
The mutex functions are similar to the
.Xr lockmgr 9
functions.
.Sh FUNCTIONS
The
.Fn mtx_init
function initializes a mutex to the unlocked state.
It is an error to use a mutex without initializing it.
The
.Fa ident
parameter is as in
.Xr tsleep 9 ,
it is a string describing the reason for a thread to be blocked.
.Pp
The
.Fn mtx_uninit
function deinitializes a mutex.
.Pp
The
.Fn mtx_lock_sh
function attempts to lock a mutex in shared mode and blocks the current
thread until it is able to do so.
The
.Fa flags
parameter is passed to
.Xr tsleep 9
if the thread must block; the
.Fa to
parameter is a timeout for the sleep.
The
.Fn mtx_lock_sh_quick
function is a version of
.Fn mtx_lock_sh
without flags or a timeout.
The
.Fn mtx_lock_sh
and
.Fn mtx_lock_sh_quick
functions return 0 on success, or the
.Fn tsleep
return code on failure.
An error can only be returned if
.Dv PCATCH
is specified in the flags.
.Pp
The
.Fn mtx_lock_ex
function attempts to lock a mutex exclusively and blocks the current thread
until it is able to do so.
The
.Fa flags
parameter is as in
.Xr tsleep 9 .
The
.Fa to
parameter is a timeout on the sleep.
The
.Fn mtx_lock_ex_quick
function is a version of
.Fn mtx_lock_ex
without flags or a timeout.
The
.Fn mtx_lock
function is a yet shorter form for exclusively locking a mutex, blocking the
current thread until acquired.
It is equivalent to
.Fn mtx_lock_ex "mtx" "0" "0" .
The
.Fn mtx_lock_ex
and
.Fn mtx_lock_ex_quick
functions return 0 on success, or the
.Fn tsleep
return code on failure.
An error can only be returned if
.Dv PCATCH
is specified in the flags.
.Pp
The
.Fn mtx_spinlock
function attempts to lock the mutex in exclusive mode and spins until it is
able to do so.
.Pp
The
.Fn mtx_lock_ex_try
and
.Fn mtx_lock_sh_try
functions attempt to lock the mutex in exclusive or shared mode, respectively.
If they are not able to, they return
.Er EAGAIN .
The
.Fn mtx_spinlock_try
function does the same but for spin mutexes.
.Pp
The
.Fn mtx_downgrade
function converts an exclusively held lock to a shared lock.
The lock must be held by the calling thread.
If the lock is already shared, this call is a no-op.
.Pp
The
.Fn mtx_upgrade_try
function attempts to convert a shared lock to an exclusive one.
The mutex must be held by the caller in the shared state.
If the upgrade is successful, this function returns 0; otherwise, it returns
.Er EDEADLK .
.Pp
The
.Fn mtx_unlock
function releases a held mutex;
it works on both exclusive and shared mutexes.
The
.Fn mtx_unlock_ex
and
.Fn mtx_unlock_sh
functions are optimized unlock paths, used when it is known that a lock is held
exclusively or in shared state.
.Pp
The
.Fn mtx_spinunlock
function releases a held spin mutex.
.Pp
The
.Fn mtx_islocked
function returns non-zero if the mutex is locked in either shared of
exclusive state by any thread.
.Fn mtx_islocked_ex
returns non-zero if the mutex is locked exclusively by any thread.
The
.Fn mtx_notlocked
function returns non-zero if the mutex is not locked.
The
.Fn mtx_owned
function returns non-zero if the mutex is exclusively locked by the calling
thread.
The
.Fn mtx_notowned
function returns non-zero if the mutex is not exclusively locked by the
calling thread.
The
.Fn mtx_lockrefs
function returns the number of shared or exclusive locks on the mutex.
.Sh FILES
The uncontended path of the
.Nm
implementation is in
.Pa /sys/sys/mutex2.h .
The data structures are in
.Pa /sys/sys/mutex.h .
The core of the spinlock implementation is in
.Pa /sys/kern/kern_mutex.c .
.Sh SEE ALSO
.Xr crit_enter 9 ,
.Xr locking 9 ,
.Xr lockmgr 9 ,
.Xr serializer 9 ,
.Xr sleep 9 ,
.Xr spinlock 9
.Sh HISTORY
Mutexes first appeared in
.Dx 2.3 .
.Sh AUTHORS
.An -nosplit
The
.Nm
implementation was written by
.An Matthew Dillon .