man/man9/spinlock.9

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.Dd October 9, 2011
.Dt SPINLOCK 9
.Os
.Sh NAME
.Nm spin_init ,
.Nm spin_lock ,
.Nm spin_lock_quick ,
.Nm spin_trylock ,
.Nm spin_uninit ,
.Nm spin_unlock ,
.Nm spin_unlock_quick ,
.Nm spin_pool_lock ,
.Nm spin_pool_unlock
.Nd core spinlocks
.Sh SYNOPSIS
.In sys/spinlock.h
.In sys/spinlock2.h
.Ft void
.Fn spin_init "struct spinlock *mtx"
.Ft void
.Fn spin_uninit "struct spinlock *mtx"
.Ft void
.Fn spin_lock "struct spinlock *mtx"
.Ft void
.Fn spin_lock_quick "globaldata_t gd" "struct spinlock *mtx"
.Ft boolean_t
.Fn spin_trylock "struct spinlock *mtx"
.Ft void
.Fn spin_unlock "struct spinlock *mtx"
.Ft void
.Fn spin_unlock_quick "globaldata_t gd" "struct spinlock *mtx"
.Sh DESCRIPTION
The
.Fa spinlock
structure and call API are defined in the
.In sys/spinlock.h
and
.In sys/spinlock2.h
header files, respectively.
.Pp
The
.Fn spin_init
function initializes a new
.Fa spinlock
structure for use.
The structure is cleaned up with
.Fn spin_uninit
when it is no longer needed.
.Pp
The
.Fn spin_lock
function obtains an exclusive
.Em read-write
spinlock.
A thread may hold any number of exclusive spinlocks but should always
be mindful of ordering deadlocks.
The
.Fn spin_trylock
function will return
.Dv TRUE
if the spinlock was successfully obtained and
.Dv FALSE
if it wasn't.
If you have the current CPU's
.Fa globaldata
pointer in hand you can call
.Fn spin_lock_quick ,
but most code will just call the normal version.
A spinlock used only for exclusive access has about the same overhead
as a mutex based on a locked bus cycle.
.Pp
A previously obtained exclusive spinlock is released by calling either
.Fn spin_unlock
or
.Fn spin_unlock_quick .
.Pp
The
.Fn spin_pool_lock
function locks a pool spinlock associated with a given address.
The spinlock's lifetime is not tied to any objects at the address.
The function returns the locked spinlock.
The
.Fn spin_pool_unlock
function unlocks a pool spinlock associated with an address.
Pool spinlocks need not be initialized.
.Sh IMPLEMENTATION NOTES
A thread may not hold any spinlock across a blocking condition or
thread switch.
LWKT tokens should be used for situations where you want an exclusive
run-time lock that will survive a blocking condition or thread switch.
Tokens will be automatically unlocked when a thread switches away and
relocked when the thread is switched back in.
If you want a lock that survives a blocking condition or thread switch
without being released, use
.Xr lockmgr 9
locks or LWKT reader/writer locks.
.Pp
.Dx Ap s
core spinlocks should only be used around small contained sections of
code.
For example, to manage a reference count or to implement higher level
locking mechanisms.
Both the token code and the
.Xr lockmgr 9
code use exclusive spinlocks internally.
Core spinlocks should not be used around large chunks of code.
.Pp
Holding one or more spinlocks will disable thread preemption by
another thread (e.g. preemption by an interrupt thread), but will not
disable FAST interrupts or IPIs.
This means that a FAST interrupt can still operate during a spinlock,
and any threaded interrupt (which is basically all interrupts except
the clock interrupt) will still be scheduled for later execution, but
will not be able to preempt the current thread.
If you wish to disable FAST interrupts and IPIs you need to enter a
critical section prior to obtaining the spinlock.
.Pp
Currently, FAST interrupts, including IPI messages, are not allowed to
acquire any spinlocks.
It is possible to work around this if
.Va mycpu->gd_spinlocks_wr
is 0.
If one
or the other is not zero, the FAST interrupt or IPI cannot acquire
any spinlocks without risking a deadlock, even if the spinlocks in
question are not related.
.Pp
A thread may hold any number of exclusive
.Em read-write
spinlocks.
.Pp
Spinlocks spin.
A thread will not block, switch away, or lose its critical section
while obtaining or releasing a spinlock.
Spinlocks do not use IPIs or other mechanisms.
They are considered to be a very low level mechanism.
.Pp
If a spinlock can not be obtained after one second a warning will be
printed on the console.
If a system panic occurs, spinlocks will succeed after one second in
order to allow the panic operation to proceed.
.Pp
If you have a complex structure such as a
.Xr vnode 9
which contains a token or
.Xr lockmgr 9
lock, it is legal to directly access the internal spinlock embedded
in those structures for other purposes as long as the spinlock is not
held when you issue the token or
.Xr lockmgr 9
operation.
.Sh FILES
The uncontended path of the spinlock implementation is in
.Pa /sys/sys/spinlock2.h .
The core of the spinlock implementation is in
.Pa /sys/kern/kern_spinlock.c .
.Sh SEE ALSO
.Xr crit_enter 9 ,
.Xr lockmgr 9 ,
.Xr serializer 9
.Sh HISTORY
A
.Nm spinlock
implementation first appeared in
.Dx 1.3 .
.Sh AUTHORS
.An -nosplit
The original
.Nm spinlock
implementation was written by
.An Jeffrey M. Hsu
and was later extended by
.An Matthew Dillon .
This manual page was written by
.An Matthew Dillon
and
.An Sascha Wildner .