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.\" $DragonFly: src/share/man/man9/spinlock.9,v 1.5 2006/06/01 19:38:06 swildner Exp $
.\"
.Dd May 27, 2006
.Os
.Dt SPINLOCK 9
.Sh NAME
.Nm spin_init ,
.Nm spin_lock_rd ,
.Nm spin_lock_rd_quick ,
.Nm spin_lock_wr ,
.Nm spin_lock_wr_quick ,
.Nm spin_trylock_wr ,
.Nm spin_uninit ,
.Nm spin_unlock_rd ,
.Nm spin_unlock_rd_quick ,
.Nm spin_unlock_wr ,
.Nm spin_unlock_wr_quick
.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_rd "struct spinlock *mtx"
.Ft void
.Fn spin_lock_rd_quick "globaldata_t gd" "struct spinlock *mtx"
.Ft void
.Fn spin_unlock_rd "struct spinlock *mtx"
.Ft void
.Fn spin_unlock_rd_quick "globaldata_t gd" "struct spinlock *mtx"
.Ft void
.Fn spin_lock_wr "struct spinlock *mtx"
.Ft void
.Fn spin_lock_wr_quick "globaldata_t gd" "struct spinlock *mtx"
.Ft boolean_t
.Fn spin_trylock_wr "struct spinlock *mtx"
.Ft void
.Fn spin_unlock_wr "struct spinlock *mtx"
.Ft void
.Fn spin_unlock_wr_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_rd
function obtains a shared
.Em read-only
spinlock.
A thread may hold only one shared lock at a time, and may not acquire any
new exclusive locks while holding a shared lock (but may already be holding
some).
A shared spinlock can be held by multiple CPUs concurrently.
If a thread attempts to obtain an exclusive spinlock while shared
references from other CPUs exist it will spin until the shared references
go away.
No new shared references will be allowed (that is, new shared requests
will also spin) while the exclusive spinlock is being acquired.
If you have the current CPU's
.Fa globaldata
pointer in hand you can call
.Fn spin_lock_rd_quick ,
but most code will just call the normal version.
Shared spinlocks reserve a bit in the spinlock's memory for each CPU
and do not clear the bit once set.
This means that once set, a shared spinlock does not need to issue a
locked read-modify-write bus cycle to the spinlock's memory, which in
turn greatly reduces conflicts between CPU caches.
The bit is cleared via a different mechanism only when an exclusive
spinlock is acquired.
The result is extremely low overheads even when a shared spinlock is
being operated upon concurrently by multiple CPUs.
.Pp
A previously obtained shared spinlock is released by calling either
.Fn spin_unlock_rd
or
.Fn spin_unlock_rd_quick .
.Pp
The
.Fn spin_lock_wr
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.
Exclusive spinlocks can only be safely
acquired if no shared spinlocks are held.
The
.Fn spin_trylock_wr
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_wr_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.
When used in a mixed shared/exclusive environment, however, additional
overhead may be incurred to obtain the exclusive spinlock.
Because shared spinlocks are left intact even after released (to
optimize shared spinlock performance), the exclusive spinlock code
must run through any shared bits it finds in the spinlock, clear them,
and check the related CPU's
.Fa globaldata
structure to determine whether it needs to spin or not.
.Pp
A previously obtained exclusive spinlock is released by calling either
.Fn spin_unlock_wr
or
.Fn spin_unlock_wr_quick .
.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.
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
and
.Va mycpu->gd_spinlocks_rd
are both 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.
However, a thread may only hold one shared
.Em read-only
spinlock, and may not acquire any new exclusive locks while it is holding
that one shared lock.
This requirement is due to the method exclusive
spinlocks use to determine when they can clear cached shared bits in
the lock.
If an exclusive lock is acquired while holding shared locks,
a deadlock can occur even if the locks are unrelated.
Always be mindful of potential deadlocks.
.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 SEE ALSO
.Xr lockmgr 9 ,
.Xr lwkt 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 .