SPINLOCK(9)

NAME
spin_init, spin_lock, spin_lock_quick, spin_trylock, spin_uninit, spin_unlock, spin_unlock_quick - core spinlocks

SYNOPSIS
#include <sys/spinlock.h>
#include <sys/spinlock2.h>
void
spin_init(struct spinlock *mtx, const char *descr);
void
spin_uninit(struct spinlock *mtx);
void
spin_lock(struct spinlock *mtx);
void
spin_lock_quick(globaldata_t gd, struct spinlock *mtx);
boolean_t
spin_trylock(struct spinlock *mtx);
void
spin_unlock(struct spinlock *mtx);
void
spin_unlock_quick(globaldata_t gd, struct spinlock *mtx);

DESCRIPTION
The spinlock structure and call API are defined in the <sys/spinlock.h> and <sys/spinlock2.h> header files, respectively.

The spin_init() function initializes a new spinlock structure for use. An initializer macro, SPINLOCK_INITIALIZER, is provided as well, taking the same arguments as spin_init(). The structure is cleaned up with spin_uninit() when it is no longer needed.

The spin_lock() function obtains an exclusive read-write spinlock. A thread may hold any number of exclusive spinlocks but should always be mindful of ordering deadlocks. The spin_trylock() function will return TRUE if the spinlock was successfully obtained and FALSE if it wasn't. If you have the current CPU's globaldata pointer in hand you can call 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.

A previously obtained exclusive spinlock is released by calling either spin_unlock() or spin_unlock_quick().

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 lockmgr(9) locks or LWKT reader/writer locks.

DragonFly'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 lockmgr(9) code use exclusive spinlocks internally. Core spinlocks should not be used around large chunks of code.

Holding one or more spinlocks will disable thread preemption by another thread (e.g. preemption by an interrupt thread), and will prevent FAST interrupts or IPIs from running. This means that a FAST interrupt, IPI 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.

A thread may hold any number of exclusive read-write spinlocks.

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.

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.

If you have a complex structure such as a vnode(9) which contains a token or 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 lockmgr(9) operation.

FILES
The uncontended path of the spinlock implementation is in /sys/sys/spinlock2.h. The core of the spinlock implementation is in /sys/kern/kern_spinlock.c.

SEE ALSO
crit_enter(9), locking(9), lockmgr(9), serializer(9)

HISTORY
A spinlock implementation first appeared in DragonFly 1.3.

AUTHORS
-nosplit The original spinlock implementation was written by Jeffrey M. Hsu and was later extended by Matthew Dillon. This manual page was written by Matthew Dillon and Sascha Wildner.