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SPINLOCK(9)           DragonFly Kernel Developer's Manual          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

     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.

DragonFly 5.5                    May 29, 2017                    DragonFly 5.5

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