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.Dd July 13, 2013.
.Dt ck_elide 3
.Sh NAME
.Nm CK_ELIDE_PROTOTYPE ,
.Nm CK_ELIDE_LOCK_ADAPTIVE ,
.Nm CK_ELIDE_UNLOCK_ADAPTIVE ,
.Nm CK_ELIDE_LOCK ,
.Nm CK_ELIDE_UNLOCK ,
.Nm CK_ELIDE_TRYLOCK_PROTOTYPE ,
.Nm CK_ELIDE_TRYLOCK
.Nd lock elision wrappers
.Sh LIBRARY
Concurrency Kit (libck, \-lck)
.Sh SYNOPSIS
.In ck_elide.h
.Pp
.Dv ck_elide_stat_t stat = CK_ELIDE_STAT_INITIALIZER;
.Pp
.Ft void
.Fn ck_elide_stat_init "ck_elide_stat_t *"
.Pp
.Dv struct ck_elide_config config = CK_ELIDE_CONFIG_DEFAULT_INITIALIZER;
.Pp
.Bd -literal -offset
struct ck_elide_config {
	unsigned short skip_busy;
	short retry_busy;
	unsigned short skip_other;
	short retry_other;
	unsigned short skip_conflict;
	short retry_conflict;
};
.Ed
.Pp
.Fn CK_ELIDE_PROTOTYPE "NAME" "TYPE" "LOCK_PREDICATE" "LOCK_FUNCTION" "UNLOCK_PREDICATE" "UNLOCK_FUNCTION"
.Fn CK_ELIDE_LOCK_ADAPTIVE "NAME" "ck_elide_stat_t *" "struct ck_elide_config *" "TYPE *"
.Fn CK_ELIDE_UNLOCK_ADAPTIVE "NAME" "ck_elide_stat_t *" "TYPE *"
.Fn CK_ELIDE_LOCK "NAME" "TYPE *"
.Fn CK_ELIDE_UNLOCK "NAME" "TYPE *"
.Fn CK_ELIDE_TRYLOCK_PROTOTYPE "NAME" "TYPE" "LOCK_PREDICATE" "TRYLOCK_FUNCTION"
.Sh DESCRIPTION
These macros implement lock elision wrappers for a user-specified single-argument
lock interface. The wrappers will attempt to elide lock acquisition, allowing
concurrent execution of critical sections that do not issue conflicting memory
operations. If any threads have successfully elided a lock acquisition,
conflicting memory operations will roll-back any side-effects of the critical
section and force every thread to retry the lock acquisition regularly.
.Pp
.Fn CK_ELIDE_LOCK ,
.Fn CK_ELIDE_UNLOCK ,
.Fn CK_ELIDE_LOCK_ADAPTIVE ,
and
.Fn CK_ELIDE_UNLOCK_ADAPTIVE
macros require
a previous
.Fn CK_ELIDE_PROTOTYPE
with the same
.Fa NAME .
Elision is attempted if the
.Fa LOCK_PREDICATE
function returns false. If
.Fa LOCK_PREDICATE
returns true then elision is aborted and
.Fa LOCK_FUNCTION
is executed instead. If any threads are in an elided critical section,
.Fa LOCK_FUNCTION
must force them to rollback through a conflicting memory operation.
The
.Fa UNLOCK_PREDICATE
function must return true if the lock is acquired by the caller, meaning
that the lock was not successfully elided. If
.Fa UNLOCK_PREDICATE
returns true, then the
.Fa UNLOCK_FUNCTION
is executed. If RTM is unsupported (no CK_F_PR_RTM macro) then
.Fn CK_ELIDE_LOCK
and
.Fn CK_ELIDE_LOCK_ADAPTIVE
will immediately call
.Fn LOCK_FUNCTION .
.Fn CK_ELIDE_UNLOCK
and
.Fn CK_ELIDE_UNLOCK_ADAPTIVE
will immediately call
.Fn UNLOCK_FUNCTION .
.Pp
.Fn CK_ELIDE_TRYLOCK
requires a previous
.Fn CK_ELIDE_TRYLOCK_PROTOTYPE
with the same name.
Elision is attempted if the
.Fa LOCK_PREDICATE
function returns false. If
.Fa LOCK_PREDICATE
returns true or if elision fails then the
operation is aborted. If RTM is unsupported
(no CK_F_PR_RTM macro) then
.Fn CK_ELIDE_TRYLOCK
will immediately call
.Fn TRYLOCK_FUNCTION .
.Pp
.Fn CK_ELIDE_LOCK_ADAPTIVE
and
.Fn CK_ELIDE_UNLOCK_ADAPTIVE
will adapt the elision behavior associated with lock operations
according to the run-time behavior of the program. This behavior
is defined by the ck_elide_config structure pointer passed to
.Fn CK_ELIDE_LOCK_ADAPTIVE .
A thread-local ck_elide_stat structure must be passed to both
.Fn CK_ELIDE_LOCK_ADAPTIVE
and
.Fn CK_ELIDE_UNLOCK_ADAPTIVE .
This structure is expected to be unique for different workloads,
may not be re-used in recursive acquisitions and must match the
lifetime of the lock it is associated with. It is safe to mix
adaptive calls with best-effort calls.
.Pp
Both ck_spinlock.h and ck_rwlock.h define ck_elide wrappers under
the ck_spinlock and ck_rwlock namespace, respectively.
.Sh EXAMPLES
This example utilizes built-in lock elision facilities in ck_rwlock and ck_spinlock.
.Bd -literal -offset indent
#include <ck_rwlock.h>
#include <ck_spinlock.h>

static ck_rwlock_t rw = CK_RWLOCK_INITIALIZER;
static struct ck_elide_config rw_config =
    CK_ELIDE_CONFIG_DEFAULT_INITIALIZER;
static __thread ck_elide_stat_t rw_stat =
    CK_ELIDE_STAT_INITIALIZER;

static ck_spinlock_t spinlock = CK_SPINLOCK_INITIALIZER;
static struct ck_elide_config spinlock_config =
    CK_ELIDE_CONFIG_DEFAULT_INITIALIZER;
static __thread ck_elide_stat_t spinlock_stat =
    CK_ELIDE_STAT_INITIALIZER;

void
function(void)
{

	/* Lock-unlock write-side lock in weak best-effort manner. */
	CK_ELIDE_LOCK(ck_rwlock_write, &rw);
	CK_ELIDE_UNLOCK(ck_rwlock_write, &rw);

	/* Attempt to acquire the write-side lock. */
	if (CK_ELIDE_TRYLOCK(ck_rwlock_write, &rw) == true)
		CK_ELIDE_UNLOCK(ck_rwlock_write, &rw);

	/* Lock-unlock read-side lock in weak best-effort manner. */
	CK_ELIDE_LOCK(ck_rwlock_read, &rw);
	CK_ELIDE_UNLOCK(ck_rwlock_read, &rw);

	/* Attempt to acquire the read-side lock. */
	if (CK_ELIDE_TRYLOCK(ck_rwlock_read, &rw) == true)
		CK_ELIDE_UNLOCK(ck_rwlock_read, &rw);

	/* Lock-unlock write-side lock in an adaptive manner. */
	CK_ELIDE_LOCK_ADAPTIVE(ck_rwlock_write, &rw_stat,
	    &rw_config, &rw);
	CK_ELIDE_UNLOCK_ADAPTIVE(ck_rwlock_write, &rw_stat,
	    &rw_config, &rw);

	/* Lock-unlock read-side lock in an adaptive manner. */
	CK_ELIDE_LOCK_ADAPTIVE(ck_rwlock_read, &rw_stat,
	    &rw_config, &rw);
	CK_ELIDE_UNLOCK_ADAPTIVE(ck_rwlock_read, &rw_stat,
	    &rw_config, &rw);

	/* Lock-unlock spinlock in weak best-effort manner. */
	CK_ELIDE_LOCK(ck_spinlock, &spinlock);
	CK_ELIDE_UNLOCK(ck_spinlock, &spinlock);

	/* Attempt to acquire the lock. */
	if (CK_ELIDE_TRYLOCK(ck_spinlock, &lock) == true)
		CK_ELIDE_UNLOCK(ck_spinlock, &spinlock);

	/* Lock-unlock spinlock in an adaptive manner. */
	CK_ELIDE_LOCK_ADAPTIVE(ck_spinlock, &spinlock_stat,
	    &spinlock_config, &spinlock);
	CK_ELIDE_UNLOCK_ADAPTIVE(ck_spinlock, &spinlock_stat,
	    &spinlock_config, &spinlock);
}
.Ed
.Pp
In this example, user-defined locking functions are provided an elision
implementation.
.Bd -literal -offset indent
/* Assume lock_t has been previously defined. */
#include <ck_elide.h>

/*
 * This function returns true if the lock is unavailable at the time
 * it was called or false if the lock is available.
 */
bool is_locked(lock_t *);

/*
 * This function acquires the supplied lock.
 */
void lock(lock_t *);

/*
 * This function releases the lock.
 */
void unlock(lock_t *);

CK_ELIDE_PROTOTYPE(my_lock, lock_t, is_locked, lock, is_locked, unlock)

static lock_t lock;

void
function(void)
{

	CK_ELIDE_LOCK(my_lock, &lock);
	CK_ELIDE_UNLOCK(my_lock, &lock);
}
.Ed
.Sh SEE ALSO
.Xr ck_rwlock 3 ,
.Xr ck_spinlock 3
.Pp
Ravi Rajwar and James R. Goodman. 2001. Speculative lock elision: enabling highly concurrent multithreaded execution. In Proceedings of the 34th annual ACM/IEEE international symposium on Microarchitecture (MICRO 34). IEEE Computer Society, Washington, DC, USA, 294-305.
.Pp
Additional information available at http://en.wikipedia.org/wiki/Transactional_Synchronization_Extensions and http://concurrencykit.org/
