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SX(9)		       FreeBSD Kernel Developer's Manual		 SX(9)

NAME
     sx, sx_init, sx_destroy, sx_slock,	sx_xlock, sx_try_slock,	sx_try_xlock,
     sx_sunlock, sx_xunlock, sx_try_upgrade, sx_downgrade, sx_assert,
     sx_unlock,	sx_xlocked, SX_SYSINIT -- kernel shared/exclusive lock

SYNOPSIS
     #include <sys/param.h>
     #include <sys/lock.h>
     #include <sys/sx.h>

     void
     sx_init(struct sx *sx, const char *description);

     void
     sx_destroy(struct sx *sx);

     void
     sx_slock(struct sx	*sx);

     void
     sx_xlock(struct sx	*sx);

     int
     sx_try_slock(struct sx *sx);

     int
     sx_try_xlock(struct sx *sx);

     void
     sx_sunlock(struct sx *sx);

     void
     sx_xunlock(struct sx *sx);

     int
     sx_try_upgrade(struct sx *sx);

     void
     sx_downgrade(struct sx *sx);

     void
     sx_assert(struct sx *sx, int what);

     int
     sx_xlocked(struct sx *sx);

   sx utility macros
     sx_unlock(struct sx *sx);

     SX_SYSINIT(name, struct sx	*sx, const char	*description);

   Kernel options
     options INVARIANTS
     options INVARIANT_SUPPORT

DESCRIPTION
     Shared/exclusive locks are	used to	protect	data that are read far more
     often than	they are written.  Mutexes are inherently more efficient than
     shared/exclusive locks, so	shared/exclusive locks should be used pru-
     dently.

     Shared/exclusive locks are	created	with sx_init(),	where sx is a pointer
     to	space for a struct sx, and description is a pointer to a null-termi-
     nated character string that describes the shared/exclusive	lock.
     Shared/exclusive locks are	destroyed with sx_destroy().  Threads acquire
     and release a shared lock by calling sx_slock() or	sx_try_slock() and
     sx_sunlock() or sx_unlock().  Threads acquire and release an exclusive
     lock by calling sx_xlock()	or sx_try_xlock() and sx_xunlock() or
     sx_unlock().  A thread can	attempt	to upgrade a currently held shared
     lock to an	exclusive lock by calling sx_try_upgrade().  A thread that has
     an	exclusive lock can downgrade it	to a shared lock by calling
     sx_downgrade().

     sx_try_slock() and	sx_try_xlock() will return 0 if	the shared/exclusive
     lock cannot be acquired immediately; otherwise the	shared/exclusive lock
     will be acquired and a non-zero value will	be returned.

     sx_try_upgrade() will return 0 if the shared lock cannot be upgraded to
     an	exclusive lock immediately; otherwise the exclusive lock will be
     acquired and a non-zero value will	be returned.

     When compiled with	options	INVARIANTS and options INVARIANT_SUPPORT, the
     sx_assert() function tests	sx for the assertions specified	in what, and
     panics if they are	not met.  The following	assertions are supported:

     SX_LOCKED	  Assert that the current thread has either a shared or	an
		  exclusive lock on the	sx lock	pointed	to by the first	argu-
		  ment.

     SX_SLOCKED	  Assert that the current thread has a shared lock on the sx
		  lock pointed to by the first argument.

     SX_XLOCKED	  Assert that the current thread has an	exclusive lock on the
		  sx lock pointed to by	the first argument.

     SX_UNLOCKED  Assert that the current thread has no	lock on	the sx lock
		  pointed to by	the first argument.

     sx_xlocked() will return non-zero if the current process holds the	exclu-
     sive lock;	otherwise, it will return zero.

     For ease of programming, sx_unlock() is provided as a macro frontend to
     the respective functions, sx_sunlock() and	sx_xunlock().  Algorithms that
     are aware of what state the lock is in should use either of the two spe-
     cific functions for a minor performance benefit.

     The SX_SYSINIT() macro is used to generate	a call to the sx_sysinit()
     routine at	system startup in order	to initialize a	given sx lock.	The
     parameters	are the	same as	sx_init() but with an additional argument,
     name, that	is used	in generating unique variable names for	the related
     structures	associated with	the lock and the sysinit routine.

     A thread may not hold both	a shared lock and an exclusive lock on the
     same lock simultaneously; attempting to do	so will	result in deadlock.

CONTEXT
     A thread may hold a shared	or exclusive lock on an	sx lock	while sleep-
     ing.  As a	result,	an sx lock may not be acquired while holding a mutex.
     Otherwise,	if one thread slept while holding an sx	lock while another
     thread blocked on the same	sx lock	after acquiring	a mutex, then the sec-
     ond thread	would effectively end up sleeping while	holding	a mutex, which
     is	not allowed.

SEE ALSO
     condvar(9), mtx_pool(9), mutex(9),	panic(9), sema(9)

BUGS
     Currently there is	no way to assert that a	lock is	not held.  This	is not
     possible in the non-WITNESS case for asserting that this thread does not
     hold a shared lock.  In the non-WITNESS case, the SX_LOCKED and
     SX_SLOCKED	assertions merely check	that some thread holds a shared	lock.
     They do not ensure	that the current thread	holds a	shared lock.

FreeBSD	10.1			January	5, 2005			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | CONTEXT | SEE ALSO | BUGS

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