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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 6.2                     January 5, 2005                    FreeBSD 6.2

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

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