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

     sx, sx_init, sx_init_flags, sx_destroy, sx_slock, sx_xlock, sx_slock_sig,
     sx_xlock_sig, sx_try_slock, sx_try_xlock, sx_sunlock, sx_xunlock,
     sx_unlock, sx_try_upgrade, sx_downgrade, sx_sleep, sx_xholder,
     sx_xlocked, sx_assert, SX_SYSINIT, SX_SYSINIT_FLAGS — kernel
     shared/exclusive lock

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

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

     sx_init_flags(struct sx *sx, const char *description, int opts);

     sx_destroy(struct sx *sx);

     sx_slock(struct sx *sx);

     sx_xlock(struct sx *sx);

     sx_slock_sig(struct sx *sx);

     sx_xlock_sig(struct sx *sx);

     sx_try_slock(struct sx *sx);

     sx_try_xlock(struct sx *sx);

     sx_sunlock(struct sx *sx);

     sx_xunlock(struct sx *sx);

     sx_unlock(struct sx *sx);

     sx_try_upgrade(struct sx *sx);

     sx_downgrade(struct sx *sx);

     sx_sleep(void *chan, struct sx *sx, int priority, const char *wmesg,
         int timo);

     struct thread *
     sx_xholder(struct sx *sx);

     sx_xlocked(const struct sx *sx);

     options INVARIANTS

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

     #include <sys/kernel.h>

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

     SX_SYSINIT_FLAGS(name, struct sx *sx, const char *desc, int flags);

     Shared/exclusive locks are used to protect data that are read far more
     often than they are written.  Shared/exclusive locks do not implement
     priority propagation like mutexes and reader/writer locks to prevent
     priority inversions, so shared/exclusive locks should be used prudently.

     Shared/exclusive locks are created with either sx_init() or
     sx_init_flags() where sx is a pointer to space for a struct sx, and
     description is a pointer to a null-terminated character string that
     describes the shared/exclusive lock.  The opts argument to
     sx_init_flags() specifies a set of optional flags to alter the behavior
     of sx.  It contains one or more of the following flags:

     SX_NOADAPTIVE  Disable adaptive spinning, rather than sleeping, for lock
                    operations while an exclusive lock holder is executing on
                    another CPU.  Adaptive spinning is the default unless the
                    kernel is compiled with options NO_ADAPTIVE_SX.

     SX_DUPOK       Witness should not log messages about duplicate locks
                    being acquired.

     SX_NOWITNESS   Instruct witness(4) to ignore this lock.

     SX_NOPROFILE   Do not profile this lock.

     SX_RECURSE     Allow threads to recursively acquire exclusive locks for

     SX_QUIET       Do not log any operations for this lock via ktr(4).

     SX_NEW         If the kernel has been compiled with options INVARIANTS,
                    sx_init() will assert that the sx has not been initialized
                    multiple times without intervening calls to sx_destroy()
                    unless this option is specified.

     Shared/exclusive locks are destroyed with sx_destroy().  The lock sx must
     not be locked by any thread when it is destroyed.

     Threads acquire and release a shared lock by calling sx_slock(),
     sx_slock_sig() or sx_try_slock() and sx_sunlock() or sx_unlock().
     Threads acquire and release an exclusive lock by calling sx_xlock(),
     sx_xlock_sig() 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_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.

     sx_slock_sig() and sx_xlock_sig() do the same as their normal versions
     but performing an interruptible sleep.  They return a non-zero value if
     the sleep has been interrupted by a signal or an interrupt, otherwise 0.

     A thread can atomically release a shared/exclusive lock while waiting for
     an event by calling sx_sleep().  For more details on the parameters to
     this function, see sleep(9).

     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.  One of the following assertions must be

     SA_LOCKED    Assert that the current thread has either a shared or an
                  exclusive lock on the sx lock pointed to by the first

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

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

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

     In addition, one of the following optional assertions may be included
     with either an SA_LOCKED, SA_SLOCKED, or SA_XLOCKED assertion:

     SA_RECURSED     Assert that the current thread has a recursed lock on sx.

     SA_NOTRECURSED  Assert that the current thread does not have a recursed
                     lock on sx.

     sx_xholder() will return a pointer to the thread which currently holds an
     exclusive lock on sx.  If no thread holds an exclusive lock on sx, then
     NULL is returned instead.

     sx_xlocked() will return non-zero if the current thread holds the
     exclusive 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
     specific 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.  The
     SX_SYSINIT_FLAGS() macro can similarly be used to initialize a given sx
     lock using sx_init_flags().

     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.

     A thread may hold a shared or exclusive lock on an sx lock while
     sleeping.  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 second thread would effectively end up sleeping while holding a
     mutex, which is not allowed.

     lock(9), locking(9), mutex(9), panic(9), rwlock(9), sema(9)

     A kernel without WITNESS cannot assert whether the current thread does or
     does not hold a shared lock.  SA_LOCKED and SA_SLOCKED can only assert
     that any thread holds a shared lock.  They cannot ensure that the current
     thread holds a shared lock.  Further, SA_UNLOCKED can only assert that
     the current thread does not hold an exclusive lock.

BSD                            November 11, 2017                           BSD