rwlock, rw_init, rw_init_flags, rw_destroy, rw_rlock, rw_wlock, rw_runlock, rw_wunlock, rw_unlock,

       The rwlock facility was written by John Baldwin.  This manual page was written by Gleb Smirnoff.

       A  kernel  without  WITNESS  cannot  assert whether the current thread does or does not hold a read lock.
       RA_LOCKED and RA_RLOCKED can only assert that any thread holds a read lock.  They cannot ensure that  the
       current  thread holds a read lock.  Further, RA_UNLOCKED can only assert that the current thread does not
       hold a write lock.

       Reader/writer is a bit of an awkward name.  An rwlock can also  be  called  a  “Robert  Watson”  lock  if
       desired.

Debian                                          November 11, 2017                                      RWLOCK(9)

       Reader/writer locks allow shared access to protected data by multiple threads, or exclusive access  by  a
       single  thread.   The  threads with shared access are known as readers since they only read the protected
       data.  A thread with exclusive access is known as a writer since it can modify protected data.

       Although reader/writer locks look very  similar  to  sx(9)  locks,  their  usage  pattern  is  different.
       Reader/writer  locks  can  be  treated as mutexes (see mutex(9)) with shared/exclusive semantics.  Unlike
       sx(9), an rwlock can be locked while holding a non-spin  mutex,  and  an  rwlock  cannot  be  held  while
       sleeping.   The  rwlock locks have priority propagation like mutexes, but priority can be propagated only
       to writers.  This limitation comes from the fact that readers are anonymous.  Another important  property
       is that readers can always recurse, and exclusive locks can be made recursive selectively.

   MacrosandFunctionsrw_init(structrwlock*rw, constchar*name)
               Initialize  structure  located  at  rw  as  reader/writer  lock,  described  by  name  name.  The
               description is used solely for debugging purposes.  This function must be called before any other
               operations on the lock.

       rw_init_flags(structrwlock*rw, constchar*name, intopts)
               Initialize the rw lock just like the rw_init() function, but specifying a set of  optional  flags
               to  alter  the  behaviour  of  rw,  through  the  opts  argument.  It contains one or more of the
               following flags:

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

               RW_NOPROFILE  Do not profile this lock.

               RW_NOWITNESS  Instruct witness(4) to ignore this lock.

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

               RW_RECURSE    Allow threads to recursively acquire exclusive locks for rw.

               RW_NEW        If the kernel has been compiled with optionINVARIANTS, rw_init_flags() will assert
                             that the rw has not been initialized multiple times without  intervening  calls  to
                             rw_destroy() unless this option is specified.

       rw_rlock(structrwlock*rw)
               Lock  rw  as a reader.  If any thread holds this lock exclusively, the current thread blocks, and
               its priority is propagated to the exclusive holder.  The rw_rlock() function can be  called  when
               the thread has already acquired reader access on rw.  This is called “recursing on a lock”.

       rw_wlock(structrwlock*rw)
               Lock rw as a writer.  If there are any shared owners of the lock, the current thread blocks.  The
               rw_wlock() function can be called recursively only if rw has been initialized with the RW_RECURSE
               option enabled.

       rw_try_rlock(structrwlock*rw)
               Try  to lock rw as a reader.  This function will return true if the operation succeeds, otherwise
               0 will be returned.

       rw_try_wlock(structrwlock*rw)
               Try to lock rw as a writer.  This function will return true if the operation succeeds,  otherwise
               0 will be returned.

       rw_runlock(structrwlock*rw)
               This function releases a shared lock previously acquired by rw_rlock().

       rw_wunlock(structrwlock*rw)
               This function releases an exclusive lock previously acquired by rw_wlock().

       rw_unlock(structrwlock*rw)
               This  function  releases  a  shared  lock  previously acquired by rw_rlock() or an exclusive lock
               previously acquired by rw_wlock().

       rw_try_upgrade(structrwlock*rw)
               Attempt to upgrade a single shared lock to an exclusive lock.  The current  thread  must  hold  a
               shared  lock  of  rw.  This will only succeed if the current thread holds the only shared lock on
               rw, and it only holds a single shared lock.  If the attempt succeeds rw_try_upgrade() will return
               a non-zero value, and the current thread will hold an  exclusive  lock.   If  the  attempt  fails
               rw_try_upgrade() will return zero, and the current thread will still hold a shared lock.

       rw_downgrade(structrwlock*rw)
               Convert  an  exclusive lock into a single shared lock.  The current thread must hold an exclusive
               lock of rw.

       rw_sleep(void*chan, structrwlock*rw, intpriority, constchar*wmesg, inttimo)
               Atomically release rw while waiting for an event.  For more details on  the  parameters  to  this
               function, see sleep(9).

       rw_initialized(conststructrwlock*rw)
               This function returns non-zero if rw has been initialized, and zero otherwise.

       rw_destroy(structrwlock*rw)
               This  functions  destroys  a  lock  previously  initialized  with rw_init().  The rw lock must be
               unlocked.

       rw_wowned(conststructrwlock*rw)
               This function returns a non-zero value if the current thread owns an exclusive lock on rw.

       rw_assert(conststructrwlock*rw, intwhat)
               This function allows assertions specified in what to be made about rw.  If the assertions are not
               true and the kernel is compiled with optionsINVARIANTS and optionsINVARIANT_SUPPORT, the kernel
               will panic.  Currently the following base assertions are supported:

               RA_LOCKED    Assert that current thread holds either a shared or exclusive lock of rw.

               RA_RLOCKED   Assert that current thread holds a shared lock of rw.

               RA_WLOCKED   Assert that current thread holds an exclusive lock of rw.

               RA_UNLOCKED  Assert that current thread holds neither a shared nor exclusive lock of rw.

               In addition, one of the following optional flags may be specified with RA_LOCKED, RA_RLOCKED,  or
               RA_WLOCKED:

               RA_RECURSED     Assert that the current thread holds a recursive lock of rw.

               RA_NOTRECURSED  Assert that the current thread does not hold a recursive lock of rw.

       These functions appeared in FreeBSD 7.0.

       rwlock,  rw_init,  rw_init_flags,  rw_destroy,  rw_rlock,  rw_wlock,  rw_runlock,  rw_wunlock, rw_unlock,
       rw_try_rlock, rw_try_upgrade, rw_try_wlock, rw_downgrade, rw_sleep, rw_initialized, rw_wowned, rw_assert,
       RW_SYSINIT, RW_SYSINIT_FLAGS — kernel reader/writer lock

locking(9), mutex(9), panic(9), sema(9), sx(9)

#include<sys/param.h>#include<sys/lock.h>#include<sys/rwlock.h>voidrw_init(structrwlock*rw, constchar*name);

       voidrw_init_flags(structrwlock*rw, constchar*name, intopts);

       voidrw_destroy(structrwlock*rw);

       voidrw_rlock(structrwlock*rw);

       voidrw_wlock(structrwlock*rw);

       intrw_try_rlock(structrwlock*rw);

       intrw_try_wlock(structrwlock*rw);

       voidrw_runlock(structrwlock*rw);

       voidrw_wunlock(structrwlock*rw);

       voidrw_unlock(structrwlock*rw);

       intrw_try_upgrade(structrwlock*rw);

       voidrw_downgrade(structrwlock*rw);

       intrw_sleep(void*chan, structrwlock*rw, intpriority, constchar*wmesg, inttimo);

       intrw_initialized(conststructrwlock*rw);

       intrw_wowned(conststructrwlock*rw);

       optionsINVARIANTSoptionsINVARIANT_SUPPORTvoidrw_assert(conststructrwlock*rw, intwhat);

       #include<sys/kernel.h>RW_SYSINIT(name, structrwlock*rw, constchar*desc);

       RW_SYSINIT_FLAGS(name, structrwlock*rw, constchar*desc, intflags);