Command Section
RMLOCK(9)              FreeBSD Kernel Developer's Manual             RMLOCK(9)

     rmlock, rm_init, rm_init_flags, rm_destroy, rm_rlock, rm_try_rlock,
     rm_wlock, rm_runlock, rm_wunlock, rm_wowned, rm_sleep, rm_assert,
     RM_SYSINIT - kernel reader/writer lock optimized for read-mostly access

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

     rm_init(struct rmlock *rm, const char *name);

     rm_init_flags(struct rmlock *rm, const char *name, int opts);

     rm_destroy(struct rmlock *rm);

     rm_rlock(struct rmlock *rm, struct rm_priotracker* tracker);

     rm_try_rlock(struct rmlock *rm, struct rm_priotracker* tracker);

     rm_wlock(struct rmlock *rm);

     rm_runlock(struct rmlock *rm, struct rm_priotracker* tracker);

     rm_wunlock(struct rmlock *rm);

     rm_wowned(const struct rmlock *rm);

     rm_sleep(void *wchan, struct rmlock *rm, int priority, const char *wmesg,
         int timo);

     options INVARIANTS
     rm_assert(struct rmlock *rm, int what);

     #include <sys/kernel.h>

     RM_SYSINIT(name, struct rmlock *rm, const char *desc, int opts);

     Read-mostly 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.

     Read-mostly locks are designed to be efficient for locks almost
     exclusively used as reader locks and as such should be used for
     protecting data that rarely changes.  Acquiring an exclusive lock after
     the lock has been locked for shared access is an expensive operation.

     Normal read-mostly locks are similar to rwlock(9) locks and follow the
     same lock ordering rules as rwlock(9) locks.  Read-mostly locks have full
     priority propagation like mutexes.  Unlike rwlock(9), read-mostly locks
     propagate priority to both readers and writers.  This is implemented via
     the rm_priotracker structure argument supplied to rm_rlock() and
     rm_runlock().  Readers can recurse if the lock is initialized with the
     RM_RECURSE option; however, writers are never allowed to recurse.

     Sleepable read-mostly locks are created by passing RM_SLEEPABLE to
     rm_init_flags().  Unlike normal read-mostly locks, sleepable read-mostly
     locks follow the same lock ordering rules as sx(9) locks.  Sleepable
     read-mostly locks do not propagate priority to writers, but they do
     propagate priority to readers.  Writers are permitted to sleep while
     holding a read-mostly lock, but readers are not.  Unlike other sleepable
     locks such as sx(9) locks, readers must use try operations on other
     sleepable locks to avoid sleeping.

   Macros and Functions
     rm_init(struct rmlock *rm, const char *name)
             Initialize the read-mostly lock rm.  The name description is used
             solely for debugging purposes.  This function must be called
             before any other operations on the lock.

     rm_init_flags(struct rmlock *rm, const char *name, int opts)
             Similar to rm_init(), initialize the read-mostly lock rm with a
             set of optional flags.  The opts arguments contains one or more
             of the following flags:

             RM_NOWITNESS      Instruct witness(4) to ignore this lock.

             RM_RECURSE        Allow threads to recursively acquire shared
                               locks for rm.

             RM_SLEEPABLE      Create a sleepable read-mostly lock.

             RM_NEW            If the kernel has been compiled with option
                               INVARIANTS, rm_init_flags() will assert that
                               the rm has not been initialized multiple times
                               without intervening calls to rm_destroy()
                               unless this option is specified.

     rm_rlock(struct rmlock *rm, struct rm_priotracker* tracker)
             Lock rm as a reader using tracker to track read owners of a lock
             for priority propagation.  This data structure is only used
             internally by rmlock and must persist until rm_runlock() has been
             called.  This data structure can be allocated on the stack since
             readers cannot sleep.  If any thread holds this lock exclusively,
             the current thread blocks, and its priority is propagated to the
             exclusive holder.  If the lock was initialized with the
             RM_RECURSE option the rm_rlock() function can be called when the
             current thread has already acquired reader access on rm.

     rm_try_rlock(struct rmlock *rm, struct rm_priotracker* tracker)
             Try to lock rm as a reader.  rm_try_rlock() will return 0 if the
             lock cannot be acquired immediately; otherwise, the lock will be
             acquired and a non-zero value will be returned.  Note that
             rm_try_rlock() may fail even while the lock is not currently held
             by a writer.  If the lock was initialized with the RM_RECURSE
             option, rm_try_rlock() will succeed if the current thread has
             already acquired reader access.

     rm_wlock(struct rmlock *rm)
             Lock rm as a writer.  If there are any shared owners of the lock,
             the current thread blocks.  The rm_wlock() function cannot be
             called recursively.

     rm_runlock(struct rmlock *rm, struct rm_priotracker* tracker)
             This function releases a shared lock previously acquired by
             rm_rlock().  The tracker argument must match the tracker argument
             used for acquiring the shared lock

     rm_wunlock(struct rmlock *rm)
             This function releases an exclusive lock previously acquired by

     rm_destroy(struct rmlock *rm)
             This functions destroys a lock previously initialized with
             rm_init().  The rm lock must be unlocked.

     rm_wowned(const struct rmlock *rm)
             This function returns a non-zero value if the current thread owns
             an exclusive lock on rm.

     rm_sleep(void *wchan, struct rmlock *rm, int priority, const char *wmesg,
             int timo)
             This function atomically releases rm while waiting for an event.
             The rm lock must be exclusively locked.  For more details on the
             parameters to this function, see sleep(9).

     rm_assert(struct rmlock *rm, int what)
             This function asserts that the rm lock is in the state specified
             by what.  If the assertions are not true and the kernel is
             compiled with options INVARIANTS and options INVARIANT_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 rm.

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

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

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

             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 rm.

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

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

     These functions appeared in FreeBSD 7.0.

     The rmlock facility was written by Stephan Uphoff.  This manual page was
     written by Gleb Smirnoff for rwlock and modified to reflect rmlock by
     Stephan Uphoff.

     The rmlock implementation is currently not optimized for single processor

     rm_try_rlock() can fail transiently even when there is no writer, while
     another reader updates the state on the local CPU.

     The rmlock implementation uses a single per CPU list shared by all
     rmlocks in the system.  If rmlocks become popular, hashing to multiple
     per CPU queues may be needed to speed up the writer lock process.

FreeBSD 11.1-RELEASE-p4        December 13, 2014       FreeBSD 11.1-RELEASE-p4
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