Provided by: freebsd-manpages_10.1~RC1-1_all bug


     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 patterns


     #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

     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_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

     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_wlock().

     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

     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

             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

             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), sleep(9), sema(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 systems.

     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.