RMLOCK(9) BSD 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, RM_SYSINIT_FLAGS — 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);



rm_assert(struct rmlock *rm, int what);

#include <sys/kernel.h>

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

RM_SYSINIT_FLAGS(name, struct rmlock *rm, const char *desc, int flags);


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:


Instruct witness(4) to ignore this lock.


Allow threads to recursively acquire shared locks for rm.


Create a sleepable read-mostly lock.


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_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 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:


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


Assert that current thread holds a shared lock of rm.


Assert that current thread holds an exclusive lock of rm.


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:


Assert that the current thread holds a recursive lock of rm.


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 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.

BSD November 11, 2017 BSD