RMLOCK(9)

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

SYNOPSIS
#include <sys/param.h>
#include <sys/lock.h>
#include <sys/rmlock.h>
void
rm_init(struct rmlock *rm, const char *name);
void
rm_init_flags(struct rmlock *rm, const char *name, int opts);
void
rm_destroy(struct rmlock *rm);
void
rm_rlock(struct rmlock *rm, struct rm_priotracker* tracker);
int
rm_try_rlock(struct rmlock *rm, struct rm_priotracker* tracker);
void
rm_wlock(struct rmlock *rm);
void
rm_runlock(struct rmlock *rm, struct rm_priotracker* tracker);
void
rm_wunlock(struct rmlock *rm);
int
rm_wowned(struct rmlock *rm);
#include <sys/kernel.h>
RM_SYSINIT(name, struct rmlock *rm, const char *desc, int opts);

DESCRIPTION
Mostly reader 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 had been locked for shared access is an expensive operation.

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 unless initialized with RM_SLEEPABLE. Unlike sx(9), an rmlock can be locked while holding a non-spin mutex, and an rmlock cannot be held while sleeping, again unless initialized with RM_SLEEPABLE. The rmlock locks have full priority propagation like mutexes. The rm_priotracker structure argument supplied in rm_rlock() and rm_runlock() is used to keep track of the read owner(s). Another important property is that shared holders of rmlock can recurse if the lock has been initialized with the LO_RECURSABLE option, however exclusive locks are not allowed to recurse.

  Macros and Functions

rm_init(struct rmlock *rm, const char *name)
    Initialize structure located at rm as mostly reader lock, described by name. 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)
    Initialize the rm lock just like the rm_init() function, but specifying a set of optional flags to alter the behaviour of rm, through the opts argument. It contains one or more of the following flags:

    RM_NOWITNESS
        Instruct witness(4) to ignore this lock.

    RM_RECURSE
        Allow threads to recursively acquire exclusive locks for rm.

    RM_SLEEPABLE
        Allow writers to sleep while holding the lock. Readers must not sleep while holding the lock and can avoid to sleep on taking the lock by using rm_try_rlock() instead of rm_rlock().

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 rmlocks cannot be held while sleeping. 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 LO_RECURSABLE option the rm_rlock() function can be called when the thread has already acquired reader access on rm. This is called “recursing on a lock”.

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.

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

SEE ALSO
locking(9), mutex(9), panic(9), rwlock(9), sema(9), sx(9)

HISTORY
These functions appeared in FreeBSD 7.0.

AUTHORS
-nosplit 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.

BUGS
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.

The rmlock can currently not be used as a lock argument for condition variable wait functions.