Provided by: freebsd-manpages_12.2-2_all 
NAME
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
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(const struct rmlock *rm);
int
rm_sleep(void *wchan, struct rmlock *rm, int priority, const char *wmesg, int timo);
options INVARIANTS
options INVARIANT_SUPPORT
void
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);
DESCRIPTION
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_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:
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.
SEE ALSO
locking(9), mutex(9), panic(9), rwlock(9), sema(9), sleep(9), sx(9)
HISTORY
These functions appeared in FreeBSD 7.0.
AUTHORS
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.
Debian November 11, 2017 RMLOCK(9)