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RMLOCK(9) FreeBSD Kernel Developer's Manual 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_sleep, rm_assert,
RM_SYSINIT, RM_SYSINIT_FLAGS, rms_init, rms_destroy, rms_rlock,
rms_wlock, rms_runlock, rms_wunlock - 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);
void
rms_init(struct rmslock *rms, const char *name);
rms_wlock(struct rmslock *rms);
void
rms_runlock(struct rmslock *rms);
void
rms_wunlock(struct rmslock *rms);
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.
Sleeping for writers can be allowed by passing RM_SLEEPABLE to
rm_init_flags(). It changes lock ordering rules to the same as for sx(9)
locks. They do not propagate priority to writers, but they do propagate
priority to readers. Note that readers are not permitted to sleep
regardless of the flag.
Sleepable read-mostly locks (created with rms_init()) allow sleeping for
both readers and writers, but don't do priority propagation for either.
They follow sx(9) lock ordering.
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
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.
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.
rms_init(struct rmslock *rms, const char *name)
Initialize the sleepable read-mostly lock rms. The name
description is used as wmesg parameter to the msleep(9) routine.
This function must be called before any other operations on the
lock.
rms_rlock(struct rmlock *rm)
Lock rms as a reader. If any thread holds this lock exclusively,
the current thread blocks.
rms_wlock(struct rmslock *rms)
Lock rms as a writer. If the lock is already taken, the current
thread blocks. The rms_wlock() function cannot be called
recursively.
rms_runlock(struct rmslock *rms)
This function releases a shared lock previously acquired by
rms_rlock().
rms_wunlock(struct rmslock *rms)
This function releases an exclusive lock previously acquired by
rms_wlock().
rms_destroy(struct rmslock *rms)
This functions destroys a lock previously initialized with
rms_init(). The rms lock must be unlocked.
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.
FreeBSD 14.0-RELEASE-p11 April 12, 2021 FreeBSD 14.0-RELEASE-p11