path: root/share/man/man9/rmlock.9

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.\" Copyright (c) 2006 Gleb Smirnoff <glebius@FreeBSD.org>
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.\" Based on rwlock.9 man page

.Dd August 22, 2007
.Dt RMLOCK 9
.Os
.Sh NAME
.Nm rmlock ,
.Nm rm_init ,
.Nm rm_destroy ,
.Nm rm_rlock ,
.Nm rm_wlock ,
.Nm rm_runlock ,
.Nm rm_wunlock ,
.Nm RM_SYSINIT
.Nd mostly read lock - a kernel reader/writer lock optimized for mostly read access patterns
.Sh SYNOPSIS
.In sys/param.h
.In sys/lock.h
.In sys/rmlock.h
.Ft void
.Fn rm_init "struct rmlock *rm" "const char *name" "int opts"
.Ft void
.Fn rm_destroy "struct rmlock *rm"
.Ft void
.Fn rm_rlock "struct rmlock *rm"  "struct rm_priotracker* tracker"
.Ft void
.Fn rm_wlock "struct rmlock *rm"
.Ft void
.Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
.Ft void
.Fn rm_wunlock "struct rmlock *rm"
.Ft int

.In sys/kernel.h
.Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" "int opts"
.Sh 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
.Em readers
since they only read the protected data.
A thread with exclusive access is known as a
.Em writer
since it can modify protected data.
.Pp
Read mostly locks are designed to be efficient for locks allmost 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.   
.Pp
Although reader/writer locks look very similar to
.Xr sx 9
locks, their usage pattern is different.
Reader/writer locks can be treated as mutexes (see
.Xr mutex 9 )
with shared/exclusive semantics.
Unlike
.Xr sx 9 ,
an
.Nm
can be locked while holding a non-spin mutex, and an
.Nm
cannot be held while sleeping.
The
.Nm
locks have full priority propagation like mutexes. The rm_priotracker structure argument supplied
in rmrlock and rmrunlock is used to keep track of the read owner(s).
Another important property is that shared holders of
.Nm
can recurse if the lock has been initialized with the LO_RECURSABLE option,
however exclusive locks are not allowed to recurse.
.Ss Macros and Functions
.Bl -tag -width indent
.It Fn rm_init "struct rmlock *rm" "const char *name" "int opts"
Initialize structure located at
.Fa rm
as mostly reader lock, described by name
.Fa name .
Optioally allowing readers to recurse by setting LO_RECURSABLE in
.Fa opts
The name description is used solely for debugging purposes.
This function must be called before any other operations
on the lock.
.It Fn rm_rlock "struct rmlock *rm"  "struct rm_priotracker* tracker"
Lock
.Fa rm
as a reader. Using
.Fa 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
.Fn rm_rlock
function can be called when the thread has already acquired reader
access on
.Fa rm .
This is called
.Dq "recursing on a lock" .
.It Fn rm_wlock "struct rmlock *rm"
Lock
.Fa rm
as a writer.
If there are any shared owners of the lock, the current thread blocks.
The
.Fn rm_wlock
function cannot be called recursively.
.It Fn rm_runlock "struct rmlock *rm"  "struct rm_priotracker* tracker"
This function releases a shared lock previously acquired by
.Fn rm_rlock .
The
.Fa tracker
argument must match the 
.Fa tracker 
argument used for acquiring the shared lock 
.It Fn rm_wunlock "struct rmlock *rm"
This function releases an exclusive lock previously acquired by
.Fn rm_wlock .
.It Fn rm_destroy "struct rmlock *rm"
This functions destroys a lock previously initialized with
.Fn rm_init .
The
.Fa rm
lock must be unlocked.
.El
.El
.Sh SEE ALSO
.Xr locking 9 ,
.Xr mutex 9 ,
.Xr panic 9 ,
.Xr rwlock 9,
.Xr sema 9 ,
.Xr sx 9
.Sh HISTORY
These
functions appeared in
.Fx 7.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
facility was written by
.An "Stephan Uphoff" .
This manual page was written by
.An "Gleb Smirnoff"
for rwlock and modifed to reflect rmlock by
.An "Stephan Uphoff" .
.Sh BUGS
.Dv Uniprocessor Systems Optimization:
rmlock is not currently optimized for single processor systems

.Dv Number of rmlocks in the system:
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. 

.Dv condvar: The rm lock can currently not be used as a lock argument for condition variable wait functions.