xref: /freebsd/share/man/man9/rwlock.9 (revision f1acac210b090db922adcfacc01b9158646fad9c)

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.Dd April 19, 2006
.Dt RWLOCK 9
.Os
.Sh NAME
.Nm rwlock ,
.Nm rw_init ,
.Nm rw_rlock ,
.Nm rw_wlock ,
.Nm rw_assert ,
.Nm rw_runlock ,
.Nm rw_wunlock ,
.Nm rw_initialized ,
.Nm rw_destroy ,
.Nm RW_SYSINIT ,
.Nd kernel reader/writer lock
.Sh SYNOPSIS
.In sys/param.h
.In sys/lock.h
.In sys/rwlock.h
.Ft void
.Fn rw_init "struct rwlock *rw" "const char *name"
.Ft void
.Fn rw_rlock "struct rwlock *rw"
.Ft void
.Fn rw_wlock "struct rwlock *rw"
.Ft void
.Fn rw_runlock "struct rwlock *rw"
.Ft void
.Fn rw_wunlock "struct rwlock *rw"
.Ft int
.Fn rw_try_upgrade "struct rwlock *rw"
.Ft void
.Fn rw_downgrade "struct rwlock *rw"
.Ft int
.Fn rw_initialized "struct rwlock *rw"
.Ft void
.Fn rw_destroy "struct rwlock *rw"
.Pp
.Cd "options INVARIANTS"
.Cd "options INVARIANT_SUPPORT"
.Ft void
.Fn rw_assert "struct rwlock *rw" "int what"
.In sys/kernel.h
.Fn RW_SYSINIT "name" "struct rwlock *rw" "const char *desc"
.Sh DESCRIPTION
Reader/writer 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
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 priority propagation like mutexes, but priority
can be propagated only to an exclusive holder.
This limitation comes from the fact that shared owners
are anonymous.
Another important property is that shared holders of
.Nm
can recurse,
but exclusive locks are not allowed to recurse.
.Ss Macros and Functions
.Bl -tag -width indent
.It Fn rw_init "struct rwlock *rw" "const char *name"
Initialize structure located at
.Fa rw
as reader/writer lock, described by name
.Fa name .
The description is used solely for debugging purposes.
This function must be called before any other operations
on the lock.
.It Fn rw_rlock "struct rwlock *rw"
Lock
.Fa rw
as a reader.
If any thread holds this lock exclusively, the current thread blocks,
and its priority is propagated to the exclusive holder.
The
.Fn rw_rlock
function can be called when the thread has already acquired reader
access on
.Fa rw .
This is called
.Dq "recursing on a lock" .
.It Fn rw_wlock "struct rwlock *rw"
Lock
.Fa rw
as a writer.
If there are any shared owners of the lock, the current thread blocks.
The
.Fn rw_wlock
function cannot be called recursively.
.It Fn rw_runlock "struct rwlock *rw"
This function releases a shared lock previously acquired by
.Fn rw_rlock .
.It Fn rw_wunlock "struct rwlock *rw"
This function releases an exclusive lock previously acquired by
.Fn rw_wlock .
.It Fn rw_try_upgrade "struct rwlock *rw"
Attempt to upgrade a single shared lock to an exclusive lock.
The current thread must hold a shared lock of
.Fa rw .
This will only succeed if the current thread holds the only shared lock on
.Fa rw ,
and it only holds a single shared lock.
If the attempt succeeds
.Fn rw_try_upgrade
will return a non-zero value,
and the current thread will hold an exclusive lock.
If the attempt fails
.Fn rw_try_upgrade
will return zero,
and the current thread will still hold a shared lock.
.It Fn rw_downgrade "struct rwlock *rw"
Convert an exclusive lock into a single shared lock.
The current thread must hold an exclusive lock of
.Fa rw .
.It Fn rw_initialized "struct rwlock *rw"
This function returns non-zero if
.Fa rw
has been initialized, and zero otherwise.
.It Fn rw_destroy "struct rwlock *rw"
This functions destroys a lock previously initialized with
.Fn rw_init .
The
.Fa rw
lock must be unlocked.
.It Fn rw_assert "struct rwlock *rw" "int what"
This function allows assertions specified in
.Fa what
to be made about
.Fa rw .
If the assertions are not true and the kernel is compiled
with
.Cd "options INVARIANTS"
and
.Cd "options INVARIANT_SUPPORT" ,
the kernel will panic.
Currently the following assertions are supported:
.Bl -tag -width ".Dv RA_UNLOCKED"
.It Dv RA_LOCKED
Assert that current thread holds either a shared or exclusive lock
of
.Fa rw .
.It Dv RA_RLOCKED
Assert that current thread holds a shared lock of
.Fa rw .
.It Dv RA_WLOCKED
Assert that current thread holds an exclusive lock of
.Fa rw .
.It Dv RA_UNLOCKED
Assert that current thread holds neither a shared nor exclusive lock of
.Fa rw .
.El
.El
.Sh SEE ALSO
.Xr mutex 9 ,
.Xr panic 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 "John Baldwin" .
This manual page was written by
.An "Gleb Smirnoff" .
.Sh BUGS
If
.Dv WITNESS
is not included in the kernel,
then it is impossible to assert that the current thread does or does not
hold a shared lock.
In the
.No non - Ns Dv WITNESS
case, the
.Dv RA_LOCKED
and
.Dv RA_RLOCKED
assertions merely check that some thread holds a shared lock.
.Pp
Reader/writer is a bit of an awkward name.
An
.Nm
can also be called a
.Dq Robert Watson
lock if desired.