Kernel Functions for Drivers rwlock(9F)
NAME
rwlock, rw_init, rw_destroy, rw_enter, rw_exit, rw_tryenter,
rw_downgrade, rw_tryupgrade, rw_read_locked - readers/writer
lock functionsSYNOPSIS
#include
void rw_init(krwlock_t *rwlp, char *name, krw_type_t type, void *arg);
void rw_destroy(krwlock_t *rwlp);
void rw_enter(krwlock_t *rwlp, krw_t enter_type);
void rw_exit(krwlock_t *rwlp);
int rw_tryenter(krwlock_t *rwlp, krw_t enter_type);
void rw_downgrade(krwlock_t *rwlp);
int rw_tryupgrade(krwlock_t *rwlp);
int rw_read_locked(krwlock_t *rwlp);
INTERFACE LEVEL
Solaris DDI specific (Solaris DDI).PARAMETERS
rwlp Pointer to a krwlock_t readers/writer lock.
name Descriptive string. This is obsolete andshould be NULL. (Non-null strings are legal,
but they're a waste of kernel memory.) type Type of readers/writer lock.arg Type-specific argument for initialization
function.SunOS 5.11 Last change: 16 Jan 2006 1
Kernel Functions for Drivers rwlock(9F)enter_type One of the values RW_READER or RW_WRITER,
indicating whether the lock is to be acquirednon-exclusively (RW_READER) or exclusively
(RW_WRITER).
DESCRIPTION
A multiple-readers, single-writer lock is represented by the
krwlock_t data type. This type of lock will allow many
threads to have simultaneous read-only access to an object.
Only one thread may have write access at any one time. An object which is searched more frequently than it is changed is a good candidate for a readers/writer lock. Readers/writer locks are slightly more expensive than mutex locks, and the advantage of multiple read access may not occur if the lock will only be held for a short time.The rw_init() function initializes a readers/writer lock. It
is an error to initialize a lock more than once. The typeargument should be set to RW_DRIVER. If the lock is used by
the interrupt handler, the type-specific argument, arg,
should be the interrupt priority returned fromddi_intr_get_pri(9F) or ddi_intr_get_softint_pri(9F). Note
that arg should be the value of the interrupt priority castby calling the DDI_INTR_PRI macro. If the lock is not used
by any interrupt handler, the argument should be NULL.The rw_destroy() function releases any resources that might
have been allocated by rw_init(). It should be called before
freeing the memory containing the lock. The lock must not be held by any thread when it is destroyed.The rw_enter() function acquires the lock, and blocks if
necessary. If enter_type is RW_READER, the caller blocks if
there is a writer or a thread attempting to enter for writ-
ing. If enter_type is RW_WRITER, the caller blocks if any
thread holds the lock. NOTE: It is a programming error for any thread to acquire an rwlock it already holds, even as a reader. Doing so can deadlock the system: if thread R acquires the lock as a reader, then thread W tries to acquire the lock as a writer,W will set write-wanted and block. When R tries to get its
second read hold on the lock, it will honor the write-wanted
bit and block waiting for W; but W cannot run until R drops the lock. Thus threads R and W deadlock.SunOS 5.11 Last change: 16 Jan 2006 2
Kernel Functions for Drivers rwlock(9F)The rw_exit() function releases the lock and may wake up one
or more threads waiting on the lock.The rw_tryenter() function attempts to enter the lock, like
rw_enter(), but never blocks. It returns a non-zero value if
the lock was successfully entered, and zero otherwise. A thread which holds the lock exclusively (entered withRW_WRITER), may call rw_downgrade() to convert to holding
the lock non-exclusively (as if entered with RW_READER). One
or more waiting readers may be unblocked.The rw_tryupgrade() function can be called by a thread which
holds the lock for reading to attempt to convert to holding it for writing. This upgrade can only succeed if no other thread is holding the lock and no other thread is blocked waiting to acquire the lock for writing.The rw_read_locked() function returns non-zero if the cal-
ling thread holds the lock for read, and zero if the caller holds the lock for write. The caller must hold the lock. Thesystem may panic if rw_read_locked() is called for a lock
that isn't held by the caller.RETURN VALUES
0 rw_tryenter() could not obtain the lock without
blocking.0 rw_tryupgrade() was unable to perform the
upgrade because of other threads holding or waiting to hold the lock.0 rw_read_locked() returns 0 if the lock is held
by the caller for write.non-zero from rw_read_locked() if the lock is held by the
caller for read.non-zero successful return from rw_tryenter() or
rw_tryupgrade().
CONTEXT
SunOS 5.11 Last change: 16 Jan 2006 3
Kernel Functions for Drivers rwlock(9F)These functions can be called from user, interrupt, or ker-
nel context, except for rw_init() and rw_destroy(), which
can be called from user context only.SEE ALSO
condvar(9F), ddi_intr_alloc(9F), ddi_intr_add_handler(9F),
ddi_intr_get_pri(9F), ddi_intr_get_softint_pri(9F),
mutex(9F), semaphore(9F) Writing Device Drivers NOTESCompiling with _LOCKTEST or _MPSTATS defined no longer has
any effect. To gather lock statistics, see lockstat(1M).SunOS 5.11 Last change: 16 Jan 2006 4