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man pages section 9: DDI and DKI Kernel Functions

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Updated: Thursday, June 13, 2019



rwlock, rw_create, rw_init, rw_destroy, rw_enter, rw_exit, rw_tryenter, rw_downgrade, rw_tryupgrade, rw_read_locked - readers/writer lock functions


#include <sys/ksynch.h>

void rw_init(krwlock_t *rwlp, char *name, krw_type_t type, void *arg);
int rw_create(krwlock_t *rwlp, int kmflags, 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).



Pointer to a krwlock_t readers/writer lock.


Descriptive string. This is obsolete and should be NULL. (Non-null strings are legal, but they're a waste of kernel memory.)


Type of readers/writer lock. RW_DEFAULT and RW_DRIVER for non-NUMA rwlock. RW_NUMA_DEFAULT for NUMA aware rwlock and RW_NUMA_READ_REENTRANT for reader re-entrant NUMA aware rwlock.


Type-specific argument for initialization function.


Determines whether caller can sleep for memory while allocating it for NUMA aware rwlock. Possible flags are KM_SLEEP to allow sleeping until memory is available, or KM_NOSLEEP to return failure (-1) immediately if memory is not available.


One of the values RW_READER or RW_WRITER, indicating whether the lock is to be acquired non-exclusively (RW_READER) or exclusively (RW_WRITER).


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 type argument 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 from ddi_intr_get_pri(9F) or ddi_intr_get_softint_pri(9F). Note that arg should be the value of the interrupt priority cast by calling the DDI_INTR_PRI macro. If the lock is not used by any interrupt handler, the argument should be NULL.

The rw_create() function creates and initializes readers/writer lcok. flag, determines whether the caller can sleep while allocating memory for rwlock. KM_SLEEP should be used to allow sleeping until memory is available, or KM_NOSLEEP to return with failure (-1) immediately if memory is not available. If RW_DRIVER is passed in type then it initializes a non-NUMA rwlock which is same as calling rw_init with type RW_DRIVER. If RW_NUMA_DEFAULT is passed in type, then it initializes a NUMA aware rwlock. If RW_NUMA_READ_REENTRANT is passed in type, then it initializes a reader re-entrant NUMA aware rwlock. Reader re-entrancy feature allows threads who have already obtained a read lock to re-enter the lock as a reader again even if there are waiting writers. Callers who have not obtained a read lock give waiting writers priority. However, NUMA aware RWlock does not allow re-entrant writers, nor does it allow a re-entrant mix of reads and writes (that is, it does not allow a caller who has already obtained a read lock to be able to then grab a write lock without first dropping all read locks, and vice versa). It is necessary to call rw_destroy for NUMA aware RWlock to avoid memory leaks.

Note -  RW_NUMA_DEFAULT and RW_NUMA_READ_REENTRANT cannot be passed in rw_init as type. rw_init and rw_create are same if type is RW_DRIVER or RW_DEFAULT.

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 writing. If enter_type is RW_WRITER, the caller blocks if any thread holds the lock.

NOTE: If the lock is not reader re-entrant NUMA aware rwlock, then 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.

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. In case of reader re-entrant NUMA aware RWlock, rw_tryupgrade can only succeed if the lock is held only once.

A thread which holds the lock exclusively (entered with RW_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 calling thread holds the lock for read, and zero if the caller holds the lock for write. The caller must hold the lock. The system may panic if rw_read_locked() is called for a lock that isn't held by the caller.

Return Values


rw_create() was successful.


rw_create() was not successful.


rw_tryenter() could not obtain the lock without blocking.


rw_tryupgrade() was unable to perform the upgrade because of other threads holding or waiting to hold the lock.


rw_read_locked() returns 0 if the lock is held by the caller for write.


from rw_read_locked() if the lock is held by the caller for read.


successful return from rw_tryenter() or rw_tryupgrade().


These functions can be called from user, interrupt, or kernel context, except for rw_init() and rw_destroy(), which can be called from user context only.

See Also

condvar(9F), ddi_intr_add_handler(9F), ddi_intr_alloc(9F), ddi_intr_get_pri(9F), ddi_intr_get_softint_pri(9F), mutex(9F), semaphore(9F)

Writing Device Drivers


Compiling with _LOCKTEST or _MPSTATS defined no longer has any effect. To gather lock statistics, see lockstat(8).