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If applicable, add the following below this CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your own identifying information: Portions Copyright [yyyy] [name of copyright owner] .TH RWLOCK 9F "Sep 19, 2013" .SH NAME rwlock, rw_init, rw_destroy, rw_enter, rw_exit, rw_tryenter, rw_downgrade, rw_tryupgrade, rw_read_locked \- readers/writer lock functions .SH SYNOPSIS .nf #include \fBvoid\fR \fBrw_init\fR(\fBkrwlock_t *\fR\fIrwlp\fR, \fBchar *\fR\fIname\fR, \fBkrw_type_t\fR \fItype\fR, \fBvoid *\fR\fIarg\fR); .fi .LP .nf \fBvoid\fR \fBrw_destroy\fR(\fBkrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBvoid\fR \fBrw_enter\fR(\fBkrwlock_t *\fR\fIrwlp\fR, \fBkrw_t\fR \fIenter_type\fR); .fi .LP .nf \fBvoid\fR \fBrw_exit\fR(\fBkrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBint\fR \fBrw_tryenter\fR(\fBkrwlock_t *\fR\fIrwlp\fR, \fBkrw_t\fR \fIenter_type\fR); .fi .LP .nf \fBvoid\fR \fBrw_downgrade\fR(\fBkrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBint\fR \fBrw_tryupgrade\fR(\fBkrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBint\fR \fBrw_read_locked\fR(\fBkrwlock_t *\fR\fIrwlp\fR); .fi .SH INTERFACE LEVEL illumos DDI specific (illumos DDI). .SH PARAMETERS .ne 2 .na \fB\fIrwlp\fR\fR .ad .RS 14n Pointer to a \fBkrwlock_t\fR readers/writer lock. .RE .sp .ne 2 .na \fB\fIname\fR\fR .ad .RS 14n Descriptive string. This is obsolete and should be \fBNULL\fR. (Non-null strings are legal, but they're a waste of kernel memory.) .RE .sp .ne 2 .na \fB\fItype\fR\fR .ad .RS 14n Type of readers/writer lock. .RE .sp .ne 2 .na \fB\fIarg\fR\fR .ad .RS 14n Type-specific argument for initialization function. .RE .sp .ne 2 .na \fB\fIenter_type\fR\fR .ad .RS 14n One of the values \fBRW_WRITER\fR, \fBRW_READER\fR or \fBRW_READER_STARVEWRITER\fR, indicating whether the lock is to be acquired exclusively (\fBRW_WRITER\fR), non-exclusively (\fBRW_READER\fR) or non-exclusively without regard to any threads that may be blocked on exclusive access (\fBRW_READER_STARVEWRITER\fR). .RE .SH DESCRIPTION A multiple-readers, single-writer lock is represented by the \fBkrwlock_t\fR 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 that is searched more frequently than it is changed is a good candidate for a readers/writer lock. .sp .LP 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. .sp .LP The \fBrw_init()\fR function initializes a readers/writer lock. It is an error to initialize a lock more than once. The \fItype\fR argument should be set to \fBRW_DRIVER\fR. If the lock is used by the interrupt handler, the type-specific argument, \fIarg\fR, should be the interrupt priority returned from \fBddi_intr_get_pri\fR(9F) or \fBddi_intr_get_softint_pri\fR(9F). Note that arg should be the value of the interrupt priority cast by calling the \fBDDI_INTR_PRI\fR macro. If the lock is not used by any interrupt handler, the argument should be \fINULL.\fR .sp .LP The \fBrw_destroy()\fR function releases any resources that might have been allocated by \fBrw_init()\fR. It should be called before freeing the memory containing the lock. The lock must not be held by any thread when it is destroyed. .sp .LP The \fBrw_enter()\fR function acquires the lock, and blocks if necessary. If \fIenter_type\fR is \fBRW_WRITER\fR, the caller blocks if any thread holds the lock. If \fIenter_type\fR is \fBRW_READER\fR, the caller blocks if there is a writer or a thread attempting to enter for writing. If \fIenter_type\fR is \fBRW_READER_STARVEWRITER\fR, the caller blocks only if there is a writer; if the lock is held for reading and a thread is blocked attempting to enter for writing, the caller will acquire the lock as a reader instead of blocking on the pending writer. .sp .LP NOTE: It is a programming error for any thread to acquire an rwlock as \fBRW_READER\fR that it already holds. Doing so can deadlock the system: if thread R acquires the lock as \fBRW_READER\fR, 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. To opt out of this behavior -- that is, to safely allow a lock to be grabbed recursively as a reader -- the lock should be acquired as \fBRW_READER_STARVEWRITER\fR, which will allow R to get its second read hold without regard for the write-wanted bit set by W. Note that the \fBRW_READER_STARVEWRITER\fR behavior will starve writers in the presence of infinite readers; it should be used with care, and only where the default \fBRW_READER\fR behavior is unacceptable. .sp .LP The \fBrw_exit()\fR function releases the lock and may wake up one or more threads waiting on the lock. .sp .LP The \fBrw_tryenter()\fR function attempts to enter the lock, like \fBrw_enter()\fR, but never blocks. It returns a non-zero value if the lock was successfully entered, and zero otherwise. .sp .LP A thread that holds the lock exclusively (entered with \fBRW_WRITER\fR), may call \fBrw_downgrade()\fR to convert to holding the lock non-exclusively (as if entered with \fBRW_READER\fR). One or more waiting readers may be unblocked. .sp .LP The \fBrw_tryupgrade()\fR function can be called by a thread that 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. .sp .LP The \fBrw_read_locked()\fR 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 \fBrw_read_locked()\fR is called for a lock that isn't held by the caller. .SH RETURN VALUES .ne 2 .na \fB\fB0\fR\fR .ad .RS 12n \fBrw_tryenter()\fR could not obtain the lock without blocking. .RE .sp .ne 2 .na \fB\fB0\fR\fR .ad .RS 12n \fBrw_tryupgrade()\fR was unable to perform the upgrade because of other threads holding or waiting to hold the lock. .RE .sp .ne 2 .na \fB\fB0\fR\fR .ad .RS 12n \fBrw_read_locked()\fR returns \fB0\fR if the lock is held by the caller for write. .RE .sp .ne 2 .na \fB\fBnon-zero\fR\fR .ad .RS 12n from \fBrw_read_locked()\fR if the lock is held by the caller for read. .RE .sp .ne 2 .na \fB\fBnon-zero\fR\fR .ad .RS 12n successful return from \fBrw_tryenter()\fR or \fBrw_tryupgrade()\fR. .RE .SH CONTEXT These functions can be called from user, interrupt, or kernel context, except for \fBrw_init()\fR and \fBrw_destroy()\fR, which can be called from user context only. .SH SEE ALSO .BR condvar (9F), .BR ddi_intr_add_handler (9F), .BR ddi_intr_alloc (9F), .BR ddi_intr_get_pri (9F), .BR ddi_intr_get_softint_pri (9F), .BR mutex (9F), .BR semaphore (9F) .sp .LP \fIWriting Device Drivers\fR .SH NOTES Compiling with \fB_LOCKTEST\fR or \fB_MPSTATS\fR defined no longer has any effect. To gather lock statistics, see \fBlockstat\fR(8).