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Neither the name of The DragonFly Project nor the names of its .\" contributors may be used to endorse or promote products derived .\" from this software without specific, prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS .\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT .\" LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS .\" FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE .\" COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING, .\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED .\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, .\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT .\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .Dd October 9, 2011 .Dt SPINLOCK 9 .Os .Sh NAME .Nm spin_init , .Nm spin_lock , .Nm spin_lock_quick , .Nm spin_trylock , .Nm spin_uninit , .Nm spin_unlock , .Nm spin_unlock_quick , .Nm spin_pool_lock , .Nm spin_pool_unlock .Nd core spinlocks .Sh SYNOPSIS .In sys/spinlock.h .In sys/spinlock2.h .Ft void .Fn spin_init "struct spinlock *mtx" .Ft void .Fn spin_uninit "struct spinlock *mtx" .Ft void .Fn spin_lock "struct spinlock *mtx" .Ft void .Fn spin_lock_quick "globaldata_t gd" "struct spinlock *mtx" .Ft boolean_t .Fn spin_trylock "struct spinlock *mtx" .Ft void .Fn spin_unlock "struct spinlock *mtx" .Ft void .Fn spin_unlock_quick "globaldata_t gd" "struct spinlock *mtx" .Sh DESCRIPTION The .Fa spinlock structure and call API are defined in the .In sys/spinlock.h and .In sys/spinlock2.h header files, respectively. .Pp The .Fn spin_init function initializes a new .Fa spinlock structure for use. The structure is cleaned up with .Fn spin_uninit when it is no longer needed. .Pp The .Fn spin_lock function obtains an exclusive .Em read-write spinlock. A thread may hold any number of exclusive spinlocks but should always be mindful of ordering deadlocks. The .Fn spin_trylock function will return .Dv TRUE if the spinlock was successfully obtained and .Dv FALSE if it wasn't. If you have the current CPU's .Fa globaldata pointer in hand you can call .Fn spin_lock_quick , but most code will just call the normal version. A spinlock used only for exclusive access has about the same overhead as a mutex based on a locked bus cycle. .Pp A previously obtained exclusive spinlock is released by calling either .Fn spin_unlock or .Fn spin_unlock_quick . .Pp The .Fn spin_pool_lock function locks a pool spinlock associated with a given address. The spinlock's lifetime is not tied to any objects at the address. The function returns the locked spinlock. The .Fn spin_pool_unlock function unlocks a pool spinlock associated with an address. Pool spinlocks need not be initialized. .Sh IMPLEMENTATION NOTES A thread may not hold any spinlock across a blocking condition or thread switch. LWKT tokens should be used for situations where you want an exclusive run-time lock that will survive a blocking condition or thread switch. Tokens will be automatically unlocked when a thread switches away and relocked when the thread is switched back in. If you want a lock that survives a blocking condition or thread switch without being released, use .Xr lockmgr 9 locks or LWKT reader/writer locks. .Pp .Dx Ap s core spinlocks should only be used around small contained sections of code. For example, to manage a reference count or to implement higher level locking mechanisms. Both the token code and the .Xr lockmgr 9 code use exclusive spinlocks internally. Core spinlocks should not be used around large chunks of code. .Pp Holding one or more spinlocks will disable thread preemption by another thread (e.g. preemption by an interrupt thread), but will not disable FAST interrupts or IPIs. This means that a FAST interrupt can still operate during a spinlock, and any threaded interrupt (which is basically all interrupts except the clock interrupt) will still be scheduled for later execution, but will not be able to preempt the current thread. If you wish to disable FAST interrupts and IPIs you need to enter a critical section prior to obtaining the spinlock. .Pp Currently, FAST interrupts, including IPI messages, are not allowed to acquire any spinlocks. It is possible to work around this if .Va mycpu->gd_spinlocks_wr is 0. If one or the other is not zero, the FAST interrupt or IPI cannot acquire any spinlocks without risking a deadlock, even if the spinlocks in question are not related. .Pp A thread may hold any number of exclusive .Em read-write spinlocks. .Pp Spinlocks spin. A thread will not block, switch away, or lose its critical section while obtaining or releasing a spinlock. Spinlocks do not use IPIs or other mechanisms. They are considered to be a very low level mechanism. .Pp If a spinlock can not be obtained after one second a warning will be printed on the console. If a system panic occurs, spinlocks will succeed after one second in order to allow the panic operation to proceed. .Pp If you have a complex structure such as a .Xr vnode 9 which contains a token or .Xr lockmgr 9 lock, it is legal to directly access the internal spinlock embedded in those structures for other purposes as long as the spinlock is not held when you issue the token or .Xr lockmgr 9 operation. .Sh FILES The uncontended path of the spinlock implementation is in .Pa /sys/sys/spinlock2.h . The core of the spinlock implementation is in .Pa /sys/kern/kern_spinlock.c . .Sh SEE ALSO .Xr crit_enter 9 , .Xr lockmgr 9 , .Xr serializer 9 .Sh HISTORY A .Nm spinlock implementation first appeared in .Dx 1.3 . .Sh AUTHORS .An -nosplit The original .Nm spinlock implementation was written by .An Jeffrey M. Hsu and was later extended by .An Matthew Dillon . This manual page was written by .An Matthew Dillon and .An Sascha Wildner .