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$FreeBSD: head/share/man/man9/locking.9 167519 2007-03-14 04:00:24Z julian $
.Dd March 14, 2007 .Dt LOCKING 9 .Os .Sh NAME .Nm locking , .Nd kernel synchronization primitives .Sh SYNOPSIS All sorts of stuff to go here.
p .Sh DESCRIPTION The .Em FreeBSD kernel is written to run across multiple CPUs and as such requires several different sychronization primatives to allow the developers to safely access and manipulate the many data types required.
p These include: l -enum t Spin Mutexes t Sleep Mutexes t pool Mutexes t Shared-Exclusive locks t Reader-Writer locks t Turnstyles t Semaphores t Condition variables t Sleep/wakeup t Giant t Lockmanager locks .El
p The primnatives interact and have a number of rules regarding how they can and can not be combined. Ther eare too many for the average humanmind and they keep changing. (if you disagree, please write replacement text) :-)
p Some of these primatives may be used at the low (interrupt) level and some may not.
p There are strict ordering requirements and for some of the types this is checked using the .Xr witness 4 code.
p .Ss SPIN Mutexes Mutexes are the basic primative. You either hold it or you don't. If you don't own it then you just spin, waiting for the holder (on another CPU) to release it. Hopefully they are doing something fast. You can not do anythign that deschedules the thread while you are holding a SPIN mutex. .Ss Sleep Mutexes Basically sleep (regular) mutexes will deschedule the thread if the mutex can not be acquired. As in spin mutexes, you either get it or you don't. You may call the .Xr sleep 9 call .Fn msleep or the new .Fn mtx_sleep variant. These will atomically drop the mutex and reacquire it as part of waking up. .Ss Pool Mutexes A variant of SLEEP mutexes where the allocation of the mutex is handled more by the system. .Ss Sx_locks Shared/exclusive locks are used to protect data that are read far more often than they are written. Mutexes are inherently more efficient than shared/exclusive locks, so shared/exclusive locks should be used prudently. A thread may hold a shared or exclusive lock on an .Em sx_lock lock while sleeping. As a result, an .Em sx_lockm lock may not be acquired while holding a mutex. Otherwise, if one thread slept while holding an .Em sx_lockm lock while another thread blocked on the same .Em sx_lockm lock after acquiring a mutex, then the second thread would effectively end up sleeping while holding a mutex, which is not allowed. .Ss Rw_locks 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.
p 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 .Em rw_lock can be locked while holding a non-spin mutex, and an .Em rw_lock cannot be held while sleeping. The .Em rw_lock 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 .Em rw_lock can recurse, but exclusive locks are not allowed to recurse. .Ss Turnstyless Turnstiles are used to hold a queue of threads blocked on non-sleepable locks. Sleepable locks use condition variables to implement their queues. Turnstiles differ from a sleep queue in that turnstile queue's are assigned to a lock held by an owning thread. Thus, when one thread is enqueued onto a turnstile, it can lend its priority to the owning thread. .Ss Semaphores .Ss Condition variables Condition variables are used in conjunction with mutexes to wait for conditions to occur. A thread must hold the mutex before calling the .Fn cv_wait* , functions. When a thread waits on a condition, the mutex is atomically released before the thread is blocked, then reacquired before the function call returns. .Ss Giant Giant is a special instance of a sleep lock. it has several special characteristics. .Ss Sleep/wakeup The functions .Fn tsleep , .Fn msleep , .Fn msleep_spin , .Fn pause , .Fn wakeup , and .Fn wakeup_one handle event-based thread blocking. If a thread must wait for an external event, it is put to sleep by .Fn tsleep , .Fn msleep , .Fn msleep_spin , or .Fn pause . Threads may also wait using one of the locking primitive sleep routines .Xr mtx_sleep 9 , .Xr rw_sleep 9 , or .Xr sx_sleep 9 .
p The parameter .Fa chan is an arbitrary address that uniquely identifies the event on which the thread is being put to sleep. All threads sleeping on a single .Fa chan are woken up later by .Fn wakeup , often called from inside an interrupt routine, to indicate that the resource the thread was blocking on is available now.
p Several of the sleep functions including .Fn msleep , .Fn msleep_spin , and the locking primitive sleep routines specify an additional lock parameter. The lock will be released before sleeping and reacquired before the sleep routine returns. If .Fa priority includes the .Dv PDROP flag, then the lock will not be reacquired before returning. The lock is used to ensure that a condition can be checked atomically, and that the current thread can be suspended without missing a change to the condition, or an associated wakeup. In addition, all of the sleep routines will fully drop the .Va Giant mutex (even if recursed) while the thread is suspended and will reacquire the .Va Giant mutex before the function returns.
p .Ss lockmanager locks Largly deprecated. See the .Xr lock 9 page for more information. I don't know what the downsides are but I'm sure someone will fill in this part. .Sh Interaction tables. The following table shows what you can and can not do if you hold one of the synchronisation primatives discussed here: (someone who knows what they are talking about should write this table) l -column ".Ic xxxxxxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXX" -offset indent t Xo .Em "You have: You want:" Ta Spin_mtx Ta Slp_mtx Ta sx_lock Ta rw_lock Ta sleep .Xc t Ic SPIN mutex Ta ok Ta no Ta no Ta no Ta no-3 t Ic Sleep mutex Ta ok Ta ok-1 Ta no Ta ok Ta no-3 t Ic sx_lock Ta ok Ta no Ta ?? Ta no Ta ok-4 t Ic rw_lock Ta ok Ta no Ta no Ta ok-2 Ta no-3 .El
p .Em *1 Recursion is defined per lock. lock order is important.
p .Em *2 readers can recurse tough writers can not. lock order is important.
p .Em *3 There are calls atomically release this primative when going to sleep and reacquire it on wakeup (e.g. .Fn mtx_sleep , .Fn rw-sleep and .Fn msleep_spin).
p .Em *4 One can also use .Fn sx_sleep which atomically release this primative when going to sleep and reacquire it on wakeup.
p .Sh SEE ALSO .Xr condvar 9 , .Xr lock 9 .Xr mtx_pool 9 , .Xr rwlock 9 , .Xr sema 9 , .Xr sleep 9 , .Xr sx 9 .Xr LOCK_PROFILING 9 , .Xr WITNESS 9 , .Sh HISTORY These functions appeared in sx 4.1 through .Fx 7.0