LOCKING(9) MidnightBSD Kernel Developer’s Manual LOCKING(9)


locking — kernel synchronization primitives


All sorts of stuff to go here.


The FreeBSD kernel is written to run across multiple CPUs and as such requires several different synchronization primitives to allow the developers to safely access and manipulate the many data types required.

These include:


Spin Mutexes


Sleep Mutexes


pool Mutexes


Shared-Exclusive locks


Reader-Writer locks






Condition variables






Lockmanager locks

The primitives interact and have a number of rules regarding how they can and can not be combined. There are too many for the average human mind and they keep changing. (if you disagree, please write replacement text) :-)

Some of these primitives may be used at the low (interrupt) level and some may not.

There are strict ordering requirements and for some of the types this is checked using the witness(4) code.

SPIN Mutexes
Mutexes are the basic primitive. 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 must not do anything that deschedules the thread while you are holding a SPIN mutex.

Basically (regular) mutexes will deschedule the thread if the mutex can not be acquired. A non-spin mutex can be considered to be equivalent to getting a write lock on an rw_lock (see below), and in fact non-spin mutexes and rw_locks may soon become the same thing. As in spin mutexes, you either get it or you don’t. You may only call the sleep(9) call via msleep() or the new mtx_sleep() variant. These will atomically drop the mutex and reacquire it as part of waking up. This is often however a BAD idea because it generally relies on you having such a good knowledge of all the call graph above you and what assumptions it is making that there are a lot of ways to make hard-to-find mistakes. For example you MUST re-test all the assumptions you made before, all the way up the call graph to where you got the lock. You can not just assume that mtx_sleep can be inserted anywhere. If any caller above you has any mutex or rwlock, your sleep, will cause a panic. If the sleep only happens rarely it may be years before the bad code path is found.

Pool Mutexes
A variant of regular mutexes where the allocation of the mutex is handled more by the system.

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 readers since they should only read the protected data. A thread with exclusive access is known as a writer since it may modify protected data.

Although reader/writer locks look very similar to sx(9) (see below) locks, their usage pattern is different. Reader/writer locks can be treated as mutexes (see above and mutex(9)) with shared/exclusive semantics. More specifically, regular mutexes can be considered to be equivalent to a write-lock on an rw_lock. In the future this may in fact become literally the fact. An rw_lock can be locked while holding a regular mutex, but can not be held while sleeping. The 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 rw_lock can recurse, but exclusive locks are not allowed to recurse. This ability should not be used lightly and may go away. Users of recursion in any locks should be prepared to defend their decision against vigorous criticism.

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. The main reason for using an sx_lock is that a thread may hold a shared or exclusive lock on an sx_lock lock while sleeping. As a consequence of this however, an sx_lock lock may not be acquired while holding a mutex. The reason for this is that, if one thread slept while holding an sx_lock lock while another thread blocked on the same sx_lock lock after acquiring a mutex, then the second thread would effectively end up sleeping while holding a mutex, which is not allowed. The sx_lock should be considered to be closely related to sleep(9). In fact it could in some cases be considered a conditional sleep.

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. If this sounds confusing, we need to describe it better.

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 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.

Giant is a special instance of a sleep lock. It has several special characteristics.


It is recursive.


Drivers can request that Giant be locked around them, but this is going away.


You can sleep while it has recursed, but other recursive locks cannot.


Giant must be locked first before other locks.


There are places in the kernel that drop Giant and pick it back up again. Sleep locks will do this before sleeping. Parts of the Network or VM code may do this as well, depending on the setting of a sysctl. This means that you cannot count on Giant keeping other code from running if your code sleeps, even if you want it to.

The functions tsleep(), msleep(), msleep_spin(), pause(), wakeup(), and wakeup_one() handle event-based thread blocking. If a thread must wait for an external event, it is put to sleep by tsleep(), msleep(), msleep_spin(), or pause(). Threads may also wait using one of the locking primitive sleep routines mtx_sleep(9), rw_sleep(9), or sx_sleep(9).

The parameter 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 chan are woken up later by wakeup(), often called from inside an interrupt routine, to indicate that the resource the thread was blocking on is available now.

Several of the sleep functions including msleep(), 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 priority includes the 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 Giant mutex (even if recursed) while the thread is suspended and will reacquire the Giant mutex before the function returns.

lockmanager locks
Largely deprecated. See the lock(9) page for more information. I don’t know what the downsides are but I’m sure someone will fill in this part.

Usage tables.

Interaction table.
The following table shows what you can and can not do if you hold one of the synchronization primitives discussed here: (someone who knows what they are talking about should write this table)

You have: You want:

SPIN mutex
ok no no no no-3
Sleep mutex
ok ok-1 no ok no-3
ok no ok-2 no ok-4
ok ok no ok-2 no-3

*1 Recursion is defined per lock. Lock order is important.

*2 readers can recurse though writers can not. Lock order is important.

*3 There are calls atomically release this primitive when going to sleep and reacquire it on wakeup (e.g. mtx_sleep(), rw_sleep() and msleep_spin() ).

*4 Though one can sleep holding an sx lock, one can also use sx_sleep() which atomically release this primitive when going to sleep and reacquire it on wakeup.

Context mode table.
The next table shows what can be used in different contexts. At this time this is a rather easy to remember table.


interrupt: ok no no no no
idle: ok no no no no


condvar(9), lock(9), mtx_pool(9), mutex(9), rwlock(9), sema(9), sleep(9), sx(9), LOCK_PROFILING(9), WITNESS(9)


These functions appeared in BSD/OS 4.1 through FreeBSD 7.0

MidnightBSD 0.3 March 14, 2007 MidnightBSD 0.3