LOCKING(9)						   BSD Kernel Developer's Manual						LOCKING(9)

NAME

     locking -- kernel synchronization primitives

DESCRIPTION

     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.

   Mutexes
     Mutexes (also called "sleep mutexes") are the most commonly used synchronization primitive in the kernel.	Thread acquires (locks) a mutex
     before accessing data shared with other threads (including interrupt threads), and releases (unlocks) it afterwards.  If the mutex cannot be
     acquired, the thread requesting it will sleep.  Mutexes fully support priority propagation.

     See mutex(9) for details.

   Spin mutexes
     Spin mutexes are variation of basic mutexes; the main difference between the two is that spin mutexes never sleep - instead, they spin, wait-
     ing for the thread holding the lock, which runs on another CPU, to release it.  Differently from ordinary mutex, spin mutexes disable inter-
     rupts when acquired.  Since disabling interrupts is expensive, they are also generally slower.  Spin mutexes should be used only when necces-
     sary, e.g. to protect data shared with interrupt filter code (see bus_setup_intr(9) for details).

   Pool mutexes
     With most synchronisaton primitives, such as mutexes, programmer must provide a piece of allocated memory to hold the primitive.  For exam-
     ple, a mutex may be embedded inside the structure it protects.  Pool mutex is a variant of mutex without this requirement - to lock or unlock
     a pool mutex, one uses address of the structure being protected with it, not the mutex itself.  Pool mutexes are seldom used.

     See mtx_pool(9) for details.

   Reader/writer 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 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.

     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. 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.

     See rwlock(9) for details.

   Read-mostly locks
     Mostly reader locks are similar to reader/writer locks but optimized for very infrequent write locking.  Read-mostly locks implement full
     priority propagation by tracking shared owners using a caller-supplied tracker data structure.

     See rmlock(9) for details.

   Shared/exclusive locks
     Shared/exclusive locks are similar to reader/writer locks; the main difference between them is that shared/exclusive locks may be held during
     unbounded sleep (and may thus perform an unbounded sleep).  They are inherently less efficient than mutexes, reader/writer locks and read-
     mostly locks.  They don't support priority propagation.  They should be considered to be closely related to sleep(9).  In fact it could in
     some cases be considered a conditional sleep.

     See sx(9) for details.

   Counting semaphores
     Counting semaphores provide a mechanism for synchronizing access to a pool of resources.  Unlike mutexes, semaphores do not have the concept
     of an owner, so they can be useful in situations where one thread needs to acquire a resource, and another thread needs to release it.  They
     are largely deprecated.

     See sema(9) for details.

   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.

     See condvar(9) for details.

   Giant
     Giant is an instance of a mutex, with some special characteristics:

     1.   It is recursive.

     2.   Drivers and filesystems can request that Giant be locked around them by not marking themselves MPSAFE.  Note that infrastructure to do
	  this is slowly going away as non-MPSAFE drivers either became properly locked or disappear.

     3.   Giant must be locked first before other locks.

     4.   It is OK to hold Giant while performing unbounded sleep; in such case, Giant will be dropped before sleeping and picked up after wakeup.

     5.   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 net-
	  work 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.

   Sleep/wakeup
     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 sleep-
     ing 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 parame-
     ter.  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 cur-
     rent 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.

     See sleep(9) for details.

   Lockmanager locks
     Shared/exclusive locks, used mostly in VFS(9), in particular as a vnode(9) lock.  They have features other lock types don't have, such as
     sleep timeout, writer starvation avoidance, draining, and interlock mutex, but this makes them complicated to implement; for this reason,
     they are deprecated.

     See lock(9) for details.

INTERACTIONS

     The primitives interact and have a number of rules regarding how they can and can not be combined.  Many of these rules are checked using the
     witness(4) code.

   Bounded vs. unbounded sleep
     The following primitives perform bounded sleep: mutexes, pool mutexes, reader/writer locks and read-mostly locks.

     The following primitives block (perform unbounded sleep): shared/exclusive locks, counting semaphores, condition variables, sleep/wakeup and
     lockmanager locks.

     It is an error to do any operation that could result in any kind of sleep while holding spin mutex.

     As a general rule, it is an error to do any operation that could result in unbounded sleep while holding any primitive from the 'bounded
     sleep' group.  For example, it is an error to try to acquire shared/exclusive lock while holding mutex, or to try to allocate memory with
     M_WAITOK while holding read-write lock.

     As a special case, it is possible to call sleep() or mtx_sleep() while holding a single mutex.  It will atomically drop that mutex and reac-
     quire it as part of waking up.  This is often a bad idea because it generally relies on the programmer having good knowledge of all of the
     call graph above the place where mtx_sleep() is being called and assumptions the calling code has made.  Because the lock gets dropped during
     sleep, one one must re-test all the assumptions that were made before, all the way up the call graph to the place where the lock was
     acquired.

     It is an error to do any operation that could result in any kind of sleep when running inside an interrupt filter.

     It is an error to do any operation that could result in unbounded sleep when running inside an interrupt thread.

   Interaction table
     The following table shows what you can and can not do while holding one of the synchronization primitives discussed:

	   You have: You want: spin mtx  mutex	 sx	 rwlock  rmlock sleep
	   spin mtx	       ok-1	 no	 no	 no	 no	no-3
	   mutex	       ok	 ok-1	 no	 ok	 ok	no-3
	   sx		       ok	 ok	 ok-2	 ok	 ok	ok-4
	   rwlock	       ok	 ok	 no	 ok-2	 ok	no-3
	   rmlock	       ok	 ok	 no	 ok	 ok-2	no

     *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 that 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 will 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.

	   Context:	       spin mtx  mutex	 sx	 rwlock  rmlock sleep
	   interrupt filter:   ok	 no	 no	 no	 no	no
	   ithread:	       ok	 ok	 no	 ok	 ok	no
	   callout:	       ok	 ok	 no	 ok	 no	no
	   syscall:	       ok	 ok	 ok	 ok	 ok	ok

SEE ALSO

     witness(4), condvar(9), lock(9), mtx_pool(9), mutex(9), rmlock(9), rwlock(9), sema(9), sleep(9), sx(9), LOCK_PROFILING(9)

HISTORY

     These functions appeared in BSD/OS 4.1 through FreeBSD 7.0

BUGS

     There are too many locking primitives to choose from.

BSD
								 February 15, 2010							       BSD