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LOCKING(9)	       FreeBSD 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	devel-
     opers to safely access and	manipulate the many data types required.

   Mutexes
     Mutexes (also called "sleep mutexes") are the most	commonly used synchro-
     nization 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, waiting
     for the thread holding the	lock, which runs on another CPU, to release
     it.  Differently from ordinary mutex, spin	mutexes	disable	interrupts
     when acquired.  Since disabling interrupts	is expensive, they are also
     generally slower.	Spin mutexes should be used only when necessary, e.g.
     to	protect	data shared with interrupt filter code (see bus_setup_intr(9)
     for details).

   Pool	mutexes
     With most synchronization primitives, such	as mutexes, programmer must
     provide a piece of	allocated memory to hold the primitive.	 For example,
     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 propa-
     gated 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 dif-
     ference 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 con-
     ditions 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	prop-
	  erly 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	lock-
     ing 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.

     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/exclu-
     sive 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 reacquire
     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
     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-5	 ok	 ok-2	no-5

     *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.

     *5	Read-mostly locks can be initialized to	support	sleeping while holding
     a write lock.  See	rmlock(9) for details.

   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
	   interrupt thread:   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), BUS_SETUP_INTR(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.

FreeBSD	10.1			 May 25, 2012			  FreeBSD 10.1

NAME | DESCRIPTION | INTERACTIONS | SEE ALSO | HISTORY | BUGS

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