path: root/share/man/man9/locking.9

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.Dd January 29, 2010
.Dt LOCKING 9
.Os
.Sh NAME
.Nm locking
.Nd kernel synchronization primitives
.Sh DESCRIPTION
The
.Em 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.
.Pp
These include:
.Bl -enum
.It
Mutexes
.It
Spin mutexes
.It
Pool mutexes
.It
Shared/exclusive locks
.It
Reader/writer locks
.It
Read-mostly locks
.It
Counting semaphores
.It
Condition variables
.It
Sleep/wakeup
.It
Giant
.It
Lockmanager locks
.El
.Pp
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
.Xr witness 4
code.
.Pp
.Ss Mutexes
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.
.Pp
See the
.Xr mutex 9
page for more information.
.Ss 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 only be used to protect data shared with primary
(INTR_FILTER) interrupt code.
You 
.Em must not
do anything that deschedules the thread while you
are holding a spin mutex.
.Ss Pool mutexes
With most synchronisaton 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.
.Pp
See the
.Xr mtx_pool 9
page for more information.
.Ss 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
.Em readers
since they should only read the protected data.
A thread with exclusive access is known as a
.Em writer
since it may modify protected data.
.Pp
Reader/writer locks can be treated as mutexes (see above and
.Xr mutex 9 )
with shared/exclusive semantics.
More specifically, regular mutexes can be 
considered to be equivalent to a write-lock on an
.Em rw_lock.
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.
This ability should not be used lightly and 
.Em may go away.
.Pp
See the
.Xr rwlock 9
page for more information.
.Ss Read-mostly locks
Mostly reader locks are similar to
.Em reader/writer
locks but optimized for very infrequent write locking.
.Em Read-mostly
locks implement full priority propagation by tracking shared owners
using a lock user supplied
.Em tracker
data structure.
.Pp
See the
.Xr rmlock 9
page for more information.
.Ss 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
.Xr sleep 9 .
In fact it could in some cases be 
considered a conditional sleep.
.Pp
See the
.Xr sx 9
page for more information.
.Ss 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.
.Pp
See the
.Xr sema 9
page for more information.
.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.
.Pp
See the
.Xr condvar 9
page for more information.
.Ss Giant
Giant is an instance of a mutex, with some special characteristics:
.Bl -enum
.It
It is recursive.
.It
Drivers can request that Giant be locked around them, but this is
going away.
.It
You can sleep while it has recursed, but other recursive locks cannot.
.It
Giant must be locked first before other locks.
.It
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.
.El
.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 .
.Pp
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.
.Pp
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.
.Pp
See the
.Xr sleep 9
page for more information.
.Pp
.Ss Lockmanager locks
Shared/exclusive locks, used mostly in
.Xr VFS 9 ,
in particular as a
.Xr 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.
.Pp
See the
.Xr lock 9
page for more information.
.Sh INTERACTIONS
.Ss Bounded vs. unbounded sleep
The following primitives perform bounded sleep: mutexes, pool mutexes,
reader/writer locks and read-mostly locks.
.Pp
The following primitives block (perform unbounded sleep): shared/exclusive locks,
counting semaphores, condition variables, sleep/wakeup and lockmanager locks.
.Pp
It is an error to do any operation that could result in any kind of sleep while
holding spin mutex.
.Pp
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.
.Pp
As a special case, it is possible to call
.Fn sleep 9
or
.Fn mtx_sleep 9
while holding a mutex.
It 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.
.Ss 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)
.Bl -column ".Ic xxxxxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXX" -offset indent
.It Xo
.Em "You have: You want:" Ta spin mtx Ta mutex Ta sx Ta rwlock Ta rmlock Ta sleep
.Xc
.It Ic spin mtx  Ta \&ok-1 Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no-3
.It Ic mutex     Ta \&ok Ta \&ok-1 Ta \&no Ta \&ok Ta \&ok Ta \&no-3
.It Ic sxlock    Ta \&ok Ta \&ok Ta \&ok-2 Ta \&ok Ta \&ok Ta \&ok-4
.It Ic rwlock    Ta \&ok Ta \&ok Ta \&no Ta \&ok-2 Ta \&ok Ta \&no-3
.It Ic rmlock    Ta \&ok Ta \&ok Ta \&no Ta \&ok Ta \&ok-2 Ta \&no
.El
.Pp
.Em *1
Recursion is defined per lock.
Lock order is important.
.Pp
.Em *2
readers can recurse though writers can not.
Lock order is important.
.Pp
.Em *3
There are calls atomically release this primitive when going to sleep
and reacquire it on wakeup (e.g.
.Fn mtx_sleep ,
.Fn rw_sleep
and
.Fn msleep_spin
).
.Pp
.Em *4
Though one can sleep holding an sx lock, one can also use
.Fn sx_sleep
which atomically release this primitive when going to sleep and
reacquire it on wakeup.
.Ss 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.
.Bl -column ".Ic Xxxxxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXX" -offset indent
.It Xo
.Em "Context:" Ta spin mtx Ta mutex Ta sx Ta rwlock Ta rmlock Ta sleep
.Xc
.It interrupt:  Ta \&ok Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no 
.It idle:  Ta \&ok Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no 
.El
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr lock 9 ,
.Xr mtx_pool 9 ,
.Xr mutex 9 ,
.Xr rmlock 9 ,
.Xr rwlock 9 ,
.Xr sema 9 ,
.Xr sleep 9 ,
.Xr sx 9 ,
.Xr witness 9 ,
.Xr LOCK_PROFILING 9
.Sh HISTORY
These
functions appeared in
.Bsx 4.1
through
.Fx 7.0
.Sh BUGS
There are too many locking primitives to choose from.