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.Dd February 3, 2023
.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 provides
several different synchronization primitives to allow developers
to safely access and manipulate many data types.
.Ss Mutexes
Mutexes (also called "blocking mutexes") are the most commonly used
synchronization primitive in the kernel.
A 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 wait.
Mutexes are adaptive by default, meaning that
if the owner of a contended mutex is currently running on another CPU,
then a thread attempting to acquire the mutex will spin rather than yielding
the processor.
Mutexes fully support priority propagation.
.Pp
See
.Xr mutex 9
for details.
.Ss Spin Mutexes
Spin mutexes are a variation of basic mutexes; the main difference between
the two is that spin mutexes never block.
Instead, they spin while waiting for the lock to be released.
To avoid deadlock, a thread that holds a spin mutex must never yield its CPU.
Unlike ordinary mutexes, spin mutexes disable interrupts when acquired.
Since disabling interrupts can be expensive, they are generally slower to
acquire and release.
Spin mutexes should be used only when absolutely necessary,
e.g. to protect data shared
with interrupt filter code (see
.Xr bus_setup_intr 9
for details),
or for scheduler internals.
.Ss Mutex Pools
With most synchronization primitives, such as mutexes, the programmer must
provide memory to hold the primitive.
For example, a mutex may be embedded inside the structure it protects.
Mutex pools provide a preallocated set of mutexes to avoid this
requirement.
Note that mutexes from a pool may only be used as leaf locks.
.Pp
See
.Xr mtx_pool 9
for details.
.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.
Reader/writer locks support priority propagation like mutexes,
but priority is propagated only to an exclusive holder.
This limitation comes from the fact that shared owners
are anonymous.
.Pp
See
.Xr rwlock 9
for details.
.Ss Read-Mostly Locks
Read-mostly 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 caller-supplied
.Em tracker
data structure.
.Pp
See
.Xr rmlock 9
for details.
.Ss Sleepable Read-Mostly Locks
Sleepable read-mostly locks are a variation on read-mostly locks.
Threads holding an exclusive lock may sleep,
but threads holding a shared lock may not.
Priority is propagated to shared owners but not to exclusive owners.
.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.
Acquiring a contested shared/exclusive lock can perform an unbounded sleep.
These locks do not support priority propagation.
.Pp
See
.Xr sx 9
for details.
.Ss Lockmanager locks
Lockmanager locks are sleepable shared/exclusive locks used mostly in
.Xr VFS 9
.Po
as a
.Xr vnode 9
lock
.Pc
and in the buffer cache
.Po
.Xr BUF_LOCK 9
.Pc .
They have features other lock types do not have such as sleep
timeouts, blocking upgrades,
writer starvation avoidance, draining, and an interlock mutex,
but this makes them complicated both to use and to implement;
for this reason, they should be avoided.
.Pp
See
.Xr lock 9
for details.
.Ss Non-blocking synchronization
The kernel has two facilities,
.Xr epoch 9
and
.Xr smr 9 ,
which can be used to provide read-only access to a data structure while one or
more writers are concurrently modifying the data structure.
Specifically, readers using
.Xr epoch 9
and
.Xr smr 9
to synchronize accesses do not block writers, in contrast with reader/writer
locks, and they help ensure that memory freed by writers is not reused until
all readers which may be accessing it have finished.
Thus, they are a useful building block in the construction of lock-free
data structures.
.Pp
These facilities are difficult to use correctly and should be avoided
in preference to traditional mutual exclusion-based synchronization,
except when performance or non-blocking guarantees are a major concern.
.Pp
See
.Xr epoch 9
and
.Xr smr 9
for details.
.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
.Xr sema 9
for details.
.Ss Condition variables
Condition variables are used in conjunction with locks to wait for
a condition to become true.
A thread must hold the associated lock before calling one of the
.Fn cv_wait ,
functions.
When a thread waits on a condition, the lock
is atomically released before the thread yields the processor
and reacquired before the function call returns.
Condition variables may be used with blocking mutexes,
reader/writer locks, read-mostly locks, and shared/exclusive locks.
.Pp
See
.Xr condvar 9
for details.
.Ss Sleep/Wakeup
The functions
.Fn tsleep ,
.Fn msleep ,
.Fn msleep_spin ,
.Fn pause ,
.Fn wakeup ,
and
.Fn wakeup_one
also handle event-based thread blocking.
Unlike condition variables,
arbitrary addresses may be used as wait channels and a dedicated
structure does not need to be allocated.
However, care must be taken to ensure that wait channel addresses are
unique to an event.
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
.Pq often called from inside an interrupt routine
to indicate that the
event the thread was blocking on has occurred.
.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
.Pq even if recursed
while the thread is suspended and will reacquire the
.Va Giant
mutex
.Pq restoring any recursion
before the function returns.
.Pp
The
.Fn pause
function is a special sleep function that waits for a specified
amount of time to pass before the thread resumes execution.
This sleep cannot be terminated early by either an explicit
.Fn wakeup
or a signal.
.Pp
See
.Xr sleep 9
for details.
.Ss Giant
Giant is a special mutex used to protect data structures that do not
yet have their own locks.
Since it provides semantics akin to the old
.Xr spl 9
interface,
Giant has special characteristics:
.Bl -enum
.It
It is recursive.
.It
Drivers 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.
.It
Giant must be locked before other non-sleepable locks.
.It
Giant is dropped during unbounded sleeps and reacquired after wakeup.
.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.
This means that you cannot count on Giant keeping other code from
running if your code sleeps, even if you want it to.
.El
.Sh INTERACTIONS
The primitives can interact and have a number of rules regarding how
they can and can not be combined.
Many of these rules are checked by
.Xr witness 4 .
.Ss Bounded vs. Unbounded Sleep
In a bounded sleep
.Po also referred to as
.Dq blocking
.Pc
the only resource needed to resume execution of a thread
is CPU time for the owner of a lock that the thread is waiting to acquire.
In an unbounded sleep
.Po
often referred to as simply
.Dq sleeping
.Pc
a thread waits for an external event or for a condition
to become true.
In particular,
a dependency chain of threads in bounded sleeps should always make forward
progress,
since there is always CPU time available.
This requires that no thread in a bounded sleep is waiting for a lock held
by a thread in an unbounded sleep.
To avoid priority inversions,
a thread in a bounded sleep lends its priority to the owner of the lock
that it is waiting for.
.Pp
The following primitives perform bounded sleeps:
mutexes, reader/writer locks and read-mostly locks.
.Pp
The following primitives perform unbounded sleeps:
sleepable read-mostly locks, shared/exclusive locks, lockmanager locks,
counting semaphores, condition variables, and sleep/wakeup.
.Ss General Principles
.Bl -bullet
.It
It is an error to do any operation that could result in yielding the processor
while holding a spin mutex.
.It
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 a shared/exclusive lock while
holding a mutex, or to try to allocate memory with M_WAITOK while holding a
reader/writer lock.
.Pp
Note that the lock passed to one of the
.Fn sleep
or
.Fn cv_wait
functions is dropped before the thread enters the unbounded sleep and does
not violate this rule.
.It
It is an error to do any operation that could result in yielding of
the processor when running inside an interrupt filter.
.It
It is an error to do any operation that could result in unbounded sleep when
running inside an interrupt thread.
.El
.Ss Interaction table
The following table shows what you can and can not do while holding
one of the locking primitives discussed.
Note that
.Dq sleep
includes
.Fn sema_wait ,
.Fn sema_timedwait ,
any of the
.Fn cv_wait
functions,
and any of the
.Fn sleep
functions.
.Bl -column ".Ic xxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXXXX" ".Xr XXXXXX" -offset 3n
.It Em "       You want:" Ta spin mtx Ta mutex/rw Ta rmlock Ta sleep rm Ta sx/lk Ta sleep
.It Em "You have:     " Ta -------- Ta -------- Ta ------ Ta -------- Ta ------ Ta ------
.It spin mtx  Ta \&ok Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no-1
.It mutex/rw  Ta \&ok Ta \&ok Ta \&ok Ta \&no Ta \&no Ta \&no-1
.It rmlock    Ta \&ok Ta \&ok Ta \&ok Ta \&no Ta \&no Ta \&no-1
.It sleep rm  Ta \&ok Ta \&ok Ta \&ok Ta \&ok-2 Ta \&ok-2 Ta \&ok-2/3
.It sx        Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&ok-3
.It lockmgr   Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&ok
.El
.Pp
.Em *1
There are calls that atomically release this primitive when going to sleep
and reacquire it on wakeup
.Po
.Fn mtx_sleep ,
.Fn rw_sleep ,
.Fn msleep_spin ,
etc.
.Pc .
.Pp
.Em *2
These cases are only allowed while holding a write lock on a sleepable
read-mostly lock.
.Pp
.Em *3
Though one can sleep while holding this lock,
one can also use a
.Fn sleep
function to atomically release this primitive when going to sleep and
reacquire it on wakeup.
.Pp
Note that non-blocking try operations on locks are always permitted.
.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 XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXXXX" ".Xr XXXXXX" -offset 3n
.It Em "Context:"  Ta spin mtx Ta mutex/rw Ta rmlock Ta sleep rm Ta sx/lk Ta sleep
.It interrupt filter:  Ta \&ok Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no
.It interrupt thread:  Ta \&ok Ta \&ok Ta \&ok Ta \&no Ta \&no Ta \&no
.It callout:    Ta \&ok Ta \&ok Ta \&ok Ta \&no Ta \&no Ta \&no
.It direct callout:  Ta \&ok Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no
.It system call:    Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&ok
.El
.Sh SEE ALSO
.Xr lockstat 1 ,
.Xr witness 4 ,
.Xr atomic 9 ,
.Xr BUS_SETUP_INTR 9 ,
.Xr callout 9 ,
.Xr condvar 9 ,
.Xr epoch 9 ,
.Xr lock 9 ,
.Xr LOCK_PROFILING 9 ,
.Xr mtx_pool 9 ,
.Xr mutex 9 ,
.Xr rmlock 9 ,
.Xr rwlock 9 ,
.Xr sema 9 ,
.Xr sleep 9 ,
.Xr smr 9 ,
.Xr sx 9
.Sh BUGS
There are too many locking primitives to choose from.