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     locking — kernel synchronization primitives


     The 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.

     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.

     See mutex(9) for details.

   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
     bus_setup_intr(9) for details), or for scheduler internals.

   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.

     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.  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.

     See rwlock(9) for details.

   Read-Mostly Locks
     Read-mostly 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.

   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.

   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

     See sx(9) for details.

   Lockmanager locks
     Lockmanager locks are sleepable shared/exclusive locks used mostly in VFS(9) (as a vnode(9)
     lock) and in the buffer cache (BUF_LOCK(9)).  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.

     See lock(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 locks to wait for a condition to become
     true.  A thread must hold the associated lock before calling one of the 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.

     See condvar(9) for details.

     The functions tsleep(), msleep(), msleep_spin(), pause(), wakeup(), and 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 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 event the
     thread was blocking on has occurred.

     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 (restoring any recursion) before the function

     The 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 wakeup() or a signal.

     See sleep(9) for details.

     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 spl(9) interface, Giant has special

     1.   It is recursive.

     2.   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.

     3.   Giant must be locked before other non-sleepable locks.

     4.   Giant is dropped during unbounded sleeps and reacquired 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 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.


     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 witness(4).

   Bounded vs. Unbounded Sleep
     In a bounded sleep (also referred to as “blocking”) 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 (often referred to as simply “sleeping”) 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.

     The following primitives perform bounded sleeps: mutexes, reader/writer locks and read-
     mostly locks.

     The following primitives perform unbounded sleeps: sleepable read-mostly locks,
     shared/exclusive locks, lockmanager locks, counting semaphores, condition variables, and

   General Principles
     ·   It is an error to do any operation that could result in yielding the processor while
         holding a spin mutex.

     ·   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.

         Note that the lock passed to one of the sleep() or cv_wait() functions is dropped before
         the thread enters the unbounded sleep and does not violate this rule.

     ·   It is an error to do any operation that could result in yielding of the processor 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 locking
     primitives discussed.  Note that “sleep” includes sema_wait(), sema_timedwait(), any of the
     cv_wait() functions, and any of the sleep() functions.

               You want: spin mtx  mutex/rw  rmlock  sleep rm  sx/lk  sleep
        You have:        --------  --------  ------  --------  ------ ------
        spin mtx         ok        no        no      no        no     no-1
        mutex/rw         ok        ok        ok      no        no     no-1
        rmlock           ok        ok        ok      no        no     no-1
        sleep rm         ok        ok        ok      ok-2      ok-2   ok-2/3
        sx               ok        ok        ok      ok        ok     ok-3
        lockmgr          ok        ok        ok      ok        ok     ok

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

     *2 These cases are only allowed while holding a write lock on a sleepable read-mostly lock.

     *3 Though one can sleep while holding this lock, one can also use a sleep() function to
     atomically release this primitive when going to sleep and reacquire it on wakeup.

     Note that non-blocking try operations on locks are always permitted.

   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/rw  rmlock  sleep rm  sx/lk  sleep
        interrupt filter:   ok        no        no      no        no     no
        interrupt thread:   ok        ok        ok      no        no     no
        callout:            ok        ok        ok      no        no     no
        system call:        ok        ok        ok      ok        ok     ok


     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)


     These functions appeared in BSD/OS 4.1 through FreeBSD 7.0.


     There are too many locking primitives to choose from.