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NAME

     locking - kernel synchronization primitives

SYNOPSIS

     All sorts of stuff to go here.

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.

     These include:

     1.   Spin Mutexes

     2.   Sleep Mutexes

     3.   pool Mutexes

     4.   Shared-Exclusive locks

     5.   Reader-Writer locks

     6.   Turnstiles

     7.   Semaphores

     8.   Condition variables

     9.   Sleep/wakeup

     10.  Giant

     11.  Lockmanager locks

     The primitives interact and have a number of rules regarding how they can
     and can not be combined.  There are too many for the average human mind
     and they keep changing.  (if you disagree, please write replacement text)
     :-)

     Some of these primitives may be used at the low (interrupt) level and
     some may not.

     There are strict ordering requirements and for some of the types this is
     checked using the witness(4) code.

   SPIN Mutexes
     Mutexes are the basic primitive.  You either hold it or you don’t.  If
     you don’t own it then you just spin, waiting for the holder (on another
     CPU) to release it.  Hopefully they are doing something fast.  You must
     not do anything that deschedules the thread while you are holding a SPIN
     mutex.

   Mutexes
     Basically (regular) mutexes will deschedule the thread if the mutex can
     not be acquired.  A non-spin mutex can be considered to be equivalent to
     getting a write lock on an rw_lock (see below), and in fact non-spin
     mutexes and rw_locks may soon become the same thing.  As in spin mutexes,
     you either get it or you don’t.  You may only call the sleep(9) call via
     msleep() or the new mtx_sleep() variant.  These 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.

   Pool Mutexes
     A variant of regular mutexes where the allocation of the mutex is handled
     more by the system.

   Rw_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.

     Although reader/writer locks look very similar to sx(9) (see below)
     locks, their usage pattern is different.  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. In the future this may in fact
     become literally the fact.  An rw_lock can be locked while holding a
     regular mutex, but can not be held while sleeping.  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. Users
     of recursion in any locks should be prepared to defend their decision
     against vigorous criticism.

   Sx_locks
     Shared/exclusive locks are used to protect data that are read far more
     often than they are written.  Mutexes are inherently more efficient than
     shared/exclusive locks, so shared/exclusive locks should be used
     prudently.  The main reason for using an sx_lock is that a thread may
     hold a shared or exclusive lock on an sx_lock lock while sleeping.  As a
     consequence of this however, an sx_lock lock may not be acquired while
     holding a mutex.  The reason for this is that, if one thread slept while
     holding an sx_lock lock while another thread blocked on the same sx_lock
     lock after acquiring a mutex, then the second thread would effectively
     end up sleeping while holding a mutex, which is not allowed.  The sx_lock
     should be considered to be closely related to sleep(9).  In fact it could
     in some cases be considered a conditional sleep.

   Turnstiles
     Turnstiles are used to hold a queue of threads blocked on non-sleepable
     locks.  Sleepable locks use condition variables to implement their
     queues.  Turnstiles differ from a sleep queue in that turnstile queue’s
     are assigned to a lock held by an owning thread.  Thus, when one thread
     is enqueued onto a turnstile, it can lend its priority to the owning
     thread.  If this sounds confusing, we need to describe it better.

   Semaphores
   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.

   Giant
     Giant is a special instance of a sleep lock.  It has several special
     characteristics.

     1.   It is recursive.

     2.   Drivers can request that Giant be locked around them, but this is
          going away.

     3.   You can sleep while it has recursed, but other recursive locks
          cannot.

     4.   Giant must be locked first before other locks.

     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, 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 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.

   lockmanager locks
     Largely deprecated.  See the lock(9) page for more information.  I don’t
     know what the downsides are but I’m sure someone will fill in this part.

Usage tables.

   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)

           You have:  You want: Spin_mtx  Slp_mtx sx_lock rw_lock sleep
           SPIN mutex           ok-1      no      no      no      no-3
           Sleep mutex          ok        ok-1    no      ok      no-3
           sx_lock              ok        ok      ok-2    ok      ok-4
           rw_lock              ok        ok      no      ok-2    no-3

     *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 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 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  Slp_mtx sx_lock rw_lock sleep
           interrupt:           ok        no      no      no      no
           idle:                ok        no      no      no      no

SEE ALSO

     condvar(9), lock(9), mtx_pool(9), mutex(9), rwlock(9), sema(9), sleep(9),
     sx(9), LOCK_PROFILING(9), WITNESS(9)

HISTORY

     These functions appeared in BSD/OS 4.1 through FreeBSD 7.0