locking/lglocks: Add documentation of current lglocks implementation

lglock - local/global locks for mostly local access patterns

------------------------------------------------------------

Origin: Nick Piggin's VFS scalability series introduced during

	2.6.35++ [1] [2]

Location: kernel/locking/lglock.c

	include/linux/lglock.h

Users: currently only the VFS and stop_machine related code

Design Goal:

------------

Improve scalability of globally used large data sets that are

distributed over all CPUs as per_cpu elements.

To manage global data structures that are partitioned over all CPUs

as per_cpu elements but can be mostly handled by CPU local actions

lglock will be used where the majority of accesses are cpu local

reading and occasional cpu local writing with very infrequent

global write access.

* deal with things locally whenever possible

	- very fast access to the local per_cpu data

	- reasonably fast access to specific per_cpu data on a different

	  CPU

* while making global action possible when needed

	- by expensive access to all CPUs locks - effectively

	  resulting in a globally visible critical section.

Design:

-------

Basically it is an array of per_cpu spinlocks with the

lg_local_lock/unlock accessing the local CPUs lock object and the

lg_local_lock_cpu/unlock_cpu accessing a remote CPUs lock object

the lg_local_lock has to disable preemption as migration protection so

that the reference to the local CPUs lock does not go out of scope.

Due to the lg_local_lock/unlock only touching cpu-local resources it

is fast. Taking the local lock on a different CPU will be more

expensive but still relatively cheap.

One can relax the migration constraints by acquiring the current

CPUs lock with lg_local_lock_cpu, remember the cpu, and release that

lock at the end of the critical section even if migrated. This should

give most of the performance benefits without inhibiting migration

though needs careful considerations for nesting of lglocks and

consideration of deadlocks with lg_global_lock.

The lg_global_lock/unlock locks all underlying spinlocks of all

possible CPUs (including those off-line). The preemption disable/enable

are needed in the non-RT kernels to prevent deadlocks like:

                     on cpu 1

              task A          task B

         lg_global_lock

           got cpu 0 lock

                 <<<< preempt <<<<

                         lg_local_lock_cpu for cpu 0

                           spin on cpu 0 lock

On -RT this deadlock scenario is resolved by the arch_spin_locks in the

lglocks being replaced by rt_mutexes which resolve the above deadlock

by boosting the lock-holder.

Implementation:

---------------

The initial lglock implementation from Nick Piggin used some complex

macros to generate the lglock/brlock in lglock.h - they were later

turned into a set of functions by Andi Kleen [7]. The change to functions

was motivated by the presence of multiple lock users and also by them

being easier to maintain than the generating macros. This change to

functions is also the basis to eliminated the restriction of not

being initializeable in kernel modules (the remaining problem is that

locks are not explicitly initialized - see lockdep-design.txt)

Declaration and initialization:

-------------------------------

  #include <linux/lglock.h>

  DEFINE_LGLOCK(name)

  or:

  DEFINE_STATIC_LGLOCK(name);

  lg_lock_init(&name, "lockdep_name_string");

  on UP this is mapped to DEFINE_SPINLOCK(name) in both cases, note

  also that as of 3.18-rc6 all declaration in use are of the _STATIC_

  variant (and it seems that the non-static was never in use).

  lg_lock_init is initializing the lockdep map only.

Usage:

------

From the locking semantics it is a spinlock. It could be called a

locality aware spinlock. lg_local_* behaves like a per_cpu

spinlock and lg_global_* like a global spinlock.

No surprises in the API.

  lg_local_lock(*lglock);

     access to protected per_cpu object on this CPU

  lg_local_unlock(*lglock);

  lg_local_lock_cpu(*lglock, cpu);

     access to protected per_cpu object on other CPU cpu

  lg_local_unlock_cpu(*lglock, cpu);

  lg_global_lock(*lglock);

     access all protected per_cpu objects on all CPUs

  lg_global_unlock(*lglock);

  There are no _trylock variants of the lglocks.

Note that the lg_global_lock/unlock has to iterate over all possible

CPUs rather than the actually present CPUs or a CPU could go off-line

with a held lock [4] and that makes it very expensive. A discussion on

these issues can be found at [5]

Constraints:

------------

  * currently the declaration of lglocks in kernel modules is not

    possible, though this should be doable with little change.

  * lglocks are not recursive.

  * suitable for code that can do most operations on the CPU local

    data and will very rarely need the global lock

  * lg_global_lock/unlock is *very* expensive and does not scale

  * on UP systems all lg_* primitives are simply spinlocks

  * in PREEMPT_RT the spinlock becomes an rt-mutex and can sleep but

    does not change the tasks state while sleeping [6].

  * in PREEMPT_RT the preempt_disable/enable in lg_local_lock/unlock

    is downgraded to a migrate_disable/enable, the other

    preempt_disable/enable are downgraded to barriers [6].

    The deadlock noted for non-RT above is resolved due to rt_mutexes

    boosting the lock-holder in this case which arch_spin_locks do

    not do.

lglocks were designed for very specific problems in the VFS and probably

only are the right answer in these corner cases. Any new user that looks

at lglocks probably wants to look at the seqlock and RCU alternatives as

her first choice. There are also efforts to resolve the RCU issues that

currently prevent using RCU in place of view remaining lglocks.

Note on brlock history:

-----------------------

The 'Big Reader' read-write spinlocks were originally introduced by

Ingo Molnar in 2000 (2.4/2.5 kernel series) and removed in 2003. They

later were introduced by the VFS scalability patch set in 2.6 series

again as the "big reader lock" brlock [2] variant of lglock which has

been replaced by seqlock primitives or by RCU based primitives in the

3.13 kernel series as was suggested in [3] in 2003. The brlock was

entirely removed in the 3.13 kernel series.

Link: 1 http://lkml.org/lkml/2010/8/2/81

Link: 2 http://lwn.net/Articles/401738/

Link: 3 http://lkml.org/lkml/2003/3/9/205

Link: 4 https://lkml.org/lkml/2011/8/24/185

Link: 5 http://lkml.org/lkml/2011/12/18/189

Link: 6 https://www.kernel.org/pub/linux/kernel/projects/rt/

        patch series - lglocks-rt.patch.patch

Link: 7 http://lkml.org/lkml/2012/3/5/26