locking/lglocks: Add documentation of current lglocks implementation

Local/global locks are currently not documented anywhere other
than in an somewhat out-of-date LWN article - this is an attempt
to document the current state of lglocks.

This patch is against linux-next 3.18.0-rc6
Signed-off-by: NNicholas Mc Guire <der.herr@hofr.at>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Carsten Emde <c.emde@osadl.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20141208083326.GA29895@opentech.atSigned-off-by: NIngo Molnar <mingo@kernel.org>

Documentation/locking/lglock.txt

+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