+++ /dev/null
-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
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