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STM support in OCaml
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Date: 2006-03-11 (19:42)
From: David MENTRE <dmentre@l...>
Subject: Deadlock free locking scheme (was: Re: [Caml-list] STM support in OCaml)

Michael Hicks <mwh@cs.umd.edu> writes:

> There's a longer answer, but one short answer is: check out AtomCaml  at
> http://www.cs.washington.edu/homes/miker/atomcaml/

Thank you for the interesting paper, I did not know about that work. Was
there an announcement on caml-list?

By the way, as a reply to initial Asfand Yar Quazi's question, there is
a simple way to implement deadlock free locking scheme, both for byte
and native code: call a locking routine that always try to lock the
needed locks *in the same order*. By using the high-order properties of
OCaml, this is very easy for the programmer to use it.

Such a locking scheme can be used in the following way (imagine you have
four bases, named like "participant" or "position", each one protected
with a multiple-reader/single-writer lock):

let do_atomic_work () =
  let unlock =
     lock_bases { no_locks with participant = Read_lock; 
                                position = Read_lock; } in
  let result = do_processing () in
  unlock ();

Compared to AtomCaml approach, it is like if the do_processing code is
executed in a Commit phase.

Please find below the code that imlements this scheme (using Reader and
Writer mutex[1]). It could probably be optimized for speed (using
precomputed locking and unlocking functions stored in a hash table) or
improved to support more locks (using an ordered set as input to lock
function), but the general scheme is there.

\chapter{Control of data bases access}

Module [[Basesctrl]] controls the locks to access the data bases. 

Each of the four data bases (Participants, Classification, Delegation,
Position) can be accessed for reading or for writing. A reader/writer
lock (see chapter \ref{module:rwmutex}) protects each one of them.

All locking and unlocking of those bases are done through this
module. We can thus control the locking and unlocking order and thus
guarantee the absence of deadlocks.

The locking of the bases is done through the [[lock]] function. It locks
bases as desired and then return an unlocking function that should be
called to cancel locking.

\section{Data structures}

Each lock can either be taken for reading ([[Read_lock]]), for writing
([[Write_lock]]) or not taken at all ([[No_lock]]).

type lock_type =
  | Read_lock
  | Write_lock
  | No_lock

We define a data structure [[t]] that indicates, for each action on the
database, the desired (un)locking for each base.

type t = {
    participant : lock_type;
    classification : lock_type;
    delegation : lock_type;
    position : lock_type;

We define a default lock state [[no_locks]] where no locks are taken.

let no_locks = {
  participant = No_lock;
  classification = No_lock;
  delegation = No_lock;
  position = No_lock;

We also define the actual set of locks that protect bases. We chose a
[[Writer_preference]] to give priority to data base modifications that
should be less frequent that data base reading.

let pref = Rwmutex.Writer_preference

let participant_lock = Rwmutex.create pref
let classification_lock = Rwmutex.create pref
let delegation_lock = Rwmutex.create pref
let position_lock = Rwmutex.create pref

\section{Base unlocking}

Helper function [[unlock_a_base]] is used to unlock [[lock]] with
[[desired]] unlocking.

let unlock_a_base desired lock =
  match desired with
  | Read_lock -> Rwmutex.read_unlock lock
  | Write_lock -> Rwmutex.write_unlock lock
  | No_lock -> ()

Function [[create_unlock]] returns a function that, when called,
unlocks the bases as specified in the [[what]] argument.

let create_unlock what =
  fun () -> 
    unlock_a_base what.position position_lock;
    unlock_a_base what.delegation delegation_lock;
    unlock_a_base what.classification classification_lock;
    unlock_a_base what.participant participant_lock

\section{Base locking}

Helper function [[lock_a_base]] is used to lock [[lock]] with
[[desired]] locking.

let lock_a_base desired lock =
  match desired with
  | Read_lock -> Rwmutex.read_lock lock
  | Write_lock -> Rwmutex.write_lock lock
  | No_lock -> ()

Function [[lock_bases]] is called to lock bases. The [[what]] argument
gives for each base the desired locking. This function returns a
function that should be called to unlock the bases.

To avoid deadlocks, the locking order is the reverse as used in the
unlocking procedure (see \codechunkref{code:create_unlock}).

let lock_bases what =
  lock_a_base what.participant participant_lock;
  lock_a_base what.classification classification_lock;
  lock_a_base what.delegation delegation_lock;
  lock_a_base what.position position_lock;
  create_unlock what

Best wishes,

[1]  http://www.linux-france.org/~dmentre/code/ocaml_thread_synchro.tar.gz

pub  1024D/A3AD7A2A 2004-10-03 David MENTRE <dmentre@linux-france.org>
 5996 CC46 4612 9CA4 3562  D7AC 6C67 9E96 A3AD 7A2A