Author: Armin Rigo <[email protected]>
Branch: extradoc
Changeset: r4686:0eba9dda7165
Date: 2012-08-17 16:13 +0200
http://bitbucket.org/pypy/extradoc/changeset/0eba9dda7165/
Log: Document Validate().
diff --git a/talk/stm2012/stmimpl.rst b/talk/stm2012/stmimpl.rst
--- a/talk/stm2012/stmimpl.rst
+++ b/talk/stm2012/stmimpl.rst
@@ -154,7 +154,7 @@
- All barriers ensure that ``global_to_local`` satisfies the following
property for any local object ``L``: either ``L`` was created by
- this transaction (``L->h_revision == NULL``) or else satisfy
+ this transaction (``L->h_revision == NULL``) or else satisfies
``global_to_local[L->h_revision] == L``.
@@ -183,8 +183,8 @@
while (v := R->h_revision) & 1: # "has a more recent revision"
R = v & ~ 1
if v > start_time: # object too recent?
- ValidateFast() # try to move start_time forward
- return LatestGlobalRevision(G) # restart searching from G
+ Validate(global_cur_time) # try to move start_time forward
+ return LatestGlobalRevision(R) # restart searching from R
PossiblyUpdateChain(G, R, ...) # see below
return R
@@ -359,6 +359,31 @@
value are all interchangeable as far as correctness goes.
+Validation
+------------------------------------
+
+``Validate(cur_time)`` is called during a transaction to update
+``start_time``, as well as during committing. It makes sure that none
+of the read objects have been modified between ``start_time`` and the
+new current time, ``cur_time``::
+
+ def Validate(cur_time):
+ for R in list_of_read_objects:
+ if R->h_revision & 1:
+ AbortTransaction()
+ start_time = cur_time
+
+Note that if such an object is modified by another commit, then this
+transaction will eventually fail --- the next time ``Validate`` is
+called, which may be during our own attempt to commit. But
+``LatestGlobalRevision`` also calls ``Validate`` whenever it sees an
+object more recent than ``start_time``. It is never possible that new
+object revisions may be added by other CPUs with a time lower than or
+equal to ``start_time``. So this guarantees consistency: the program
+will never see during the same transaction two different versions of the
+same object.
+
+
Committing
------------------------------------
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