Author: Armin Rigo <[email protected]>
Branch: extradoc
Changeset: r4709:0be10b1d49fe
Date: 2012-08-19 12:41 +0200
http://bitbucket.org/pypy/extradoc/changeset/0be10b1d49fe/
Log: Another deadlock to work around here.
diff --git a/talk/stm2012/stmimpl.rst b/talk/stm2012/stmimpl.rst
--- a/talk/stm2012/stmimpl.rst
+++ b/talk/stm2012/stmimpl.rst
@@ -391,7 +391,15 @@
AbortTransaction() # "has a more recent revision"
if v >= LOCKED: # locked
if v != my_lock: # and not by me
- spin loop retry # jump back to the "v = ..." line
+ spin loop retry OR # jump back to the "v = ..." line
+ AbortTransaction() # ...or just abort
+
+The choice of waiting or aborting when encountering a read of a locked
+object needs to be done carefully to avoid deadlocks. Always aborting
+would be correct, but a bit too restrictive. Always entering a spin
+loop could lead to deadlocks with two transactions that each locked
+objects from the other's ``list_of_read_objects``. So for the purposes
+of this explanation we will always assume that it aborts.
Committing
@@ -508,12 +516,13 @@
``smp_wmb`` is a "write memory barrier": it means "make sure the
previous writes are sent to the main memory before the succeeding
writes". On x86 it is just a "compiler fence", preventing the compiler
-from doing optimizations that would move ``R->h_revision`` earlier. On
-non-x86 CPUs, it is actually a real CPU instruction, needed because the
-CPU doesn't normally send to main memory the writes in the original
-program order. (In that situation, it could be more efficiently done by
-splitting the loop in two: first update all local objects, then only do
-one ``smp_wmb``, and then update all the ``R->h_revision`` fields.)
+from doing optimizations that would move the assignment to
+``R->h_revision`` earlier. On non-x86 CPUs, it is actually a real CPU
+instruction, needed because the CPU doesn't normally send to main memory
+the writes in the original program order. (In that situation, it could
+be more efficiently done by splitting the loop in two: first update all
+local objects, then only do one ``smp_wmb``, and then update all the
+``R->h_revision`` fields.)
Note that the Linux documentation pushes forward the need to pair
``smp_wmb`` with either ``smp_read_barrier_depends`` or ``smp_rmb``. In
_______________________________________________
pypy-commit mailing list
[email protected]
http://mail.python.org/mailman/listinfo/pypy-commit
