Repository : ssh://darcs.haskell.org//srv/darcs/ghc On branch : master
http://hackage.haskell.org/trac/ghc/changeset/a006ecdfd381fa75ab16ddb66c3a2b247f359eb8 >--------------------------------------------------------------- commit a006ecdfd381fa75ab16ddb66c3a2b247f359eb8 Author: Simon Marlow <marlo...@gmail.com> Date: Tue Dec 18 09:10:26 2012 +0000 A better fix for #7493 (see comment for details) >--------------------------------------------------------------- rts/STM.c | 60 ++++++++++++++++++++++++++++++++++++++++++------------------ 1 files changed, 42 insertions(+), 18 deletions(-) diff --git a/rts/STM.c b/rts/STM.c index 0a4d0b2..62ced25 100644 --- a/rts/STM.c +++ b/rts/STM.c @@ -705,32 +705,56 @@ static void merge_update_into(Capability *cap, /*......................................................................*/ static void merge_read_into(Capability *cap, - StgTRecHeader *t, + StgTRecHeader *trec, StgTVar *tvar, - StgClosure *expected_value) { + StgClosure *expected_value) +{ int found; + StgTRecHeader *t; - // Look for an entry in this trec found = FALSE; - FOR_EACH_ENTRY(t, e, { - StgTVar *s; - s = e -> tvar; - if (s == tvar) { - found = TRUE; - if (e -> expected_value != expected_value) { - // Must abort if the two entries start from different values - TRACE("%p : read entries inconsistent at %p (%p vs %p)", - t, tvar, e -> expected_value, expected_value); - t -> state = TREC_CONDEMNED; + + // + // See #7493 + // + // We need to look for an existing entry *anywhere* in the stack of + // nested transactions. Otherwise, in stmCommitNestedTransaction() + // we can't tell the difference between + // + // (1) a read-only entry + // (2) an entry that writes back the original value + // + // Since in both cases e->new_value == e->expected_value. But in (1) + // we want to do nothing, and in (2) we want to update e->new_value + // in the outer transaction. + // + // Here we deal with the first possibility: we never create a + // read-only entry in an inner transaction if there is an existing + // outer entry; so we never have an inner read and an outer update. + // So then in stmCommitNestedTransaction() we know we can always + // write e->new_value over the outer entry, because the inner entry + // is the most up to date. + // + for (t = trec; !found && t != NO_TREC; t = t -> enclosing_trec) + { + FOR_EACH_ENTRY(t, e, { + if (e -> tvar == tvar) { + found = TRUE; + if (e -> expected_value != expected_value) { + // Must abort if the two entries start from different values + TRACE("%p : read entries inconsistent at %p (%p vs %p)", + t, tvar, e -> expected_value, expected_value); + t -> state = TREC_CONDEMNED; + } + BREAK_FOR_EACH; } - BREAK_FOR_EACH; - } - }); + }); + } if (!found) { - // No entry so far in this trec + // No entry found TRecEntry *ne; - ne = get_new_entry(cap, t); + ne = get_new_entry(cap, trec); ne -> tvar = tvar; ne -> expected_value = expected_value; ne -> new_value = expected_value; _______________________________________________ Cvs-ghc mailing list Cvs-ghc@haskell.org http://www.haskell.org/mailman/listinfo/cvs-ghc
