Commit f184d9ca authored by Simon Marlow's avatar Simon Marlow

Fix a bug in the handling of nested orElse

Exposed by the following snippet, courtesy of Bas van Dijk and Patrick
Palka on libraries@haskell.org:

import Control.Concurrent.STM
main = do
  x <- atomically $ do
         t <- newTVar 1
         writeTVar t 2
         ((readTVar t >> retry) `orElse` return ()) `orElse` return ()
         readTVar t
  print x
parent 332e6812
......@@ -1460,10 +1460,28 @@ StgBool stmCommitNestedTransaction(Capability *cap, StgTRecHeader *trec) {
StgTVar *s;
s = e -> tvar;
if (entry_is_update(e)) {
// Careful! We might have a read entry here that we don't want
// to spam over the update entry in the enclosing TRec. e.g. in
//
// t <- newTVar 1
// writeTVar t 2
// ((readTVar t >> retry) `orElse` return ()) `orElse` return ()
//
// - the innermost txn first aborts, giving us a read-only entry
// with e->expected_value == e->new_value == 1
// - the inner orElse commits into the outer orElse, which
// lands us here. If we unconditionally did
// merge_update_into(), then we would overwrite the outer
// TRec's update, so we must check whether the entry is an
// update or not, and if not, just do merge_read_into.
//
if (entry_is_update(e)) {
unlock_tvar(cap, trec, s, e -> expected_value, FALSE);
}
merge_update_into(cap, et, s, e -> expected_value, e -> new_value);
merge_update_into(cap, et, s, e -> expected_value, e -> new_value);
} else {
merge_read_into(cap, et, s, e -> expected_value);
}
ACQ_ASSERT(s -> current_value != (StgClosure *)trec);
});
} else {
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment