Explain why the new setup can't deadlock

This commit is contained in:
Ben Kimock 2024-07-19 17:43:33 -04:00
parent 3623c76acb
commit 107cf981d5

View File

@ -206,9 +206,21 @@ impl<'s> AllocDecodingSession<'s> {
(alloc_kind, decoder.position())
});
// We are going to hold this lock during the entire decoding of this allocation, which may
// require that we decode other allocations. This cannot deadlock for two reasons:
//
// At the time of writing, it is only possible to create an allocation that contains a pointer
// to itself using the const_allocate intrinsic (which is for testing only), and even attempting
// to evaluate such consts blows the stack. If we ever grow a mechanism for producing
// cyclic allocations, we will need a new strategy for decoding that doesn't bring back
// https://github.com/rust-lang/rust/issues/126741.
//
// It is also impossible to create two allocations (call them A and B) where A is a pointer to B, and B
// is a pointer to A, because attempting to evaluate either of those consts will produce a
// query cycle, failing compilation.
let mut entry = self.state.decoding_state[idx].lock();
// Check the decoding state to see if it's already decoded or if we should
// decode it here.
let mut entry = self.state.decoding_state[idx].lock();
if let State::Done(alloc_id) = *entry {
return alloc_id;
}