Auto merge of rust-lang#127442 - saethlin:alloc-decoding-lock, r=oli-obk

Try to fix ICE from re-interning an AllocId with different allocation contents

As far as I can tell, based on my investigation in rust-lang#126741, the racy decoding scheme implemented here was never fully correct, but the arrangement of Allocations that's required to ICE the compiler requires some very specific MIR optimizations to create. As far as I can tell, GVN likes to create the problematic pattern, which is why we're noticing this problem now.

So the solution here is to not do racy decoding. If two threads race to decoding an AllocId, one of them is going to sit on a lock until the other is done.

Author: bors

compiler/rustc_middle/src/mir/interpret/mod.rs

@@ -12,15 +12,13 @@ use std::fmt;
 use std::io;
 use std::io::{Read, Write};
 use std::num::NonZero;
-use std::sync::atomic::{AtomicU32, Ordering};
 
-use smallvec::{smallvec, SmallVec};
 use tracing::{debug, trace};
 
 use rustc_ast::LitKind;
 use rustc_attr::InlineAttr;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_data_structures::sync::{HashMapExt, Lock};
+use rustc_data_structures::sync::Lock;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
@@ -159,14 +157,9 @@ pub fn specialized_encode_alloc_id<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>>(
     }
 }
 
-// Used to avoid infinite recursion when decoding cyclic allocations.
-type DecodingSessionId = NonZero<u32>;
-
 #[derive(Clone)]
 enum State {
     Empty,
-    InProgressNonAlloc(SmallVec<[DecodingSessionId; 1]>),
-    InProgress(SmallVec<[DecodingSessionId; 1]>, AllocId),
     Done(AllocId),
 }
 
@@ -180,13 +173,7 @@ pub struct AllocDecodingState {
 impl AllocDecodingState {
     #[inline]
     pub fn new_decoding_session(&self) -> AllocDecodingSession<'_> {
-        static DECODER_SESSION_ID: AtomicU32 = AtomicU32::new(0);
-        let counter = DECODER_SESSION_ID.fetch_add(1, Ordering::SeqCst);
-
-        // Make sure this is never zero.
-        let session_id = DecodingSessionId::new((counter & 0x7FFFFFFF) + 1).unwrap();
-
-        AllocDecodingSession { state: self, session_id }
+        AllocDecodingSession { state: self }
     }
 
     pub fn new(data_offsets: Vec<u64>) -> Self {
@@ -200,7 +187,6 @@ impl AllocDecodingState {
 #[derive(Copy, Clone)]
 pub struct AllocDecodingSession<'s> {
     state: &'s AllocDecodingState,
-    session_id: DecodingSessionId,
 }
 
 impl<'s> AllocDecodingSession<'s> {
@@ -220,106 +206,62 @@ 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 alloc_id = {
-            let mut entry = self.state.decoding_state[idx].lock();
-
-            match *entry {
-                State::Done(alloc_id) => {
-                    return alloc_id;
-                }
-                ref mut entry @ State::Empty => {
-                    // We are allowed to decode.
-                    match alloc_kind {
-                        AllocDiscriminant::Alloc => {
-                            // If this is an allocation, we need to reserve an
-                            // `AllocId` so we can decode cyclic graphs.
-                            let alloc_id = decoder.interner().reserve_alloc_id();
-                            *entry = State::InProgress(smallvec![self.session_id], alloc_id);
-                            Some(alloc_id)
-                        }
-                        AllocDiscriminant::Fn
-                        | AllocDiscriminant::Static
-                        | AllocDiscriminant::VTable => {
-                            // Fns and statics cannot be cyclic, and their `AllocId`
-                            // is determined later by interning.
-                            *entry = State::InProgressNonAlloc(smallvec![self.session_id]);
-                            None
-                        }
-                    }
-                }
-                State::InProgressNonAlloc(ref mut sessions) => {
-                    if sessions.contains(&self.session_id) {
-                        bug!("this should be unreachable");
-                    } else {
-                        // Start decoding concurrently.
-                        sessions.push(self.session_id);
-                        None
-                    }
-                }
-                State::InProgress(ref mut sessions, alloc_id) => {
-                    if sessions.contains(&self.session_id) {
-                        // Don't recurse.
-                        return alloc_id;
-                    } else {
-                        // Start decoding concurrently.
-                        sessions.push(self.session_id);
-                        Some(alloc_id)
-                    }
-                }
-            }
-        };
+        if let State::Done(alloc_id) = *entry {
+            return alloc_id;
+        }
 
         // Now decode the actual data.
         let alloc_id = decoder.with_position(pos, |decoder| {
             match alloc_kind {
                 AllocDiscriminant::Alloc => {
+                    trace!("creating memory alloc ID");
                     let alloc = <ConstAllocation<'tcx> as Decodable<_>>::decode(decoder);
-                    // We already have a reserved `AllocId`.
-                    let alloc_id = alloc_id.unwrap();
-                    trace!("decoded alloc {:?}: {:#?}", alloc_id, alloc);
-                    decoder.interner().set_alloc_id_same_memory(alloc_id, alloc);
-                    alloc_id
+                    trace!("decoded alloc {:?}", alloc);
+                    decoder.interner().reserve_and_set_memory_alloc(alloc)
                 }
                 AllocDiscriminant::Fn => {
-                    assert!(alloc_id.is_none());
                     trace!("creating fn alloc ID");
                     let instance = ty::Instance::decode(decoder);
                     trace!("decoded fn alloc instance: {:?}", instance);
                     let unique = bool::decode(decoder);
                     // Here we cannot call `reserve_and_set_fn_alloc` as that would use a query, which
                     // is not possible in this context. That's why the allocation stores
                     // whether it is unique or not.
-                    let alloc_id =
-                        decoder.interner().reserve_and_set_fn_alloc_internal(instance, unique);
-                    alloc_id
+                    decoder.interner().reserve_and_set_fn_alloc_internal(instance, unique)
                 }
                 AllocDiscriminant::VTable => {
-                    assert!(alloc_id.is_none());
                     trace!("creating vtable alloc ID");
                     let ty = <Ty<'_> as Decodable<D>>::decode(decoder);
                     let poly_trait_ref =
                         <Option<ty::PolyExistentialTraitRef<'_>> as Decodable<D>>::decode(decoder);
                     trace!("decoded vtable alloc instance: {ty:?}, {poly_trait_ref:?}");
-                    let alloc_id =
-                        decoder.interner().reserve_and_set_vtable_alloc(ty, poly_trait_ref);
-                    alloc_id
+                    decoder.interner().reserve_and_set_vtable_alloc(ty, poly_trait_ref)
                 }
                 AllocDiscriminant::Static => {
-                    assert!(alloc_id.is_none());
                     trace!("creating extern static alloc ID");
                     let did = <DefId as Decodable<D>>::decode(decoder);
                     trace!("decoded static def-ID: {:?}", did);
-                    let alloc_id = decoder.interner().reserve_and_set_static_alloc(did);
-                    alloc_id
+                    decoder.interner().reserve_and_set_static_alloc(did)
                 }
             }
         });
 
-        self.state.decoding_state[idx].with_lock(|entry| {
-            *entry = State::Done(alloc_id);
-        });
+        *entry = State::Done(alloc_id);
 
         alloc_id
     }
@@ -563,12 +505,6 @@ impl<'tcx> TyCtxt<'tcx> {
             bug!("tried to set allocation ID {id:?}, but it was already existing as {old:#?}");
         }
     }
-
-    /// Freezes an `AllocId` created with `reserve` by pointing it at an `Allocation`. May be called
-    /// twice for the same `(AllocId, Allocation)` pair.
-    fn set_alloc_id_same_memory(self, id: AllocId, mem: ConstAllocation<'tcx>) {
-        self.alloc_map.lock().alloc_map.insert_same(id, GlobalAlloc::Memory(mem));
-    }
 }
 
 ////////////////////////////////////////////////////////////////////////////////