Auto merge of #115220 - Zoxc:revive-gcx-ptr, r=oli-obk

Add a `CurrentGcx` type to let the deadlock handler access `TyCtxt`

This brings back `GCX_PTR` (previously removed in https://github.com/rust-lang/rust/pull/74969) allowing the deadlock handler access to `GlobalCtxt`. This fixes https://github.com/rust-lang/rust/issues/111522.

r? `@cjgillot`

compiler/rustc_middle/src/ty/context.rs

@@ -32,6 +32,7 @@ use crate::ty::{
 };
 use crate::ty::{GenericArg, GenericArgs, GenericArgsRef};
 use rustc_ast::{self as ast, attr};
+use rustc_data_structures::defer;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::intern::Interned;
@@ -39,7 +40,7 @@ use rustc_data_structures::profiling::SelfProfilerRef;
 use rustc_data_structures::sharded::{IntoPointer, ShardedHashMap};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::steal::Steal;
-use rustc_data_structures::sync::{self, FreezeReadGuard, Lock, Lrc, WorkerLocal};
+use rustc_data_structures::sync::{self, FreezeReadGuard, Lock, Lrc, RwLock, WorkerLocal};
 #[cfg(parallel_compiler)]
 use rustc_data_structures::sync::{DynSend, DynSync};
 use rustc_data_structures::unord::UnordSet;
@@ -723,6 +724,8 @@ pub struct GlobalCtxt<'tcx> {
 
     /// Stores memory for globals (statics/consts).
     pub(crate) alloc_map: Lock<interpret::AllocMap<'tcx>>,
+
+    current_gcx: CurrentGcx,
 }
 
 impl<'tcx> GlobalCtxt<'tcx> {
@@ -733,6 +736,19 @@ impl<'tcx> GlobalCtxt<'tcx> {
         F: FnOnce(TyCtxt<'tcx>) -> R,
     {
         let icx = tls::ImplicitCtxt::new(self);
+
+        // Reset `current_gcx` to `None` when we exit.
+        let _on_drop = defer(move || {
+            *self.current_gcx.value.write() = None;
+        });
+
+        // Set this `GlobalCtxt` as the current one.
+        {
+            let mut guard = self.current_gcx.value.write();
+            assert!(guard.is_none(), "no `GlobalCtxt` is currently set");
+            *guard = Some(self as *const _ as *const ());
+        }
+
         tls::enter_context(&icx, || f(icx.tcx))
     }
 
@@ -741,6 +757,39 @@ impl<'tcx> GlobalCtxt<'tcx> {
     }
 }
 
+/// This is used to get a reference to a `GlobalCtxt` if one is available.
+///
+/// This is needed to allow the deadlock handler access to `GlobalCtxt` to look for query cycles.
+/// It cannot use the `TLV` global because that's only guaranteed to be defined on the thread
+/// creating the `GlobalCtxt`. Other threads have access to the `TLV` only inside Rayon jobs, but
+/// the deadlock handler is not called inside such a job.
+#[derive(Clone)]
+pub struct CurrentGcx {
+    /// This stores a pointer to a `GlobalCtxt`. This is set to `Some` inside `GlobalCtxt::enter`
+    /// and reset to `None` when that function returns or unwinds.
+    value: Lrc<RwLock<Option<*const ()>>>,
+}
+
+#[cfg(parallel_compiler)]
+unsafe impl DynSend for CurrentGcx {}
+#[cfg(parallel_compiler)]
+unsafe impl DynSync for CurrentGcx {}
+
+impl CurrentGcx {
+    pub fn new() -> Self {
+        Self { value: Lrc::new(RwLock::new(None)) }
+    }
+
+    pub fn access<R>(&self, f: impl for<'tcx> FnOnce(&'tcx GlobalCtxt<'tcx>) -> R) -> R {
+        let read_guard = self.value.read();
+        let gcx: *const GlobalCtxt<'_> = read_guard.unwrap() as *const _;
+        // SAFETY: We hold the read lock for the `GlobalCtxt` pointer. That prevents
+        // `GlobalCtxt::enter` from returning as it would first acquire the write lock.
+        // This ensures the `GlobalCtxt` is live during `f`.
+        f(unsafe { &*gcx })
+    }
+}
+
 impl<'tcx> TyCtxt<'tcx> {
     /// Expects a body and returns its codegen attributes.
     ///
@@ -859,6 +908,7 @@ impl<'tcx> TyCtxt<'tcx> {
         query_kinds: &'tcx [DepKindStruct<'tcx>],
         query_system: QuerySystem<'tcx>,
         hooks: crate::hooks::Providers,
+        current_gcx: CurrentGcx,
     ) -> GlobalCtxt<'tcx> {
         let data_layout = s.target.parse_data_layout().unwrap_or_else(|err| {
             s.dcx().emit_fatal(err);
@@ -893,6 +943,7 @@ impl<'tcx> TyCtxt<'tcx> {
             canonical_param_env_cache: Default::default(),
             data_layout,
             alloc_map: Lock::new(interpret::AllocMap::new()),
+            current_gcx,
         }
     }
 

compiler/rustc_middle/src/ty/mod.rs

@@ -87,8 +87,8 @@ pub use self::consts::{
     Const, ConstData, ConstInt, ConstKind, Expr, ScalarInt, UnevaluatedConst, ValTree,
 };
 pub use self::context::{
-    tls, CtxtInterners, DeducedParamAttrs, Feed, FreeRegionInfo, GlobalCtxt, Lift, TyCtxt,
-    TyCtxtFeed,
+    tls, CtxtInterners, CurrentGcx, DeducedParamAttrs, Feed, FreeRegionInfo, GlobalCtxt, Lift,
+    TyCtxt, TyCtxtFeed,
 };
 pub use self::instance::{Instance, InstanceDef, ShortInstance, UnusedGenericParams};
 pub use self::list::List;