Add a hook before yielding for GC to save context

src/gc-interface.h

@@ -108,6 +108,9 @@ JL_DLLEXPORT const char* jl_gc_active_impl(void);
 // each GC should implement it but it will most likely not be used by other code in the runtime.
 // It still needs to be annotated with JL_DLLEXPORT since it is called from Rust by MMTk.
 JL_DLLEXPORT void jl_gc_sweep_stack_pools_and_mtarraylist_buffers(jl_ptls_t ptls) JL_NOTSAFEPOINT;
+// Notifies the GC that the given thread is about to yield for a GC. ctx is the ucontext for the thread
+// if it is already fetched by the caller, otherwise it is NULL.
+JL_DLLEXPORT void jl_gc_notify_thread_yield(jl_ptls_t ptls, void* ctx);
 
 // ========================================================================= //
 // Metrics

src/gc-mmtk.c

@@ -294,6 +294,8 @@ JL_DLLEXPORT void jl_gc_prepare_to_collect(void)
     gc_num.total_time_to_safepoint += duration;
 
     if (!jl_atomic_load_acquire(&jl_gc_disable_counter)) {
+        // This thread will yield.
+        jl_gc_notify_thread_yield(ptls, NULL);
         JL_LOCK_NOGC(&finalizers_lock); // all the other threads are stopped, so this does not make sense, right? otherwise, failing that, this seems like plausibly a deadlock
 #ifndef __clang_gcanalyzer__
         mmtk_block_thread_for_gc();
@@ -323,6 +325,10 @@ JL_DLLEXPORT void jl_gc_prepare_to_collect(void)
     errno = last_errno;
 }
 
+JL_DLLEXPORT void jl_gc_notify_thread_yield(jl_ptls_t ptls, void* ctx) {
+    // TODO: Save the context of this thread for conservative scanning.
+}
+
 // ========================================================================= //
 // GC Statistics
 // ========================================================================= //

src/gc-stock.c

@@ -3458,6 +3458,11 @@ JL_DLLEXPORT void jl_gc_collect(jl_gc_collection_t collection)
         gc_cblist_pre_gc, (collection));
 
     if (!jl_atomic_load_acquire(&jl_gc_disable_counter)) {
+        // This thread will yield.
+        // jl_gc_notify_thread_yield does nothing for the stock GC at the point, but it may be non empty in the future,
+        // and this is a place where we should call jl_gc_notify_thread_yield.
+        // TODO: This call can be removed if requested.
+        jl_gc_notify_thread_yield(ptls, NULL);
         JL_LOCK_NOGC(&finalizers_lock); // all the other threads are stopped, so this does not make sense, right? otherwise, failing that, this seems like plausibly a deadlock
 #ifndef __clang_gcanalyzer__
         if (_jl_gc_collect(ptls, collection)) {
@@ -4080,6 +4085,10 @@ JL_DLLEXPORT const char* jl_gc_active_impl(void) {
     return "Built with stock GC";
 }
 
+JL_DLLEXPORT void jl_gc_notify_thread_yield(jl_ptls_t ptls, void* ctx) {
+    // Do nothing before a thread yields
+}
+
 #ifdef __cplusplus
 }
 #endif

src/signals-unix.c

@@ -390,6 +390,8 @@ JL_NO_ASAN static void segv_handler(int sig, siginfo_t *info, void *context)
         return;
     }
     if (sig == SIGSEGV && info->si_code == SEGV_ACCERR && jl_addr_is_safepoint((uintptr_t)info->si_addr) && !is_write_fault(context)) {
+        // TODO: We should do the same for other platforms
+        jl_gc_notify_thread_yield(ct->ptls, context);
         jl_set_gc_and_wait(ct);
         // Do not raise sigint on worker thread
         if (jl_atomic_load_relaxed(&ct->tid) != 0)