From 88590e1cd28e5a9e500c78329dcc0ad3836c8bad Mon Sep 17 00:00:00 2001
From: Diogo Netto <61364108+d-netto@users.noreply.github.com>
Date: Mon, 25 Mar 2024 10:23:54 -0300
Subject: [PATCH] create phantom task for GC threads (#53815)

A common idiom used throughout the codebase is to get a pointer to
thread-local-state through `jl_current_task->ptls`.

Create a phantom task for GC threads so that we can make use of this
idiom when running in the GC threads as well.

Idea originally suggested by @vchuravy, bugs are mine.

(cherry picked from commit 9636ef72eebe0155710471fb79eb5c75cc26949b)
---
 src/partr.c | 12 ++++++++++--
 src/task.c  |  1 +
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/partr.c b/src/partr.c
index 0f3b581f5122f..cb82dc2c3d5fe 100644
--- a/src/partr.c
+++ b/src/partr.c
@@ -121,7 +121,11 @@ void jl_gc_mark_threadfun(void *arg)
 
     // initialize this thread (set tid and create heap)
     jl_ptls_t ptls = jl_init_threadtls(targ->tid);
-
+    void *stack_lo, *stack_hi;
+    jl_init_stack_limits(0, &stack_lo, &stack_hi);
+    // warning: this changes `jl_current_task`, so be careful not to call that from this function
+    jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
+    JL_GC_PROMISE_ROOTED(ct);
     // wait for all threads
     jl_gc_state_set(ptls, JL_GC_STATE_WAITING, 0);
     uv_barrier_wait(targ->barrier);
@@ -146,7 +150,11 @@ void jl_gc_sweep_threadfun(void *arg)
 
     // initialize this thread (set tid and create heap)
     jl_ptls_t ptls = jl_init_threadtls(targ->tid);
-
+    void *stack_lo, *stack_hi;
+    jl_init_stack_limits(0, &stack_lo, &stack_hi);
+    // warning: this changes `jl_current_task`, so be careful not to call that from this function
+    jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
+    JL_GC_PROMISE_ROOTED(ct);
     // wait for all threads
     jl_gc_state_set(ptls, JL_GC_STATE_WAITING, 0);
     uv_barrier_wait(targ->barrier);
diff --git a/src/task.c b/src/task.c
index 1dab8688cb079..86033a81ddf41 100644
--- a/src/task.c
+++ b/src/task.c
@@ -1685,6 +1685,7 @@ jl_task_t *jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi)
     JL_GC_PROMISE_ROOTED(ct);
     jl_set_pgcstack(&ct->gcstack);
     assert(jl_current_task == ct);
+    assert(jl_current_task->ptls == ptls);
 
 #ifdef _COMPILER_TSAN_ENABLED_
     ct->ctx.tsan_state = __tsan_get_current_fiber();