From 17a70d25678a0e8f87ec49a993e3180e8d11c6e7 Mon Sep 17 00:00:00 2001
From: Takafumi Arakaki <aka.tkf@gmail.com>
Date: Thu, 11 Nov 2021 18:04:28 -0500
Subject: [PATCH] Null out lastt->ptls after setting current task (pgcstack)

This makes it difficult for `usr2_handler` to observe null ptls.
---
 src/task.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/src/task.c b/src/task.c
index cef294551a6d4..ccab5bded2c33 100644
--- a/src/task.c
+++ b/src/task.c
@@ -424,18 +424,17 @@ static void ctx_switch(jl_task_t *lastt)
         else
 #endif
         *pt = NULL; // can't fail after here: clear the gc-root for the target task now
-        lastt->ptls = NULL;
     }
 
     // set up global state for new task and clear global state for old task
     t->ptls = ptls;
     jl_atomic_store_relaxed(&ptls->current_task, t);
     JL_GC_PROMISE_ROOTED(t);
+    jl_set_pgcstack(&t->gcstack);
     lastt->ptls = NULL;
 #ifdef MIGRATE_TASKS
     ptls->previous_task = lastt;
 #endif
-    jl_set_pgcstack(&t->gcstack);
 
     if (t->started) {
 #ifdef COPY_STACKS