[PATCH 2/6] x86/smpboot: Use current_task to get idle thread

Brian Gerst
2023-02-22T22:13:51Z

The idle_threads array is not populated during early boot. Use
current_task instead, which is initialized to init_task for the boot
CPU.

Also simplify start_cpu0(). Since the boot CPU never really goes
offline, the GSBASE is still set up and can be used for per-cpu
accesses.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
---
arch/x86/kernel/asm-offsets.c | 1 +
arch/x86/kernel/head_64.S | 7 ++-----
kernel/smpboot.c | 2 +-
3 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
index 8650f29387e0..445bce086717 100644
--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -114,6 +114,7 @@ static void __used common(void)
OFFSET(TSS_sp1, tss_struct, x86_tss.sp1);
OFFSET(TSS_sp2, tss_struct, x86_tss.sp2);
OFFSET(X86_top_of_stack, pcpu_hot, top_of_stack);
+ OFFSET(X86_current_task, pcpu_hot, current_task);
#ifdef CONFIG_CALL_DEPTH_TRACKING
OFFSET(X86_call_depth, pcpu_hot, call_depth);
#endif
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index c1253aa737ca..c32e5b06a9ce 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -315,7 +315,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
movq %rcx, early_gdt_descr_base(%rip)

/* Find the idle task stack */
- movq idle_threads(%rbx), %rcx
+ movq pcpu_hot + X86_current_task(%rbx), %rcx
movq TASK_threadsp(%rcx), %rcx
movq %rcx, initial_stack(%rip)
#endif /* CONFIG_SMP */
@@ -460,12 +460,9 @@ SYM_CODE_END(secondary_startup_64)
SYM_CODE_START(start_cpu0)
ANNOTATE_NOENDBR
UNWIND_HINT_EMPTY
- /* Load the per-cpu base for CPU#0 */
- leaq __per_cpu_offset(%rip), %rbx
- movq (%rbx), %rbx

/* Find the idle task stack */
- movq idle_threads(%rbx), %rcx
+ movq PER_CPU_VAR(pcpu_hot) + X86_current_task, %rcx
movq TASK_threadsp(%rcx), %rsp

jmp .Ljump_to_C_code
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index a18a21dff9bc..2c7396da470c 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -25,7 +25,7 @@
* For the hotplug case we keep the task structs around and reuse
* them.
*/
-DEFINE_PER_CPU(struct task_struct *, idle_threads);
+static DEFINE_PER_CPU(struct task_struct *, idle_threads);

struct task_struct *idle_thread_get(unsigned int cpu)
{
--
2.39.2