[PATCH v2] io_thread/x86: setup io_threads more like normal user space threads

[Stefan Metzmacher <metze@samba.org>]

As io_threads are fully set up USER threads it's clearer to
separate the code path from the KTHREAD logic.

The only remaining difference to user space threads is that
io_threads never return to user space again.
Instead they loop within the given worker function.

The fact that they never return to user space means they
don't have an user space thread stack. In order to
indicate that to tools like gdb we reset the stack and instruction
pointers to 0.

This allows gdb attach to user space processes using io-uring,
which like means that they have io_threads, without printing worrying
message like this:

  warning: Selected architecture i386:x86-64 is not compatible with reported target architecture i386

  warning: Architecture rejected target-supplied description

The output will be something like this:

  (gdb) info threads
    Id   Target Id                  Frame
  * 1    LWP 4863 "io_uring-cp-for" syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
    2    LWP 4864 "iou-mgr-4863"    0x0000000000000000 in ?? ()
    3    LWP 4865 "iou-wrk-4863"    0x0000000000000000 in ?? ()
  (gdb) thread 3
  [Switching to thread 3 (LWP 4865)]
  #0  0x0000000000000000 in ?? ()
  (gdb) bt
  #0  0x0000000000000000 in ?? ()
  Backtrace stopped: Cannot access memory at address 0x0

Fixes: 4727dc20e04 ("arch: setup PF_IO_WORKER threads like PF_KTHREAD")
Link: https://lore.kernel.org/io-uring/044d0bad-6888-a211-e1d3-159a4aeed52d@polymtl.ca/T/#m1bbf5727e3d4e839603f6ec7ed79c7eebfba6267
Signed-off-by: Stefan Metzmacher <metze@samba.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Thomas Gleixner <tglx@linutronix.de>
cc: Andy Lutomirski <luto@kernel.org>
cc: linux-kernel@vger.kernel.org
cc: io-uring@vger.kernel.org
cc: x86@kernel.org
---
 arch/x86/kernel/process.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 9c214d7085a4..6a64ee204897 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -161,7 +161,7 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
 #endif
 
 	/* Kernel thread ? */
-	if (unlikely(p->flags & (PF_KTHREAD | PF_IO_WORKER))) {
+	if (unlikely(p->flags & PF_KTHREAD)) {
 		memset(childregs, 0, sizeof(struct pt_regs));
 		kthread_frame_init(frame, sp, arg);
 		return 0;
@@ -177,6 +177,23 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
 	task_user_gs(p) = get_user_gs(current_pt_regs());
 #endif
 
+	if (unlikely(p->flags & PF_IO_WORKER)) {
+		/*
+		 * An IO thread is a user space thread, but it doesn't
+		 * return to ret_after_fork().
+		 *
+		 * In order to indicate that to tools like gdb,
+		 * we reset the stack and instruction pointers.
+		 *
+		 * It does the same kernel frame setup to return to a kernel
+		 * function that a kernel thread does.
+		 */
+		childregs->sp = 0;
+		childregs->ip = 0;
+		kthread_frame_init(frame, sp, arg);
+		return 0;
+	}
+
 	/* Set a new TLS for the child thread? */
 	if (clone_flags & CLONE_SETTLS)
 		ret = set_new_tls(p, tls);
-- 
2.25.1