From: Linus Torvalds <[email protected]>
To: Thomas Gleixner <[email protected]>
Cc: Andy Lutomirski <[email protected]>, Jens Axboe <[email protected]>,
Stefan Metzmacher <[email protected]>,
Linux Kernel Mailing List <[email protected]>,
io-uring <[email protected]>,
"the arch/x86 maintainers" <[email protected]>
Subject: Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads
Date: Mon, 3 May 2021 16:16:17 -0700 [thread overview]
Message-ID: <CAHk-=wi4Dyg_Z70J_hJbtFLPQDG+Zx3dP2jB5QrOdZC6W6j4Gw@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
On Mon, May 3, 2021 at 3:56 PM Thomas Gleixner <[email protected]> wrote:
>
> It's all fine that we have lots of blurb about GDB, but there is no
> reasoning why this does not affect regular kernel threads which take the
> same code path.
Actual kernel threads don't get attached to by ptrace.
> This is a half setup user space thread which is assumed to behave like a
> regular kernel thread, but is this assumption actually true?
No, no.
It's a *fully set up USER thread*.
Those IO threads used to be kernel threads. That didn't work out for
the reasons already mentioned earlier.
These days they really are fully regular user threads, they just don't
return to user space because they continue to do the IO work that they
were created for.
Maybe instead of Stefan's patch, we could do something like this:
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 43cbfc84153a..890f3992e781 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -156,7 +156,7 @@ 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;
@@ -168,6 +168,17 @@ int copy_thread(unsigned long clone_flags,
unsigned long sp, unsigned long arg,
if (sp)
childregs->sp = sp;
+ /*
+ * An IO thread is a user space thread, but it doesn't
+ * return to ret_after_fork(), it does the same kernel
+ * frame setup to return to a kernel function that
+ * a kernel thread does.
+ */
+ if (unlikely(p->flags & PF_IO_WORKER)) {
+ kthread_frame_init(frame, sp, arg);
+ return 0;
+ }
+
#ifdef CONFIG_X86_32
task_user_gs(p) = get_user_gs(current_pt_regs());
#endif
does that clarify things and make people happier?
Maybe the compiler might even notice that the
kthread_frame_init(frame, sp, arg);
return 0;
part is common code and then it will result in less generated code too.
NOTE! The above is - as usual - COMPLETELY UNTESTED. It looks obvious
enough, and it builds cleanly. But that's all I'm going to guarantee.
It's whitespace-damaged on purpose.
Linus
next prev parent reply other threads:[~2021-05-03 23:16 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <[email protected]>
2021-05-03 16:05 ` [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads Andy Lutomirski
2021-05-03 19:14 ` Linus Torvalds
2021-05-03 20:15 ` Andy Lutomirski
2021-05-03 20:21 ` Andy Lutomirski
2021-05-03 20:37 ` Linus Torvalds
2021-05-03 21:26 ` Jens Axboe
2021-05-03 21:49 ` Andy Lutomirski
2021-05-03 22:08 ` Linus Torvalds
2021-05-03 22:56 ` Thomas Gleixner
2021-05-03 23:15 ` Andy Lutomirski
2021-05-03 23:16 ` Linus Torvalds [this message]
2021-05-03 23:19 ` Linus Torvalds
2021-05-03 23:27 ` Stefan Metzmacher
2021-05-03 23:48 ` Linus Torvalds
2021-05-04 2:50 ` Jens Axboe
2021-05-04 11:39 ` Stefan Metzmacher
2021-05-04 15:53 ` Linus Torvalds
2021-05-12 4:24 ` Olivier Langlois
2021-05-12 17:44 ` Linus Torvalds
2021-05-12 20:55 ` Jens Axboe
2021-05-20 4:13 ` Olivier Langlois
2021-05-21 7:31 ` Olivier Langlois
2021-05-25 19:39 ` Olivier Langlois
2021-05-25 19:45 ` Olivier Langlois
2021-05-25 19:52 ` Jens Axboe
2021-05-25 20:23 ` Linus Torvalds
2021-05-04 8:22 ` David Laight
2021-05-04 0:01 ` Andy Lutomirski
2021-05-04 8:39 ` Peter Zijlstra
2021-05-04 15:35 ` Borislav Petkov
2021-05-04 15:55 ` Simon Marchi
2021-05-05 11:29 ` Stefan Metzmacher
2021-05-05 21:59 ` Simon Marchi
2021-05-05 22:11 ` Andy Lutomirski
2021-05-05 23:12 ` Borislav Petkov
2021-05-05 23:22 ` Andy Lutomirski
2021-05-06 1:04 ` Simon Marchi
2021-05-06 15:11 ` Andy Lutomirski
2021-05-06 9:47 ` David Laight
2021-05-06 9:53 ` David Laight
2021-05-05 22:21 ` Stefan Metzmacher
2021-05-05 23:15 ` Simon Marchi
2021-04-11 15:27 Stefan Metzmacher
2021-04-14 21:28 ` Stefan Metzmacher
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