Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads

Jens Axboe
2021-05-04T02:50:23Z

On 5/3/21 5:48 PM, Linus Torvalds wrote:
> On Mon, May 3, 2021 at 4:27 PM Stefan Metzmacher <metze@samba.org> wrote:
>>
>> If I remember correctly gdb showed bogus addresses for the backtraces of the io_threads,
>> as some regs where not cleared.
>
> Yeah, so that patch will make the IO thread have the user stack
> pointer point to the original user stack, but that stack will
> obviously be used by the original thread which means that it will
> contain random stuff on it.
>
> Doing a
>
> childregs->sp = 0;
>
> is probably a good idea for that PF_IO_WORKER case, since it really
> doesn't have - or need - a user stack.
>
> Of course, it doesn't really have - or need - any of the other user
> registers either, but once you fill in the segment stuff to make gdb
> happy, you might as well fill it all in using the same code that the
> regular case does.

I tested the below, which is the two combined, with a case that
deliberately has two types of io threads - one for SQPOLL submission,
and one that was created due to async work being needed. gdb attaches
just fine to the creator, with a slight complaint:

Attaching to process 370
[New LWP 371]
[New LWP 372]
Error while reading shared library symbols for /usr/lib/libpthread.so.0:
Cannot find user-level thread for LWP 372: generic error
0x00007f1a74675125 in clock_nanosleep@GLIBC_2.2.5 () from /usr/lib/libc.so.6
(gdb) info threads
Id Target Id Frame
* 1 LWP 370 "io_uring" 0x00007f1a74675125 in clock_nanosleep@GLIBC_2.2.5 ()
from /usr/lib/libc.so.6
2 LWP 371 "iou-sqp-370" 0x00007f1a746a7a9d in syscall () from /usr/lib/libc.so.6
3 LWP 372 "io_uring" 0x00007f1a74675125 in clock_nanosleep@GLIBC_2.2.5 ()
from /usr/lib/libc.so.6

(gdb) thread 2
[Switching to thread 2 (LWP 371)]
#0 0x00007f1a746a7a9d in syscall () from /usr/lib/libc.so.6
(gdb) bt
#0 0x00007f1a746a7a9d in syscall () from /usr/lib/libc.so.6
Backtrace stopped: Cannot access memory at address 0x0

(gdb) thread 1
[Switching to thread 1 (LWP 370)]
#0 0x00007f1a74675125 in clock_nanosleep@GLIBC_2.2.5 () from /usr/lib/libc.so.6
(gdb) bt
#0 0x00007f1a74675125 in clock_nanosleep@GLIBC_2.2.5 () from /usr/lib/libc.so.6
#1 0x00007f1a7467a357 in nanosleep () from /usr/lib/libc.so.6
#2 0x00007f1a7467a28e in sleep () from /usr/lib/libc.so.6
#3 0x000055bd41e929ba in main (argc=<optimized out>, argv=<optimized out>)
at t/io_uring.c:658

which looks very reasonable to me - no backtraces for the io threads, and
no arch complaints.

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 43cbfc84153a..58987bce90e2 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 +168,12 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
if (sp)
childregs->sp = sp;

+ if (unlikely(p->flags & PF_IO_WORKER)) {
+ childregs->sp = 0;
+ kthread_frame_init(frame, sp, arg);
+ return 0;
+ }
+
#ifdef CONFIG_X86_32
task_user_gs(p) = get_user_gs(current_pt_regs());
#endif

--
Jens Axboe