From: Stefan Metzmacher metze@samba.org
[ Upstream commit 50b7b6f29de3e18e9d6c09641256a0296361cfee ]
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: 4727dc20e042 ("arch: setup PF_IO_WORKER threads like PF_KTHREAD") Link: https://lore.kernel.org/io-uring/044d0bad-6888-a211-e1d3-159a4aeed52d@polymt... Signed-off-by: Stefan Metzmacher metze@samba.org cc: Linus Torvalds torvalds@linux-foundation.org cc: Jens Axboe axboe@kernel.dk cc: Andy Lutomirski luto@kernel.org cc: linux-kernel@vger.kernel.org cc: io-uring@vger.kernel.org cc: x86@kernel.org Link: https://lore.kernel.org/r/20210505110310.237537-1-metze@samba.org Reviewed-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kernel/process.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-)
--- a/arch/x86/kernel/process.c +++ b/arch/x86/kernel/process.c @@ -162,7 +162,7 @@ int copy_thread(unsigned long clone_flag #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; @@ -178,6 +178,23 @@ int copy_thread(unsigned long clone_flag 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);