On 04/20/2018 10:46 AM, Rajendra SY wrote:
> <patch attached>
> 
> Problem:
> GDB  "attach pid" command shows error message "Couldn't get registers:
> Device busy" because of which below gdb tests fail.
> 
> Cause:
> On FreeBSD ptrace() syscall requires LWP id to fetch info from the
> inferior process.
> The current implementation is every command is executed completely and
> then it wait's for asynchronous events.
> 
> When attach command is executed it calls inf_ptrace_attach() function
> which is currently doing just the ptrace(PT_ATTACH), it does not wait
> for the process to stop to fetch LWP info.
> The fbsd_wait() function is the one which fetches LWP info which is
> being called after the command completes.
> 
> Command execution code flow -
> gdb_do_one_event() => .. => handle_file_event() => .. =>
> execute_command() => check_frame_language_change()
> 
> The async event handling code flow where LWP info gets updated -
> gdb_one_event() => check_async_event_handlers() => ..
> ->do_target_wait() => .. => fbsd_wait()
> 
> The check, if the frame language changed via
> check_frame_language_change() function is trying to read registers
> using ptrace(pid) which fails because LWP id is not fetched.
> 
> The purposed patch tries to avoid this situation by avoiding calling
> check_frame_language_change() function just for the "attach" command.
> 
Thanks for the patch, but we need to find a better fix.

It seems to me that the problem is that we fail to mark
the thread as executing between the initial attach, and the
subsequent target_wait.  Can you give the attached patch a try?
check_frame_language_change() then becomes a nop until
the thread is marked not-executing again, after target_wait
is called.

We don't see the problem on Linux because there, the target_attach
implementation waits for the thread to stop before returning.
Still, that's supposedly hidden from the core, since the Linux
target, like most targets, is a '!to_attach_no_wait' target.

Pedro Alves