Messages in this thread | | | From | Roland McGrath <> | Subject | Re: copy_process() && ti->flags (Was: PT_DTRACE && uml) | Date | Sun, 26 Apr 2009 19:10:56 -0700 (PDT) |
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> dup_task_struct()->setup_thread_stack() copies parent's ti->flags. > > Why? Which flags should be actually copied? I must have missed > something, but whats wrong with the patch below?
I suspect it just evolved that way as the default case of how copy_process() is written: copy whole structs, and then fix them up.
Almost all the details of struct thread_info are arch-specific, so whether copy-and-fix or start-from-zero is better really has to be decided by each arch maintainer.
Your next two questions are not about UML, but about arch/x86 code. Those should be directed to the x86 maintainers, whom you did not CC.
> OK, it is wrong. On x86 we should at least copy TIF_IA32. But > why should we copy, say, TIF_DEBUG?
TIF_DEBUG is set when task->thread.debugregN fields have interesting values. The semantics today are that those values are copied, so copying TIF_DEBUG too makes the child's context switches do what they should.
This illustrates the general point: since the overall policy defaults to copying, then what actually keeps the code simpler overall is to have the same default at each substructure (and so it is for thread_struct and thread_info). If some things should be cleared, they are cleared explicitly. Hence, to clear TIF_DEBUG one would have to do it on purpose, and would be required to think about what TIF_DEBUG means and that clearing thread->debugregN goes along with it. (And at this point, one would then realize that one can't do it without changing the user-visible semantics.)
> Actually, I don't understand why don't we use TS_IA32 instead of > TIF_IA32. Only current can change this flag, perhaps it makes sense > to move it in thread_info->status.
However, it is tested by other threads asynchronously. The stated use of TS_* flags is things only tested by current or when current is stopped (e.g. TS_COMPAT). In other uses like TASK_SIZE_OF(), get_gate_vma(), etc., that might not be so. It might be so on x86 that there can never be any modification to ti->status that could momentarily clear some bit, but that is not formally said to be required.
> copy_process()->clear_tsk_thread_flag(TIF_SIGPENDING) looks unneeded > in any case...
It goes along with init_sigpending(). But it's actually potentially wrong in case of shared_pending signals. Do recalc_sigpending_tsk() if anything. As I think you intend to point out, it is always pretty harmless to leave TIF_SIGPENDING set. (get_signal_to_deliver will just figure it out.) So I think that should just be removed, independent of your other questions.
Thanks, Roland
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