On Tue, 6 Mar 2007, Roland McGrath wrote:
> > > Yeah, I guess that's right. It should still return NOTIFY_STOP when
> > > args->err has no other bits set, so notifiers aren't called with zero.
> >
> > In practice that might not work. On my machine, at least, reads of DR6
> > return ones in all the reserved bit positions.
>
> Does that mean asm("mov %1,%%dr6; mov %%dr6,%0" : "=r" (mask) : "r" (0));
> puts in mask the set of reserved bits? We could collect that value at CPU
> startup and mask it off args->err, then OR it back into vdr6.
That sounds like a rather fragile approach to avoiding a minimal amount of
work. Debug exceptions don't occur very often, and when they do it won't
matter too much if we go through some extra notifier-chain callouts.
Back to a previous topic:
> > The actual guarantee I need is that nobody will switch_to() the task while
> > my routines are running.
>
> You can't get that. It can always be woken for SIGKILL (which is a good
> thing). What you are guaranteed is that if it does, it will never return
> to user mode. So it has to be ok for switching in to use the bits in any
> intermediate state you might get them, meaning any possible garbage state
> is harmful only to user mode or is otherwise recoverable (worst case
> perhaps the exception handler has to know to ignore some traps). This is
> already true with ptrace and ->thread.debugreg, as well as the normal user
> registers. In your case, if you wanted to be paranoid you could clear
> TIF_DEBUG before you touch anything, and set it again only after you're
> done (with memory barriers as needed).
It turns out that this won't work correctly unless I use something
stronger, like a spinlock or RCU. Either one seems like overkill.
Is there any way to find out from within the
switch_to_thread_hw_breakpoint routine whether the task is in this unusual
state? (By which I mean the task is being debugged and the debugger
hasn't told it to start running.) Would (tsk->exit_code == SIGKILL) work?
If not, can we add a TIF_DEBUG_STOPPED flag? Or should I just go with a
spinlock?
Is SIGKILL the only way this can happen?
In a similar vein, I need a reliable way to know whether a task has gone
through exit_thread(). If it has, then its hw_breakpoint area has been
deallocated and a new one must not be allocated. Will (tsk->flags &
PF_EXITING) always be true once that happens?
Alan Stern
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