Re: [RFC Patch 6/9] Use virtual debug registers in process/threadhandling code

From: Alan Stern
Date: Thu Oct 16 2008 - 10:27:18 EST

On Wed, 15 Oct 2008, Roland McGrath wrote:

> +/*
> + * Handle debug registers. This must be done _after_ current
> + * is updated.
> + */
> + if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
> + switch_to_thread_hw_breakpoint(next_p);
> It would be good if we could arrange that this works before current changes.
> That way it says in __switch_to_xtra, which is off the hot path.

There's a problem with moving the switch_to_thread_hw_breakpoint() call
before current is updated. Suppose a kernel breakpoint is triggered in
between the two. The hw-breakpoint handler will see that current is
different from the task pointer stored in the chbi area, so it will
think the task pointer is leftover from an old task (lazy switching)
and will erase it. Then until the next context switch, no
user-breakpoints will be installed.

The real problem is that it's impossible to update both current and
chbi->bp_task at the same instant, so there will always be a window in
which they disagree and a breakpoint might get triggered. Since we use
lazy switching, we are forced to assume that a disagreement means that
current is correct an chbi->bp_task is old. But if you move the code
above then you'll create a window in which current is old and
chbi->bp_task is correct.

Alan Stern

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