On Thu, 15 Feb 2007, Ingo Molnar wrote:
> /*
> + * Move user-space context from one kernel thread to another.
> + * This includes registers and FPU state. Callers must make
> + * sure that neither task is running user context at the moment:
> + */
> +void
> +move_user_context(struct task_struct *new_task, struct task_struct *old_task)
> +{
> + struct pt_regs *old_regs = task_pt_regs(old_task);
> + struct pt_regs *new_regs = task_pt_regs(new_task);
> + union i387_union *tmp;
> +
> + *new_regs = *old_regs;
> + /*
> + * Flip around the FPU state too:
> + */
> + tmp = new_task->thread.i387;
> + new_task->thread.i387 = old_task->thread.i387;
> + old_task->thread.i387 = tmp;
> +}
Let's say that old_task ("prev" at the incoming schedule) has TS_USEDFPU
set. Its context gets moved to the new_task (the one returning to
userspace) *before* the __unlazy_fpu() done in __switch_to(). The
__unlazy_fpu() at the following schedule will save the state to the old
new_task context, and that fine as far as the going-to-sleep task goes.
The next fault happening in new_task (return to userspace one) will reload
a non up2date context (the one we got from old_task, but never hit by
the __unlazy_fpu() flush). Right?
- Davide
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