On Thu, 15 Feb 2007, Davide Libenzi wrote:
> 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?
Yeah. Given TS_USEDFPU set, before move_user_context():
CPU => FPUc
NTSK => FPUn
OTSK => FPUo
After move_user_context():
CPU => FPUc
NTSK => FPUo
OTSK => FPUn
After the incoming __unlazy_fpu() in __switch_to():
CPU => FPUc
NTSK => FPUo
OTSK => FPUc
After the first fault in NTSK:
CPU => FPUo
NTSK => FPUo
OTSK => FPUc
So NTSK loads a non up2date FPUo, instead of the FPUc that was the "dirty"
context to migrate (since TS_USEDFPU was set).
I think you need an early __unlazy_fpu() in that case.
- Davide
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