"Kevin D. Kissell" wrote:
>
> > > current->used_math should never be set to zero in this sort of
> > > situation. It's not an ownership flag! It marks whether the FP state
> > > in the thread structure is valid.
> >
> > Daniel, it is funny that I agree with your last statement but cannot agree
> > with your first one.
> >
> > Under the above mentioned situation, after we make the copy of FPU state
> from
> > thread structure to the saved signal context, we need to set used_math bit
> to
> > zero. This way when the signal handler uses FPU for the first time - if
> it
> > ever uses it -, the normal lazy FPU switch mechanism can kick in smoothly.
>
> We've had a lot of messages crossing here, but please
> explain yourself here why clearing used_math helps in
> this case. If, as has been proposed, the current
> thread does not own the FPU, and thus CU1 is not
> enabled, the FPU switch mechanism should kick in
> during the signal handler regardless. The signal
> handler will inherit the thread's FPU state from
> the thread context, and will muck with it, but
> if, as has been noted, the sigcontext has
> been loaded from the thread context before
> the handler is dispatched, and is restored after
> the handler executes, we're fine. The only thing
> I can see that clearing used_math would achieve
> would be to guarantee the signal handler a virgin
> FPU context.
>
Yes, that is somewhat the purpose. Essentially we want to see, at the
beginning of a signal handler execution, the process appears to have not used
FPU at all.
This requirement might be a must, because whether clearing current->used_math
bit determine which patch we will take in the do_cpu(), when signal handler
uses FPU for the first time. See the code below.
if (current->used_math) { /* Using the FPU again. */
lazy_fpu_switch(last_task_used_math);
} else { /* First time FPU user. */
init_fpu();
current->used_math = 1;
}
last_task_used_math = current;
Clearly the second path is logically the correct one.
BTW, do I see another bug here in do_cpu()? It seems that before we call
init_fpu(), we should check last_task_used_math. If it is not NULL, we should
save the FP state to the last_task_used_math. Hmm, strange ...
Jun
