Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in flush_thread()

From: Oleg Nesterov
Date: Fri Mar 13 2015 - 10:58:10 EST

On 03/13, Borislav Petkov wrote:
> On Wed, Mar 11, 2015 at 06:35:07PM +0100, Oleg Nesterov wrote:
> > drop_init_fpu() makes no sense. We need drop_fpu() and only if
> Oh, please explain why. I can try to rhyme it up as something like "we
> don't need to restore FPU context when flushing the thread" but I'm not
> sure...

Hmm. The changelog could be more clear. I'll send v2.

But please look at drop_init_fpu(). If eagerfpu == F it calls drop_fpu() and
this is what we need. flush_thread() already has the "if (!use_eager_fpu())",
we can shift drop_fpu() there.

Otherwise, if eagerfpu == T, drop_init_fpu() does restore_init_xstate() and
this just burns CPU. Until flush_thread user_has_fpu/used_math this state
restore_init_xstate() is pointless, this state will be lost after preemption.

> > + } else if (!used_math()) {
> > /* kthread execs. TODO: cleanup this horror. */
> > if (WARN_ON(init_fpu(current)))
> > force_sig(SIGKILL, current);
> Also, can we clean up the tsk/current usage here?
> We assign current to tsk and we work with it but then later use current
> again. Needlessly confusing.

Agreed, will do.


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