Re: [PATCH RT v3] arm64: fpsimd: use preemp_disable in addition to local_bh_disable()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2018-07-27 16:35:59 [+0100], Dave Martin wrote:
> On Thu, Jul 26, 2018 at 05:06:34PM +0200, Sebastian Andrzej Siewior wrote:
> > In v4.16-RT I noticed a number of warnings from task_fpsimd_load(). The
> > code disables BH and expects that it is not preemptible. On -RT the
> > task remains preemptible but remains the same CPU. This may corrupt the
> > content of the SIMD registers if the task is preempted during
> > saving/restoring those registers.
> > 
> > Add preempt_disable()/enable() to enfore the required semantic on -RT.
> 
> Does this supersede the local_lock based approach?

Yes, it does.

> That would have been nice to have, but if there are open questions about
> how to do it then I guess something like this patch makes sense as a
> stopgap solution.
> 
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> > This should work. Compiling currently gcc-6 on the box to see what
> > happens. Since the crypto disables preemption "frequently" and I don't
> > expect or see anything to worry about.
> > 
> >  arch/arm64/kernel/fpsimd.c |   30 ++++++++++++++++++++++++++++--
> >  1 file changed, 28 insertions(+), 2 deletions(-)
> > 
> > --- a/arch/arm64/kernel/fpsimd.c
> > +++ b/arch/arm64/kernel/fpsimd.c
> > @@ -157,6 +157,15 @@ static void sve_free(struct task_struct
> >  	__sve_free(task);
> >  }
> >  
> > +static void *sve_free_atomic(struct task_struct *task)
> > +{
> > +	void *sve_state = task->thread.sve_state;
> > +
> > +	WARN_ON(test_tsk_thread_flag(task, TIF_SVE));
> > +
> > +	task->thread.sve_state = NULL;
> > +	return sve_state;
> > +}
> 
> This feels a bit excessive.  Since there's only one call site, I'd
> prefer if the necessary code were simply inlined.  We wouldn't need the
> WARN either in that case, since (IIUC) it's only there to check for
> accidental misuse of this helper.
okay.

> >  /* Offset of FFR in the SVE register dump */
> >  static size_t sve_ffr_offset(int vl)
…
> I think we should have local helpers for the preempt+local_bh
> maintenance, since they're needed all over the place in this
> file.
okay.

> Cheers
> ---Dave

Sebastian