Re: [PATCH v5 3/3] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state

[Julien Grall]

Hi Catalin,

On 6/3/19 10:21 PM, Catalin Marinas wrote:
> On Mon, Jun 03, 2019 at 05:25:34PM +0100, Catalin Marinas wrote:
> > On Tue, May 21, 2019 at 06:21:39PM +0100, Julien Grall wrote:
> > > Since a softirq is supposed to check may_use_simd() anyway before
> > > attempting to use FPSIMD/SVE, there is limited reason to keep softirq
> > > disabled when touching the FPSIMD/SVE context. Instead, we can simply
> > > disable preemption and mark the FPSIMD/SVE context as in use by setting
> > > CPU's fpsimd_context_busy flag.
> > [...]
> > > +static void get_cpu_fpsimd_context(void)
> > > +{
> > > +	preempt_disable();
> > > +	__get_cpu_fpsimd_context();
> > > +}
> >
> > Is there anything that prevents a softirq being invoked between
> > preempt_disable() and __get_cpu_fpsimd_context()?
>
> Actually, it shouldn't matter as the softirq finishes using the fpsimd
> before the thread is resumed.

If the softirqs is handled in a thread (i.e ksoftirqd), then 
preempt_disable() will prevent them to run.

For softirq running on return from interrupt context, they will finish 
before using fpsimd before the thread is resumed.

Softirq running after __get_cpu_fpsimd_context() is called will not be 
able to use FPSIMD (may_use_simd() returns false).

Cheers,

--
Julien Grall