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

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Message-ID: <20190603212104.mhz7vvj7afb2p3yr@mbp>
Date:   Mon, 3 Jun 2019 22:21:05 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Julien Grall <julien.grall@....com>
Cc:     ard.biesheuvel@...aro.org, julien.thierry@....com,
        marc.zyngier@....com, Dave.Martin@....com, suzuki.poulose@....com,
        will.deacon@....com, christoffer.dall@....com,
        linux-kernel@...r.kernel.org, james.morse@....com,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 3/3] arm64/fpsimd: Don't disable softirq when touching
 FPSIMD/SVE state

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.

-- 
Catalin