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

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Date:   Tue, 4 Jun 2019 11:35:11 +0100
From:   Julien Grall <julien.grall@....com>
To:     Catalin Marinas <catalin.marinas@....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

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