linux-kernel - Re: [PATCH] arm64/cpufeature: Convert hook_lock to raw_spin_lock_t in cpu_enable

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Message-ID: <20190530120129.GD13751@fuggles.cambridge.arm.com>
Date:   Thu, 30 May 2019 13:01:29 +0100
From:   Will Deacon <will.deacon@....com>
To:     Julien Grall <julien.grall@....com>
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, tglx@...utronix.de,
        rostedt@...dmis.org, bigeasy@...utronix.de, suzuki.poulose@....com,
        catalin.marinas@....com, dave.martin@....com
Subject: Re: [PATCH] arm64/cpufeature: Convert hook_lock to raw_spin_lock_t
 in cpu_enable_ssbs()

On Thu, May 30, 2019 at 12:30:58PM +0100, Julien Grall wrote:
> cpu_enable_ssbs() is called via stop_machine() as part of the cpu_enable
> callback. A spin lock is used to ensure the hook is registered before
> the rest of the callback is executed.
> 
> On -RT spin_lock() may sleep. However, all the callees in stop_machine()
> are expected to not sleep. Therefore a raw_spin_lock() is required here.
> 
> Given this is already done under stop_machine() and the work done under
> the lock is quite small, the latency should not increase too much.
> 
> Signed-off-by: Julien Grall <julien.grall@....com>
> 
> ---
> 
> It was noticed when looking at the current use of spin_lock in
> arch/arm64. I don't have a platform calling that callback, so I have
> hacked the code to reproduce the error and check it is now fixed.
> ---
>  arch/arm64/kernel/cpufeature.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index ca27e08e3d8a..2a7159fda3ce 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1194,14 +1194,14 @@ static struct undef_hook ssbs_emulation_hook = {
>  static void cpu_enable_ssbs(const struct arm64_cpu_capabilities *__unused)
>  {
>  	static bool undef_hook_registered = false;
> -	static DEFINE_SPINLOCK(hook_lock);
> +	static DEFINE_RAW_SPINLOCK(hook_lock);
>  
> -	spin_lock(&hook_lock);
> +	raw_spin_lock(&hook_lock);
>  	if (!undef_hook_registered) {
>  		register_undef_hook(&ssbs_emulation_hook);
>  		undef_hook_registered = true;
>  	}
> -	spin_unlock(&hook_lock);
> +	raw_spin_unlock(&hook_lock);

Makes sense to me. We could probably avoid the lock entirely if we wanted
to (via atomic_dec_if_positive), but I'm not sure it's really worth it.

Will