linux-kernel - Re: [RFC PATCH 1/1] fs/namespace.c: use spinlock instead of busy loop

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Date:   Wed, 17 Jun 2020 13:26:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] fs/namespace.c: use spinlock instead of busy loop

On Wed, Jun 17, 2020 at 12:46:58PM +0206, John Ogness wrote:
> @@ -459,17 +469,39 @@ void mnt_drop_write_file(struct file *file)
>  }
>  EXPORT_SYMBOL(mnt_drop_write_file);
>  
> +static void mnt_lock_writers(struct mount *mnt)
> +{
> +#ifdef CONFIG_SMP
> +	int cpu;
> +
> +	for_each_possible_cpu(cpu) {
> +		spin_lock(&per_cpu_ptr(mnt->mnt_pcp,
> +				       cpu)->mnt_writers_lock);
> +	}
> +#else
> +	spin_lock(&mnt->mnt_writers_lock);
> +#endif
> +}
> +
> +static void mnt_unlock_writers(struct mount *mnt)
> +{
> +#ifdef CONFIG_SMP
> +	int cpu;
> +
> +	for_each_possible_cpu(cpu) {
> +		spin_unlock(&per_cpu_ptr(mnt->mnt_pcp,
> +					 cpu)->mnt_writers_lock);
> +	}
> +#else
> +	spin_unlock(&mnt->mnt_writers_lock);
> +#endif
> +}

*groan*.. this is brlock reincarnate :-/ Also broken.