linux-kernel - Re: [RFC PATCH 1/7] x86/resctrl: Add register/unregister functions for driver to hook into resctrl

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Date:   Fri, 5 May 2023 16:17:48 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        James Morse <james.morse@....com>,
        "Babu Moger" <babu.moger@....com>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/7] x86/resctrl: Add register/unregister functions
 for driver to hook into resctrl

Hi Tony,

On 4/20/2023 3:06 PM, Tony Luck wrote:
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 6ad33f355861..3e6778bde427 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -51,6 +51,9 @@ static struct kernfs_node *kn_mongrp;
>  /* Kernel fs node for "mon_data" directory under root */
>  static struct kernfs_node *kn_mondata;
>  
> +static LIST_HEAD(drivers);
> +static bool resctrl_is_mounted;
> +

fyi ...
https://lore.kernel.org/lkml/20230320172620.18254-13-james.morse@arm.com/

> +int resctrl_register_driver(struct resctrl_driver *d)
> +{
> +	mutex_lock(&rdtgroup_mutex);
> +	list_add(&d->list, &drivers);
> +
> +	if (resctrl_is_mounted)
> +		driver_up(d);
> +	mutex_unlock(&rdtgroup_mutex);
> +
> +	return 0;
> +}

Do you expect that the drivers may at any time
need to call into resctrl as opposed to relying entirely
on callbacks from resctrl? I am wondering about any potential
lock ordering issues between rdtgroup_mutex and locks
used internally by the drivers.

Reinette