Re: [PATCH v7 24/24] x86/resctrl: Separate arch and fs resctrl locks

[Reinette Chatre]

Hi James,

On 10/25/2023 11:03 AM, James Morse wrote:
> resctrl has one mutex that is taken by the architecture specific code,
> and the filesystem parts. The two interact via cpuhp, where the
> architecture code updates the domain list. Filesystem handlers that
> walk the domains list should not run concurrently with the cpuhp
> callback modifying the list.
> 
> Exposing a lock from the filesystem code means the interface is not
> cleanly defined, and creates the possibility of cross-architecture
> lock ordering headaches. The interaction only exists so that certain
> filesystem paths are serialised against CPU hotplug. The CPU hotplug
> code already has a mechanism to do this using cpus_read_lock().
> 
> MPAM's monitors have an overflow interrupt, so it needs to be possible
> to walk the domains list in irq context. RCU is ideal for this,
> but some paths need to be able to sleep to allocate memory.
> 
> Because resctrl_{on,off}line_cpu() take the rdtgroup_mutex as part
> of a cpuhp callback, cpus_read_lock() must always be taken first.
> rdtgroup_schemata_write() already does this.
> 
> Most of the filesystem code's domain list walkers are currently
> protected by the rdtgroup_mutex taken in rdtgroup_kn_lock_live().
> The exceptions are rdt_bit_usage_show() and the mon_config helpers
> which take the lock directly.
> 
> Make the domain list protected by RCU. An architecture-specific
> lock prevents concurrent writers. rdt_bit_usage_show() could
> walk the domain list using RCU, but to keep all the filesystem
> operations the same, this is changed to call cpus_read_lock().
> The mon_config helpers send multiple IPIs, take the cpus_read_lock()
> in these cases.
> 
> The other filesystem list walkers need to be able to sleep.
> Add cpus_read_lock() to rdtgroup_kn_lock_live() so that the
> cpuhp callbacks can't be invoked when file system operations are
> occurring.
> 
> Add lockdep_assert_cpus_held() in the cases where the
> rdtgroup_kn_lock_live() call isn't obvious.
> 
> Resctrl's domain online/offline calls now need to take the
> rdtgroup_mutex themselves.
> 
> Tested-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxx>
> Tested-by: Peter Newman <peternewman@xxxxxxxxxx>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxx>
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> ---

Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>

Reinette