Re: [PATCH 04/18] x86/resctrl: Move rmid allocation out of mkdir_rdt_prepare()

[James Morse <james.morse@arm.com>]

Hello!

On 10/11/2022 10:50, Shaopeng Tan (Fujitsu) wrote:
>> RMID are allocated for each monitor or control group directory, because each of
>> these needs its own RMID. For control groups,
>> rdtgroup_mkdir_ctrl_mon() later goes on to allocate the CLOSID.
>>
>> MPAM's equivalent of RMID are not an independent number, so can't be
>> allocated until the closid is known. An RMID allocation for one CLOSID may fail,
>> whereas another may succeed depending on how many monitor groups a
>> control group has.
>>
>> The RMID allocation needs to move to be after the CLOSID has been allocated.
>>
>> Move the RMID allocation out of mkdir_rdt_prepare() to occur in its caller, after
>> the mkdir_rdt_prepare() call. This allows the RMID allocator to know the
>> CLOSID.

>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> index 841294ad6263..c67083a8a5f5 100644
>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c

>> @@ -3003,12 +3004,19 @@ static int rdtgroup_mkdir_mon(struct kernfs_node
>> *parent_kn,
>>  	prgrp = rdtgrp->mon.parent;
>>  	rdtgrp->closid = prgrp->closid;
>>
>> +	ret = mkdir_rdt_prepare_rmid_alloc(rdtgrp);
>> +	if (ret) {
>> +		mkdir_rdt_prepare_clean(rdtgrp);
>> +		goto out_unlock;
>> +	}
>> +
>>  	/*
>>  	 * Add the rdtgrp to the list of rdtgrps the parent
>>  	 * ctrl_mon group has to track.
>>  	 */
>>  	list_add_tail(&rdtgrp->mon.crdtgrp_list, &prgrp->mon.crdtgrp_list);
>>
>> +out_unlock:
>>  	rdtgroup_kn_unlock(parent_kn);

> Is there any case that a lock is not unlocked here?

If you've found an example - please share!

The unlock is necessary because mkdir_rdt_prepare() calls rdtgroup_kn_lock_live(),
but expects its caller to do the unlock.

This patch allows the rmid allocation to fail separately to mkdir_rdt_prepare(), so
rdtgroup_mkdir_mon() needs to ensure the unlock() happens when it returns an error, this
work was previously done by 'out_unlock' in mkdir_rdt_prepare().

I hope this helps,

Thanks,

James