linux-kernel - Re: [PATCH v6 5/8] x86/resctrl: Unwind the errors inside rdt_enable

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Date:   Mon, 7 Aug 2023 11:19:20 -0500
From:   "Moger, Babu" <babu.moger@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Cc:     fenghua.yu@...el.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, paulmck@...nel.org, akpm@...ux-foundation.org,
        quic_neeraju@...cinc.com, rdunlap@...radead.org,
        damien.lemoal@...nsource.wdc.com, songmuchun@...edance.com,
        peterz@...radead.org, jpoimboe@...nel.org, pbonzini@...hat.com,
        chang.seok.bae@...el.com, pawan.kumar.gupta@...ux.intel.com,
        jmattson@...gle.com, daniel.sneddon@...ux.intel.com,
        sandipan.das@....com, tony.luck@...el.com, james.morse@....com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        bagasdotme@...il.com, eranian@...gle.com,
        christophe.leroy@...roup.eu, jarkko@...nel.org,
        adrian.hunter@...el.com, quic_jiles@...cinc.com,
        peternewman@...gle.com
Subject: Re: [PATCH v6 5/8] x86/resctrl: Unwind the errors inside
 rdt_enable_ctx

Hi Reinette,

On 8/4/23 15:41, Reinette Chatre wrote:
> Hi Babu,
> 
> On 7/19/2023 4:22 PM, Babu Moger wrote:
>> rdt_enable_ctx() takes care of enabling the features provided during
> 
> "rdt_enable_ctx() takes care of enabling" can just be "rdt_enable_ctx()
> enables"

Sure.

> 
>> resctrl mount. The error unwinding of rdt_enable_ctx is done from the
>> caller rdt_get_tree. This is not ideal and can cause some error unwinding
>> to be omitted.
>>
> 
> Please consistently use () to indicate function names (in
> changelog and subject).

Sure.

> 
> "This is not ideal and can cause some error unwinding to be omitted."
> is a bit vague. How about (in a new paragraph):
> "Additions to rdt_enable_ctx() are required to also modify error paths
> of rdt_enable_ctx() callers to ensure correct unwinding if errors
> are encountered after calling rdt_enable_ctx(). This is error prone."

Sure.

> 
>> Fix this by moving all the error unwinding inside rdt_enable_ctx.
> 
> "Fix" creates expectation for a "fixes" tag which is not needed here. This
> refactors code to simplify future additions.

Sure.
> 
> Even so, I do not think this solution addresses the stated problem (more
> below).
> 
>>
>> Suggested-by: Reinette Chatre <reinette.chatre@...el.com>
>> Signed-off-by: Babu Moger <babu.moger@....com>
>> ---
>>  arch/x86/kernel/cpu/resctrl/rdtgroup.c |   31 +++++++++++++++++++++++--------
>>  1 file changed, 23 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> index 3010e3a1394d..9a7204f71d2d 100644
>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> @@ -2381,15 +2381,31 @@ static int rdt_enable_ctx(struct rdt_fs_context *ctx)
>>  {
>>  	int ret = 0;
>>  
>> -	if (ctx->enable_cdpl2)
>> +	if (ctx->enable_cdpl2) {
>>  		ret = resctrl_arch_set_cdp_enabled(RDT_RESOURCE_L2, true);
>> +		if (ret)
>> +			goto out;
>> +	}
>>  
>> -	if (!ret && ctx->enable_cdpl3)
>> +	if (ctx->enable_cdpl3) {
>>  		ret = resctrl_arch_set_cdp_enabled(RDT_RESOURCE_L3, true);
>> +		if (ret)
>> +			goto out_cdpl2;
>> +	}
>>  
>> -	if (!ret && ctx->enable_mba_mbps)
>> +	if (ctx->enable_mba_mbps) {
>>  		ret = set_mba_sc(true);
>> +		if (ret)
>> +			goto out_cdpl3;
>> +	}
>>  
>> +	return 0;
>> +
>> +out_cdpl3:
>> +	resctrl_arch_set_cdp_enabled(RDT_RESOURCE_L3, false);
>> +out_cdpl2:
>> +	resctrl_arch_set_cdp_enabled(RDT_RESOURCE_L2, false);
> 
> Be careful here. There is no dependency between L3 and L2 CDP ...
> if L3 CDP was enabled it does not mean that L2 CDP was enabled also.
> Similarly, if the software controller was enabled it does not mean that
> CDP was also enabled.
> Since resctrl_arch_set_cdp_enabled() does much more than just change
> a flag value I think these should first check if it was enabled
> before disabling the feature.

Yes. Agree.
> 
>> +out:
>>  	return ret;
>>  }
>>  
>> @@ -2497,13 +2513,13 @@ static int rdt_get_tree(struct fs_context *fc)
>>  	}
>>  
>>  	ret = rdt_enable_ctx(ctx);
>> -	if (ret < 0)
>> -		goto out_cdp;
>> +	if (ret)
>> +		goto out;
>>  
>>  	ret = schemata_list_create();
>>  	if (ret) {
>>  		schemata_list_destroy();
>> -		goto out_mba;
>> +		goto out_ctx;
>>  	}
>>  
>>  	closid_init();
>> @@ -2562,10 +2578,9 @@ static int rdt_get_tree(struct fs_context *fc)
>>  	kernfs_remove(kn_info);
>>  out_schemata_free:
>>  	schemata_list_destroy();
>> -out_mba:
>> +out_ctx:
>>  	if (ctx->enable_mba_mbps)
>>  		set_mba_sc(false);
>> -out_cdp:
>>  	cdp_disable_all();
>>  out:
>>  	rdt_last_cmd_clear();
>>
> 
> The problem statement in the changelog was that rdt_get_tree() is
> doing error unwinding of rdt_enable_ctx(). Looking at the above it
> seems that the problem remains ... callers of rdt_enable_ctx()
> still need to know all internals of that function to do error
> unwind correctly. Could it perhaps be made simpler with a new
> rdt_disable_ctx() that undoes rdt_enable_ctx()? New additions
> to rdt_enable_ctx() would have more clarity where changes are
> needed and callers only need to call a single rdt_disable_ctx().
> 

Yes. We can do that.
-- 
Thanks
Babu Moger