Re: [PATCH v6 5/8] x86/resctrl: Unwind the errors inside rdt_enable_ctx

Moger, Babu
2023-08-07T16:19:37Z

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@intel.com>
>> Signed-off-by: Babu Moger <babu.moger@amd.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