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From: Reinette Chatre <reinette.chatre@intel.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: tglx@linutronix.de, fenghua.yu@intel.com, bp@alien8.de,
	tony.luck@intel.com, kuo-lang.tseng@intel.com,
	shakeelb@google.com, mingo@redhat.com, babu.moger@amd.com,
	james.morse@arm.com, hpa@zytor.com, x86@kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group
Date: Wed, 16 Dec 2020 10:26:42 -0800	[thread overview]
Message-ID: <2ff6b87f-3b1f-7b5e-2b4c-050192a12d4f@intel.com> (raw)
In-Reply-To: <jhj3605tzr1.mognet@arm.com>

Hi Valentin,

On 12/16/2020 9:41 AM, Valentin Schneider wrote:
> 
> On 14/12/20 18:38, Reinette Chatre wrote:
>>> Thinking a bit more (too much?) about it, we could limit ourselves to
>>> wrapping only reads not protected by the rdtgroup_mutex: the only two
>>> task_struct {closid, rmid} writers are
>>> - rdtgroup_move_task()
>>> - rdt_move_group_tasks()
>>> and they are both invoked while holding said mutex. Thus, a reader holding
>>> the mutex cannot race with a write, so load tearing ought to be safe.
>>
>> The reads that are not protected by the rdtgroup_mutex can be found in
>> __resctrl_sched_in(). It thus sounds to me like your proposed changes to
>> this function found in your patch [1] is what is needed?
> 
> Right.
> 
>> It is not clear
>> to me how the pairing would work in this case though. If I understand
>> correctly the goal is for the  write to the closid/rmid in the functions
>> you mention above to be paired with the reads in resctrl_sched_in() and
>> it is not clear how adding a single READ_ONCE would accomplish this
>> pairing by itself.
>>
> 
> So all the writes would need WRITE_ONCE(), but not all reads would require
> a READ_ONCE() (those that can't race with writes shouldn't need them).

Got it. I misunderstood your previous response, mistakenly thinking that 
it stated that there would be only READ_ONCE() reads without being 
paired with WRITE_ONCE() writes. Thanks for clearing that up.

> I'll go and update that patch so that you can bundle it with v2 of this
> series.

Thank you so much.

>> It is also not entirely clear to me what the problematic scenario could
>> be. If I understand correctly, the risk is (as you explained in your
>> commit message), that a CPU could have its {closid, rmid} fields read
>> locally (resctrl_sched_in()) while they are concurrently being written
>> to from another CPU (in rdtgroup_move_task() and rdt_move_group_tasks()
>> as you state above). If this happens then a task being moved may be
>> scheduled in with its old closid/rmid.
> 
> Worse, it may be scheduled with a mangled closid/rmid if the read in
> resctrl_sched_in() is torn (i.e. compiled as a sequence of multiple
> smaller-sized loads). This one of the things READ_ONCE() / WRITE_ONCE()
> try to address.

I see. This area is unfamiliar to me, thank you very much for catching 
this and helping to get it right.

> 
>> The update of closid/rmid in
>> rdtgroup_move_task()/rdt_move_group_tasks() is followed by
>> smp_call_function_xx() where the registers are updated with preemption
>> disabled and thus protected against __switch_to. If a task was thus
>> incorrectly scheduled in with old closid/rmid, would it not be corrected
>> at this point?
>>
> 
> Excluding load/store tearing, then yes, the above works fine.
> 

Thank you for this sanity check.

Reinette


  reply	other threads:[~2020-12-16 18:27 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-03 23:25 [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group Reinette Chatre
2020-12-03 23:25 ` [PATCH 1/3] x86/resctrl: Move setting task's active CPU in a mask into helpers Reinette Chatre
2020-12-07 18:29   ` Borislav Petkov
2020-12-07 21:24     ` Reinette Chatre
2020-12-08  9:49       ` Borislav Petkov
2020-12-08 16:35         ` Reinette Chatre
2020-12-09 16:47   ` James Morse
2020-12-10  0:21     ` Reinette Chatre
2020-12-03 23:25 ` [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group Reinette Chatre
2020-12-09 16:51   ` James Morse
2020-12-10  0:22     ` Reinette Chatre
2020-12-11 20:46   ` Valentin Schneider
2020-12-14 18:41     ` Reinette Chatre
2020-12-16 17:41       ` Valentin Schneider
2020-12-16 18:26         ` Reinette Chatre
2020-12-17 10:39           ` Valentin Schneider
2020-12-03 23:25 ` [PATCH 3/3] x86/resctrl: Don't move a task to the same " Reinette Chatre
2020-12-11 20:46 ` [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a " Valentin Schneider
2020-12-14 18:38   ` Reinette Chatre
2020-12-16 17:41     ` Valentin Schneider
2020-12-16 18:26       ` Reinette Chatre [this message]
2020-12-17 12:19 ` [PATCH 4/3] x86/intel_rdt: Apply READ_ONCE/WRITE_ONCE to task_struct .rmid & .closid Valentin Schneider

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