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From: Reinette Chatre <>
To: Valentin Schneider <>
Subject: Re: [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group
Date: Mon, 14 Dec 2020 10:38:58 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hi Valentin,

On 12/11/2020 12:46 PM, Valentin Schneider wrote:
> On 03/12/20 23:25, Reinette Chatre wrote:
>> Valentin's series in [2] ends by adding memory barriers to support the
>> updating of the task_struct from one CPU and the usage of the task_struct data
>> from another CPU. This work is still needed and as discussed with Valentin in
>> that thread the work would be re-evaluated by him after seeing how this series
>> turns out.

Thank you very much for taking a look.

> So the "problematic" pattern is still there: a context switch can happen
> concurrently with a write to the switching-to-tasks's {closid, rmid}.
> Accesses to these fields would thus need to be wrapped by READ_ONCE() &


> 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? 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.

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. 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?

Thank you



  reply	other threads:[~2020-12-14 18:40 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 [this message]
2020-12-16 17:41     ` Valentin Schneider
2020-12-16 18:26       ` Reinette Chatre
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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