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From: Valentin Schneider <>
To: Reinette Chatre <>
Subject: Re: [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group
Date: Wed, 16 Dec 2020 17:41:22 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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?


> 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).

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

> 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.

> 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
> Reinette
> [1]

  reply	other threads:[~2020-12-16 17:42 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 [this message]
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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