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From: Valentin Schneider <valentin.schneider@arm.com>
To: Reinette Chatre <reinette.chatre@intel.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: Fri, 11 Dec 2020 20:46:27 +0000	[thread overview]
Message-ID: <jhjk0tot6jg.mognet@arm.com> (raw)
In-Reply-To: <cover.1607036601.git.reinette.chatre@intel.com>


Hi Reinette,

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.
>

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() &
WRITE_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.

> [1]: https://lore.kernel.org/lkml/CALvZod7E9zzHwenzf7objzGKsdBmVwTgEJ0nPgs0LUFU3SN5Pw@mail.gmail.com/
> [2]: https://lore.kernel.org/lkml/20201123022433.17905-1-valentin.schneider@arm.com
>
> Fenghua Yu (3):
>   x86/resctrl: Move setting task's active CPU in a mask into helpers
>   x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to
>     resource group
>   x86/resctrl: Don't move a task to the same resource group
>
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 159 +++++++++++++------------
>  1 file changed, 82 insertions(+), 77 deletions(-)
>
>
> base-commit: b65054597872ce3aefbc6a666385eabdf9e288da

  parent reply	other threads:[~2020-12-11 21:41 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 ` Valentin Schneider [this message]
2020-12-14 18:38   ` [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a " Reinette Chatre
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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