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, stable@vger.kernel.org
Subject: Re: [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group
Date: Thu, 17 Dec 2020 10:39:43 +0000 [thread overview]
Message-ID: <jhjwnxgsols.mognet@arm.com> (raw)
In-Reply-To: <b75d780d-d067-12bf-b0e6-706dda200511@intel.com>
On 16/12/20 18:26, Reinette Chatre wrote:
> Hi Valentin,
>> So that's part paranoia and part nonsense from my end - the contents of
>> smp_call() shouldn't matter here.
>>
>> If we distill the code to:
>>
>> tsk->closid = x;
>>
>> if (task_curr(tsk))
>> smp_call(...);
>>
>> It is somewhat far fetched, but AFAICT this can be compiled as:
>>
>> if (task_curr(tsk))
>> tsk->closid = x;
>> smp_call(...);
>> else
>> tsk->closid = x;
>>
>> IOW, there could be a sequence where the closid write is ordered *after*
>> the task_curr() read.
>
> Could you please elaborate why it would be an issue is the closid write
> is ordered after the task_curr() read? task_curr() does not depend on
> the closid.
>
IMO the 'task_curr()' check only makes sense if it happens *after* the
write, the logic being: 'closid/rmid has been written to, does the task now
need interrupting?'
In the above 'else' clause, task_curr() would need to be re-evaluated after
the write: the task wasn't current *before* the write, but nothing
guarantees this still holds *after* the write.
>> With
>>
>> tsk->closid = x;
>>
>> barrier();
>>
>> if (task_curr(tsk))
>> smp_call(...);
>>
>> that explicitely cannot happen.
>>
>
>
> Reinette
next prev parent reply other threads:[~2020-12-17 10:40 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 [this message]
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
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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