linux-kernel - Re: [RFC PATCH v3 1/2] membarrier: Provide register expedited private command

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Date:   Fri, 22 Sep 2017 16:59:59 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Andrew Hunter <ahh@...gle.com>,
        Maged Michael <maged.michael@...il.com>, gromer@...gle.com,
        Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH v3 1/2] membarrier: Provide register expedited
 private command

On Tue, Sep 19, 2017 at 06:13:41PM -0400, Mathieu Desnoyers wrote:
[...]
> +static inline void membarrier_arch_sched_in(struct task_struct *prev,
> +		struct task_struct *next)
> +{
> +	/*
> +	 * Only need the full barrier when switching between processes.
> +	 */
> +	if (likely(!test_ti_thread_flag(task_thread_info(next),
> +			TIF_MEMBARRIER_PRIVATE_EXPEDITED)
> +				|| prev->mm == next->mm))

And we also don't need the smp_mb() if !prev->mm, because switching from
kernel to user will have a smp_mb() implied by mmdrop()?

> +		return;
> +
> +	/*
> +	 * The membarrier system call requires a full memory barrier
> +	 * after storing to rq->curr, before going back to user-space.
> +	 */
> +	smp_mb();
> +}

[...]

> +static inline void membarrier_fork(struct task_struct *t,
> +		unsigned long clone_flags)
> +{
> +	if (!current->mm || !t->mm)
> +		return;
> +	t->mm->membarrier_private_expedited =
> +		current->mm->membarrier_private_expedited;

Have we already done the copy of ->membarrier_private_expedited in
copy_mm()?

Regards,
Boqun

> +	membarrier_arch_fork(t, clone_flags);
> +}
> +static inline void membarrier_execve(struct task_struct *t)
> +{
> +	t->mm->membarrier_private_expedited = 0;
> +	membarrier_arch_execve(t);
> +}
> +#else
> +static inline void membarrier_sched_in(struct task_struct *prev,
> +		struct task_struct *next)
> +{
> +}
> +static inline void membarrier_fork(struct task_struct *t,
> +		unsigned long clone_flags)
> +{
> +}
> +static inline void membarrier_execve(struct task_struct *t)
> +{
> +}
> +#endif
> +
[...]

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