linux-kernel - Re: [tip:x86/mm] x86/mm: Give each mm TLB flush generation a unique ID

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Date:   Wed, 5 Jul 2017 12:58:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     riel@...hat.com, akpm@...ux-foundation.org, arjan@...ux.intel.com,
        mgorman@...e.de, hpa@...or.com, nadav.amit@...il.com,
        tglx@...utronix.de, luto@...nel.org, bp@...en8.de,
        mingo@...nel.org, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mm] x86/mm: Give each mm TLB flush generation a unique
 ID

On Wed, Jul 05, 2017 at 03:31:03AM -0700, tip-bot for Andy Lutomirski wrote:

> @@ -132,6 +135,9 @@ static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
>  static inline int init_new_context(struct task_struct *tsk,
>  				   struct mm_struct *mm)
>  {
> +	mm->context.ctx_id = atomic64_inc_return(&last_mm_ctx_id);

You could use atomic64_inc_return_relaxed() here; but since its x86
specific there is no difference.

> +	atomic64_set(&mm->context.tlb_gen, 0);
> +
>  	#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>  	if (cpu_feature_enabled(X86_FEATURE_OSPKE)) {
>  		/* pkey 0 is the default and always allocated */

> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index 50ea348..ad21353 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -57,6 +57,23 @@ static inline void invpcid_flush_all_nonglobals(void)
>  	__invpcid(0, 0, INVPCID_TYPE_ALL_NON_GLOBAL);
>  }
>  
> +static inline u64 inc_mm_tlb_gen(struct mm_struct *mm)
> +{
> +	u64 new_tlb_gen;
> +
> +	/*
> +	 * Bump the generation count.  This also serves as a full barrier
> +	 * that synchronizes with switch_mm(): callers are required to order
> +	 * their read of mm_cpumask after their writes to the paging
> +	 * structures.
> +	 */
> +	smp_mb__before_atomic();
> +	new_tlb_gen = atomic64_inc_return(&mm->context.tlb_gen);
> +	smp_mb__after_atomic();

that's wrong... smp_mb__{before,after}_atomic() are entirely superfluous
here:

 - they're no-ops on x86
 - atomic_*_return() is already fully serializing